Circuit Diagram & Explanation

Engines&Magnets

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1. Tecumseh Ignitions-Part 1, SSI Replacement

Part1, Trigger Pin SSI, 610906,610748, 610855 and Onan NB.( Replacement for the 610759 was added as Attachment 5 on 30 March 2012.)

Background

About 10 years ago, I came across a mid 1980s Chrysler transistorized ignition with parts readily available at competitive prices and realized that it would make an economical replacement ignition for small air cooled engines. There is a transistor in the Control Module, CM that switches a conventional ignition coil in place of the points. The ignition coil here has a built in ballast resistor. The Control Module needs a timing signal or trigger pulse to fire the spark. This is the Automotive Replacement Ignition presented here.

Introduction

To help you place your ignition in the various configurations, a brief history is needed. The Tecumseh engines addressed here are the older, cast iron series starting with the HH100 in about 1967, thru the HH120, OH 120, OH 140, OH 160 and OH 180. The first of these engines had true Point / Condenser Magnetos which were followed by the first generation Capacitance Discharge Ignition, CDI, or 610759 Solid State Ignition, SSI, with the electronics under the flywheel. This was followed by the now most common 610906, 610855 and 610748 which have two trigger pins on the outer rim of the flywheel. The Onan NB has an ignition like the 610748 but with a single knob cast in the flywheel. It is the configuration with the trigger pins on the flywheel that is discussed here in Section 1 and are shown in Figure 1-1, 3up.jpg.

Figure 1-1 3up.jpg

If you installed trigger pins on the flywheel of about any engine and made a suitable trigger bracket, this ignition should work. It did not work on my 1922 Ottawa Hit - Miss because I could not get up enough RPM when cranking by hand under compression for the trigger to work.

A good place to start is initial test procedures to avoid guessing, but first a few do’s and don’ts first.

Dos and Don’ts

1. Don’t ever crank an engine with out a grounded spark plug attached to the spark plug wire, see Figure 1-2 Spark Tester. The output voltage of the ignition increases until it is high enough to arc the plug gap. With out a spark plug attached, the voltage will continue to increase until it arcs somewhere else inside the ignition where it can do permanent damage. If the engine is running, never remove the spark plug wire to kill the engine; shut off the fuel.

Figure 1-2

2. Don’t assume an attitude of guessing at what has failed, buying and installing parts only to realize that it was not the part that failed. Instead, do some detective work to gather the facts which point to or isolate the failure. Start with the simple and easy tests first, even if you think the failure is elsewhere.

3. If you ask for help on one of the many internet yahoo engine or tractor clubs like http://groups.yahoo.com/group/tecumseh_engine_group or http://groups.yahoo.com/group/eskaoutboards , give a complete description of your tractor and engine with the model numbers. This will help those trying to help you to know the configuration that you are talking about.

4. If you don’t already have a Volt/Ohm meter, get an inexpensive one at Sears, Walmart or an auto parts store. This will save a lot of guessing and frustration. My $10 analog meter looks like Figure 1-3, MeterA.jpg. If you set it to Ohms and touch the leads together, the needle will go to zero, no resistance as in a short. If you leave the leads separated, the meter will read infinite (very high) resistance or an open circuit.

Figure 1-3

Testing Original Equipment

Test for Spark

Usually checking for a spark is a good place to start. Figure 1-2, SparkTest.jpg shows a home made spark tester which can help protect from causing damage. Removing the spark plug from the cylinder head provides compression release and reduces the stress on the battery, starter motor and rope puller during testing.

I use inductive timing light and have had problems with it not working on a weak sparks. This I have resolved by running the spark wire thru the pick up twice instead of just once. See Figure 2-4, TimingLight.jpg. This doubles the spark current sensed by the timing light and fires off the weak spark. Sometimes there is not a sufficient length of spark wire, so I made an adapter with the loop in it to place in series with the wire to the spark plug. When the normal timing light won’t fire off a weak spark, but with the loop does fire, I conclude the ignition has a weak spark and has failed. It is like a go / no go test.

Figure 1-4

Engine Kill Wire Check

Engines with magnetos as well as electronic ignitions like the Solid State Ignition, SSI, are shut down by grounding out the ignition. Sometimes the tractor wiring between the key switch and the engine gets shorted to ground, thus always killing the ignition. An easy way to check for this is to disconnect the kill wire at the engine to separate the tractor wiring from the engine wiring. For the 20 Amp Stator, there is a three terminal connector that comes out from behind the flywheel for this connection. The kill wire is in the center and the outer terminals are for the AC for charging the battery. See Figure 1-5, 20stator.jpg or Figure 1-6, 10Stator.jpg and 20StatorCT below. Just disconnect the connector for test. The battery won’t be charging, but that is O.K. for a short time. If you disconnect the kill wire at the engine and then get a spark, then there is a problem in the tractor wiring, not the ignition system.

Figure 1-5

Figure1-6

Figure1-7

Spark Plug Connector

Another common problem with Tecumseh engines is the connector at the spark plug. Measure the resistance from the connector to the engine block. The resistance should be 4,000 to 7,000 Ohms. If it is erratic, in the megohm range or open, cut off the boot and re-measure directly from the stranded wire. If it now checks O.K., hook the wire directly to the spark plug until all else is fixed and then replace the connector with a Briggs and Stratton 493880S or 66538S or Stens 135-081. If the spark wire is still open, it may have a poor connection at the other end of the wire. After all the other testing is completed, you can use what ever force is necessary to pull the spark wire out of the SSI and will find a small brass nail in the bottom center of the hole. Measure the resistance from that pin to ground. If it is still open, there is likely an open circuit in the secondary winding of the Pulse Transformer inside the SSI and the unit has failed. If there is the nominal resistance from the pin to ground, clean out the hole and replace the spark plug wire with new stranded core wire.

Ignition Charge Coil

There is an Ignition Charge Coil as part of the Stator under the flywheel.

It is very rare that they fail, but they have been damaged from cleaning and debris from loose flywheel magnets. It is possible that if the wrong type ignition key switch were installed and +12 Volts DC were applied by the kill switch wire instead of a ground that the Charge Coil may burn up. Before testing the Ignition Charge Coil, first disconnect the kill switch wire at the engine as above so it being grounded by the key switch will not give you miss leading information. To test this Ignition Charge Coil, measure the resistance between the engine block and the disconnected lead at the SSI. Figures 1-5, 20stator.jpg and 1-6, 20StatorCT .jpg show the 20 Amp Stator and associated circuit diagram for the 610906 SSI. The resistance of Ignition Charge Coil for the 610906 is Ohms 137 Ohms. The 10 Amp Stator for the 610748 is similar. You can also crank the engine and touch the terminal on the SSI and feel a tickle from the voltage pulses.

Beyond that, you can mail the SSI to me and I can run it on the engine simulator to verify the failure. If the tests above show no failure except there is no spark, then it is most likely that the SSI has failed and you have three options:

1. Replace the ignition system with a battery powered automotive type below.

2. Look for a replacement. Good luck.

3. See Dale Colvert’s site www.overnightsolutions.com

Automotive Type Replacement Ignition

The automotive type replacement ignition is shown in Figure 1- 8,Option 3C

Figure 1- 8

Here a conventional ignition coil ( with a built in ballast resistor ) is powered the 12 Volt battery. This requires a different key switch or other arrangement for the battery power. Since the engine has no points, the coil is switched by a transistor in the mid 1980's Chrysler Control Module, CM. The CM needs a timing pulse which is provided by a Trigger Coil that replaced the original SSI. The CM also needs 12 V battery power.

In the last count, about 500 Tecumseh engines have been converted, though most use the first generation Bobbin Trigger and second generation Modified 610906. There is a List of Material at the end. I can provide the Trigger Coil, edstoller at earthlink dot net. If you needed a complete kit including the Trigger Coil, Control Module, Ignition coil , new key switch and wire harness with all wires labeled and all connectors provided, see Mike Brooks, 585-243-7765 or brokndwn64@gmail.com .

Trigger Coil

The third generation Trigger Coil is shown in Figure 1-9, 3GenTrg.jpg

Figure 1-9

The Trigger Coil is mounted on a bracket which replaces the failed SSI. The wire that came from the SSI terminal must taped up and secured . This wire was used to charge the capacitor in the SSI, but is no longer needed and still has several hundred volts on it at high RPM. The bracket is designed to align the magnet directly over the tall trigger pin and to set the optimum air gap of about .015". The mounting hole size provides some room for adjustment, but use of a round file maybe required to position the magnet directly over the tall trigger pin and to set the optimum air gap. There is a slight difference between the L-Head and the Over Head Valve, OHV engines so we have a slightly different bracket for each type engine.

One objective in designing the trigger coil was to keep it as simple as possible to keep the costs low, but carefully optimize all the parameters for a TDC spark for easy starting at an extremely low RPM and yet have a stable transition to the Advanced spark at an RPM above what the starter motor is capable of. This is to prevent kick back. In other words, the engine has to already have started running for the spark advance to kick in. The performance here is much better than that of the first and second generation triggers. Despite the simplicity, the trigger coil requires very tight configuration control and testing during manufacture.

The Trigger Coil consists of a strong Rear Earth magnet mounted on the end of a partially threaded 1/4 inch bolt. Fiber washers are used to make a rugged spool for the coil winding. The start end of the winding is soldered directly to the bolt for the grounded end which accounts for one of the two connections. A diode is connected to the other end of the winding and to the yellow lead wire which goes to the Control Module Terminal B.

When the tall trigger pin passes the magnet, a voltage is generated which provides a timing pulse to the Control Module which causes a spark. The magnitude of the voltage and thus the transition form the TDC, starting spark to the advanced spark is determined by, the strength and direction of the magnet, the diode, the dimension of the spool, the number of turns of wire in the coil, the air gap , the RPM and somewhat on the battery voltage. Since the air gap for the short trigger pin is much wider, the critical voltage for spark advance is not reached until a higher RPM. Each trigger is tested and calibrated for a transition between 550 and 750 RPM for the air gap specified, nominally .015" .

Control Module

The Control Module with associated connections is shown in Figure 1-10, CM.jpg. Note that the CM must be grounded and mounted is a cool place. The CM runs on 12 V and quits when the battery drops below about 8 volts. So it is advisable it have a good battery.

Figure 1-10

Figure1-11

Ignition Coil

The Coil with associated connections is shown in Figure 1- 11, Coil.jpg.

If the Ignition Coil does not have an internal ballast resistor, an external ballast resistor will have to be added in series with the + lead.

Installation Options

Wire Conventions: To make it easier for us to communicate clearly, the colors of wires is defined as follows:

Red /Orange ...........................Plus 12 Volts DC

Black ......................Minus 12 Volts DC and Ground

Yellow .............................Timing / Trigger Pulse

Blue ...........................Negative side of Ignition Coil

Switch ology

There is a great variety of key switches, but one looks like this, Figure 1-12,MagSw.jpg

Details on key switches can be found at http://www.kochsales.com/?page_id=29 . In the switch ology world, you may see other letters like:

A - Accessories

I - Ignition

R - Run or Regulator or Rectifier

Installation Options Updated from December 11, 2009

Most of the ignitions being replaced are old Capacitance Discharge Ignitions , CDI or Solid State Ignitions, SSI. These ignition systems, like magnetos, get their electrical power from flywheel magnets rotating past a coil in the ignition system. To shut the ignition off, this coil is connected to ground by the key ignition switch. This is most unsuitable for the replacement ignition which gets its power from the battery and is turned on / off by connecting or disconnecting the battery power. The three installation options below are ways around this fundamental problem.

Option 1 - Use existing key switch for a magneto or CDI ignitions

Option 2 - Adding a new Toggle Switch and Pilot Light

Option 3 - Adding a new Key Switch

Option 1 - Use existing key switch for a magneto or CDI ignitions

Option 1 uses the old key switch, but leaves the M or Magneto terminal not connected. We need power for the replacement ignition both during Start and Run key switch positions. If we connected the Replacement Ignition to the L or A terminal, we would have power in the Running mode, but usually not during the Starting mode. If we connected the Replacement Ignition to the S terminal, we would have power in the Starting mode, but not during the Running mode. So we need to tap into both wires coming from the switch, one for Starting mode and the other for Running mode. If we just spliced these two connections together and ran the wire to the Replacement Ignition, we would have a new problem; the starter would run all the time. To solve this problem, a diode is placed in series with the connection to the S terminal to block the current that would flow from the Run connection. To keep Mr. Murphy form getting these two connections mixed up, a diode is placed in both connections to the switch. Again, for Mr. Murphy, the wires going from the diodes to the switch are RED and the wires going from the diodes to the replacement Ignition are ORANGE. To make the conversion to the Replacement Ignition as easy as possible, a pre- wired Diode Wire Harness can be provided which already has the diodes, with the correct polarity, soldered in it and wires labeled. The Diode Wire Harness is shown in Figure 1-13, Diodes.jpg.

Figure 1-13

Figure1-14

The replacement ignition draws about 3 Amps, so I use 6 Amp diodes for reliability. Realizing that the starter solenoid will produce a voltage spike when the current is removed, I picked a 600 Volt Peak Inverse Voltage, PIV which will be more than sufficient.

Figure 1-14, Option1.jpg Shows the wires to be added for Option 1. The existing wires are not shown and should not be disturbed unless to tap into.

Mike Brooks, brokndwn64@gmail.com , Phone 585-243-7765, offers a kit with all the components in Figure 1-14, Option1C.jpg except the pre-existing switch. The kit comes with the pre-fabricated Diode Wire Harness with only one end of the wires made up so the wires can be trimmed to fit the installation.

Option 1 Installation Instructions:

1. Turn the key switch to off and disconnect the negative battery cable.

2. Install the trigger coil making sure it is centered over the tall trigger pin, set the air gap as marked on the trigger, about .015". This controls the RPM for the transition to advanced spark. Tape up and stow the ICC connection to the stator.

3. Install the Ignition Coil in a cool place and so the spark wire reaches the plug.

4. Install the Control Module in a convenient but cool place. If installed on plastic, run a wire to ground the case.

5. Locate the wire that comes from the S terminal on the key switch and tap the RED Diode Wire Harness wire marked " To Start" into it with . See Figure 1-15 TapSplice.jpg . The other end of the wire goes to the solenoid , which might be easier to get to and should have a post for a ring terminal.

Figure 1-15

6. Locate the wire that comes from the A, L or R terminal on the key switch and tap the RED Diode Wire Harness wire marked " To Run" into it.

7. Connect the ORNGE Diode Wire Harness wire marked "Ignition Coil +" to the Ignition Coil +.

8. Connect the ORANGE Diode Wire Harness wire marked " CM-C" to the Control Module terminal C with a bullet connector.

9. Connect the YELLOW wire from the Trigger Coil to the Control Module terminal B with a bullet connector.

10. Connect the negative terminal on the Ignition Coil to the Control Module terminal D to with a bullet connector using BLUE wire.

11. Connect the spark plug wire to the Ignition Coil and uninstalled spark plug. Wrap some bare wire around the threads of the spark plug and ground the other end.

12. Turn the key switch to off and reconnect the negative battery terminal.

13. Turn the key switch on and look for a spark at the spark plug when turning the switch on and off.

14. Spin the flywheel by hand and see if you get sparks.

15. Install the spark plug and blower housing and fire up the engine.

Option 2 - Adding a new Toggle Switch and Pilot Light

Another way the Automotive Replacement Ignition can get its electrical power is directly from the battery by adding a fuse, toggle switch and pilot light, but this has the disadvantage that if the engine ignition left on, it will eventually run down the battery. A LED pilot light is added here to help remind the operator to turn off the ignition. This installation is ideal for welders and compressors that do not have the original key switch. Figure 1-16,Option2.jpg , shows a mockup of Option 2.

Figure 1-16

Most tractors have a heavy red wire that goes from the + battery terminal to the always hot post of the starter solenoid (1). ( Following the numbers on the figure.) This is a good place to pick up the needed power.(Always disconnect the battery ground before messing with the hot wires.) This connects to a 5 Amp fuse (2) and on to a new toggle switch (3). The switched power (4) goes to three devices;

1. The + side of the Ignition Coil.

2. Terminal C on the Control Module

3. The pilot light

The terminals of the Control Module can be identified by noting the transistor. The terminal closest to the transistor is A which is not used here since the ground for the Control Module is provided by mounting it of the metal of the tractor.

The pilot light has a ground that can be easily connected to a 5/16 ring terminal of the pilot light assembly. The yellow wire, 5, connects the trigger signal form the Trigger Coil to terminal B of the Control Module. Last, the BLUE wire, 6 connects the switched output of the Control Module , terminal D to the negative or - side of the Ignition Coil.

Mike Brooks, brokndwn64@gmail.com, Phone 585-243-7765 offers a kit with all the components in Figure 1-16 except the battery, battery cable and solenoid. The kit comes with the pre-fabricated wire harness with only one end of the wires made up so the wires can be trimmed to fit. The various crimp connector are provided.

Option 2 -Installation Instructions

1 Disconnect the negative battery terminal.

2. Locate a desired place on the dash board , drill a ½ hole for the new toggle switch and a 5/16 hole for the pilot light about 2 inches above the switch hole. See Figure 2-17, Switch.jpg.

Figure 1-17

3. Install the switch in the bottom hole and the pilot light above it. The 5/16 ring terminal goes on the pilot light on the back side of the dash board and is for the pilot light ground. The black wire from the pilot light to the 5/16 ring terminal then tighten to mounting nut. If the dash board is plastic, connect the black pilot light wire to a suitable ground.

4. Connect the pilot light red wire to the short red wire coming off the new toggle switch . When installing the Pilot Light, the wires have to be inserted in the 5/16 hole first. Because they are soldered to the Pilot Light, they must be of a smaller gage to fit thru the hole and they need to be spliced after installed. Splice them by t twisting them tightly together with a pliers. Then install a wire nut and tape tightly with electrical tape.

5. Connect the red wire coming off the switch with the ring terminal to the Ignition Coil +.

6. Connect the red wire coming off the switch with the bullet connector to the Control Module terminal C .

7. Connect the wire coming off the switch that has a 5 amp fuse in it to the always hot side of the solenoid using a 1/4 inch ring terminal.

8. Connect the YELLOW wire from the Trigger Coil to terminal B on the Control module.

9. Connect the Control Module terminal D to the negative terminal on the Ignition Coil with BLUE wire.

10. Check to make sure that the Pilot Light and Control Module are grounded.

11. Connect the spark plug wire to the Ignition Coil and uninstalled spark plug. Wrap some bare wire around the threads of the spark plug and ground one end.

12. Turn the new toggle switch to off and reconnect the negative battery terminal.

13. Turn the toggle switch on and look for the Pilot Light to light. You should get a spark at the spark plug when turning the switch on and off.

14. Spin the flywheel by hand and see if you get sparks.

15. Install the spark plug and blower housing and fire up the engine.

Installation Option 3 - Replacing the Key Switch

A third way to power the Automotive Replacement Ignition is to get its electrical power from a new key switch made for battery powered ignitions. Figure 1-18, Option 3C, repeated here, shows a mockup of Option 3.

Figure 1-18

Here the white wire from key switch terminal I provides power directly to both the Control Module terminal D and the + terminal of the Ignition Coil. There are too many different types of switches being replaced for me to provide details on the other wires.

Option 3 Installation Instructions

1. Disconnect the negative battery terminal.

2. Install the Trigger Coil, set the air gap to as marked on the unit, typically .015". This air gap controls the RPM for the transition to advanced spark.

3. Install the Ignition Coil in a cool place and so the spark wire reaches the plug.

4. Install the Control Module in a convenient but cool place. If installed on plastic, run a wire to ground the case.

5.Remove the old key switch and note the terminal markings on the back of the switch. Install the new key switch and note the terminal markings on the back of that also. Install the new switch connector. On the white wire ( shown here) splice two red wires and route them one to the + terminal of the Ignition Coil and terminal C of the Control Module. Splice the other wires following the markings on the back of the switch.

6. Connect the YELLOW Trigger wire to the Control Module terminal B with a bullet connector.

7. Connect the negative terminal on the Ignition Coil to the Control Module terminal D to with a bullet connector using BLUE wire.

8. Connect the spark plug wire to the Ignition Coil and uninstalled spark plug. Wrap some bare wire around the threads of the spark plug and ground other end.

9. Turn the key switch to off and reconnect the negative battery terminal.

10. Turn the new key switch to the RUN position and look for a spark at the spark plug when turning the switch on and off.

11. Turn the new key switch to the START position and look for a spark at the spark plug when turning the switch on and off.

12. Spin the flywheel by hand and see if you get sparks.

13. Install the spark plug and blower housing and fire up the engine.

Trouble Shooting Guide for all three Options.

Connect a grounded spark plug to the ignition. When power is applied and removed from the circuit, you should see a spark. This is a good over all check that the ignition is set up right.

The spark timing is determined by the physical location of the trigger pins on the flywheel. The transition from the TDC to Advanced spark as the engine increases in RPM is controlled by the air gap between the trigger pins and the Trigger Coil. It is the nature of magnetic induction that the trigger coil voltage increases with RPM. Increasing the air gap, reduces the trigger voltage causing the transition to be delayed to a higher RPM. Decreasing the air gap increases the trigger voltage and causes the transition to occur earlier or at a lower RPM.

With and with out a spark plug installed provides two conditions of compression and hence two test RPMs. With an old spark plug installed for normal compression , an inductive timing light will show the tall trigger pin just before the magnet when triggering the TDC spark. With no spark plug installed for a no compression and a higher RPM, an inductive timing light might show the short trigger pin just before the magnet when triggering the Advanced spark. If this test still shows the tall pin, the transition from the TDC, start spark to the advanced spark has not occurred yet. Before you put the blower housing on you can run a cold engine for half a minute. If you try this, you should see the spark advance as the engine begins to run.

Voltage measurements:

Be sure to use a real battery in good condition and not a battery charger or battery eliminator or you will get continuous sparks.

1. Meter black lead on ground, red lead on Ignition Coil +

A. Switch off - reading zero

B. Switch on run - reading +12 Volts DC

C. Switch on Start- reading +12 Volts DC

2. Meter black lead on ground, red lead on Control Module Terminal C

A. Switch off - reading zero

B. Switch on run - reading +12 Volts DC

C. Switch on Start- reading +12 Volts DC

Wire Check.

1. BLUE wire from Control Module Terminal D to -, negative on Ignition

Coil

2. YELLOW wire from Trigger to Pin B on Control Module

3. If the Ignition Coil does not have a built in resistor, a ballast resistor must be connected to the + terminal of the Ignition coil and the Red or Orange feed wire connected to the other end of the ballast resistor. That is the ballast resistor is in series with the coil. If the Ignition Coil needs a ballast resistor and does not have one, it will run real hot.

Ground Check.

Make sure the Control Module is grounded.

Make sure the Battery connections are clean and thght.

Trigger Check.

Again, with a grounded spark plug connected to the ignition. Spin the flywheel by hand and you should see a spark every time the trigger pins on the flywheel pass the trigger. The trigger has a diode in it so a normal Ohms check won't mean much. If your meter has a diode test feature, measure between the frame ( ground) and the YELLOW Trigger wire in both directions. You should get .48 volts in one direction which shows that you have continuity and the diode is good.

List of Materials

Several have ask me for a complete kit for the conversion to the automotive type ignition. This is more than I can deal with so Mike Brooks, 585-243-7765 or brokndwn64@gmail.com, has put together a complete and cost effective kits in the spirit of helping folks keep their old tractors going again with out delay. I have tested the kit and send him triggers. Thus Mike can provide all the parts form one source complete even with the Third Generation Trigger, Control Module, wire, various connectors, switch, coil, and coil bracket.

I can provide the Third Generation Trigger if that is all you need, edstoller at earthlink dot net. For Modified SSI Tecumseh 610906, or bobbin trigger, see the DYI Trigger below.

Control Module:

Wells............... CR 109

NAPA .............TP51SB

Ignition Coil: (* built in resistor)

Tecumseh*...............32080

Kohler* ...................231281 or 237256 ( NAPA 4151921S )

Mopar .....................4176009

Ford., Motorcraft.....D5TE-12029 Says on it" use with external resistor "

Foley-Belsaw.............5978730 ( $17.50 )

Delco Remy..........231281

STENS.................460-048 ($27.50)

Ballast resistor if required:

Wells......................CR 107 ($3.19)

NAPA.....................ICR23

For Option 3, an Ignition Switch which supplies power to the Replacement Ignition ( not one that grounds out a magneto or electronic ignition)

NAPA...................7-01854

DYI Triggers

Historically, a few industrious folks have wanted to make there own Trigger Coils. I am carrying over this dated material to preserve that opportunity. There are two options:

1. Modifying the Tecumseh 610906 SSI to recover the

Trigger Coil in it, the second generation trigger.

2. Making a Sewing Machine Bobbin Trigger, the first generation trigger.

The modification of Tecumseh 610748, 610855 SSI and Onan SSBI3 is not worth the effort because they are too rear or don’t have a printed circuit board in them to modify and their trigger coils have many more turns that the 610906. The use of a Third Generation Trigger makes more sense.

610906 Modification

The 61906 has a trigger coil in it for triggering the Silicone Controlled Rectifier, SCR, so I thought why not use that. I tried for several years and could not get it to work until one day I put a diode in series with the output for some testing and the trigger coil then worked. There are some draw backs to this approach like the transition from the TDC starting spark to the Advanced spark is very erratic over 100 or so RPM. This may or may not be a problem, since the engine, once started, ramps up to the idle RPM or higher rather quickly. Another problem is that the number of turns of wire in the trigger coil is much more that optimum so it puts out an excessively high trigger voltage. This is overcome by using a very large air gap to raise the transition RPM. The problem here is that this can compromise the initial starting spark. Not withstanding, this was made to work by crefull testing and adjustment of the air gap and a couple of hundred engines are running on this approach.

Figure 1-19

Figure 1-19, 610906Bot7.jpg shows the bottom side of a 610906 with some of the potting removed and the printed circuit board exposed. A printed circuit board is made of a phenolic board ( white here) with a layer of copper on it that has been etched to leave the copper traces that make up the wires of the device. A green coating is applied to assist in making the solder connections. Modifications to the circuit are made using cuts and jumpers. A cut in the copper trace is like cutting a wire and a jumper is adding a wire. In the modification here, all we need is the trigger coil from a failed SSI and the rest of the electronics in the SSI are disconnected and abandoned in place. Figure 1-19 shows the back of a Tecumseh 610906 SSI with some of the epoxy potting removed. The modifications include making one cut and adding jumper.

Figure 1-20

The first thing to do is to cut off the wire to spark plug. Next we need to excavate the epoxy potting being careful to stay away from the trigger coil. I use a small Butane Torch to heat the epoxy then carve it off with an old dull screw driver. ( I know this sounds a little crude for the Spark Doctor, but it works.) It helps to heat the driver as you go. One has to use caution as the chips of epoxy that come off are hot and best not be allowed to hit exposed skin. One could start in an area near the ground screw. I also use a soldering iron to remove epoxy near the end.

There are two diodes connected to the terminal that need to be disconnected. The easiest way to do this is to un-solder the wire from the PC board and push it towards the terminal. Leave a pig tail of the wire attached to the terminal as it makes it easy for a later connection. The trigger coil has one red wire and one black wire. If you expose these, you are going too far. I start the excavation away from the Trigger Coil and find the PC Board, then use the copper traces as reference to remove the epoxy until I have the trace for the red wire and the trace for the black wire exposed but not the wires them selves. Extreme caution is required to avoid damaging the trigger coil.

Figure 1-21

The black trace of the trigger coil is to be jumpered to ground. The ground screw is shown for reference and there is a large copper pad around it. Solder a short length of black wire on the black trace and the other end to ground pad. By grounding one side of the trigger coil, we will need only one wire between the trigger and the Control Module.

One cut has to be made on the trace connected to the Red wire as shown in Figure 1-19, 610906Bot7.jpg above. This separates the trigger coil from the original circuit. Now is a good time to measure the resistance of the trigger coil. Put one probe of an Ohm meter on the red trace and the other on the black trace. You should get 9 to 11 Ohms for a good 610906 trigger coil. If the coil is open, the SSI is trash. While the meter is in hand, measure the resistance between the terminal and the PCB trace where it was disconnected to make sure it is totally disconnected , open. In order for the trigger to work , we need to add a diode, ( 1N400X, where x is 2 thru 7). So using the diode as the jumper, solder the banded end to the red trace and the other end to the pig tail you left at the terminal.

Now would be a good time to mount the SSI on the engine and test it. Start with an air gap between the trigger and the tall trigger pin of .020". Use a YELLOW wire from the terminal on the SSI to Terminal B on the Control Module. Follow the instructions above, just replacing the Third Generation Trigger with the modified 610906. After the modified SSI tests good, remove the SSI and fill the cavity made from removing the epoxy with E6000 or a silicon adhesive.

Pay particular attention to the testing, as the Modified 610906 will have many more turns of wire than the Third Generation Trigger and will need a wider air gap to try to compensate for it. If you get the air gap too wide, the spark will be delayed at low RPM and might cause hard starting. If the air gap is too small, the engine might transition to advanced spark while cranking.

Making a Sewing Machine Bobbin Trigger

Mostly Copied from older material, but updated for new findings, new test equipment and test opportunities.

About 10 years ago, I had an ignition failure with a Tecumseh HH120 and wound up using a Chrysler Control Module and conventional ignition coil. My wife wound a bobbin on her sewing machine for the trigger coil and it seemed to work, however, later the modified 610906 worked much better. In this article, I am going back and re-doing this development since I have learned a lot and have better test equipment. I made about two dozen bobbin coils and tested them to see which was best, which didn’t work and why. The criteria for rating them was:

1. It must put out a spark by flipping the engine simulator flywheel by hand, a very low RPM. This is most important for easy starting an engine with a DELCO motor / generator.

2. It must put out an adanced spark above 3400 RPM which is as fast as my engine simulator will run. The original bobbin coils would sometimes fail this test and I solved the problem by adding a diode like that used on the modified 610906. However, this requires more turns of wire.

3. It must transition from a TDC spark to an advanced spark between 400 and 600 RPM, (Updated 550 to 750). The number of turns of wire and the air gap determine this transition point.

4. It must be easy to make from available material, durable and inexpensive.

As an overview, Figure 1-22, Install.jpg is a bobbin trigger installed on the engine simulator.

Figure 1-22

The Bobbin

I did most of the testing using plastic bobbins because this is what is used in the electronics industry. I also tried a metal bobbin and sometimes it worked and sometimes it did not. I think that the plastic bobbin better concentrates the changing magnetic field thru the center of the coil which is important. So we use plastic bobbins. The Class 15 are wider, thus better, than the Class 66.

The Wire

Magnet wire has a thin insulating coating on it which prevents all the winding from shorting together. I scrape or sand the insulation off the ends to make the solder connections. The amount of current drawn by the trigger is so low that the wire size is not an issue. The wire size is limited on the large size by running out of room on the bobbin. It is limited on the small size by the wire being too fine to work with. I found that 30 AGW with a measured diameter of .010 inches worked the best. Below, Figure 1-23, 400Turns is a sample of the data I gathered for this wire which was from Radio Shack. The conclusion is that 400 turns and an air gap to the tall trigger pin of .012 inches produced a spark from just flipping the flywheel by hand to 3400 RPM with a transition from the TDC spark to the advanced spark at 537 RPM. One can see how the changes in air gap effects the transition RPM.

400 Turns, 30 AGW, .010 Inches in Diameter

Figure 1-23, 400Turns.

Figure 1-24

Air Gap, Inches	Transition RPM
.008"	496
.010	500
.012	537
.024	640

The 400 Turns required 50 Feet of wire and measured a resistance of 5.5 Ohms. The handbook indicates that 100 Feet would be 10.3 Ohms which can be used to estimate the wire length. It would be acceptable to find some used wire in discarded computer or electronic equipment like a computer CRT monitor. Wire up to .013 inches in diameter should work, but if you over fill the bobbin, you may have to locate the diode on the terminal strip instead of on the bobbin.

Figure 1-24, WindMach.jpg shows the wire being wound. Note that the winding motor turns CCW and the wire is wound CW. So if you are winding the wire by hand, holding the bobbin in your left hand, you would wind the wire on CW. This determines the polarization and is important. Also note that the winding started on the side of the bobbin closest the motor. This side will go closest to the mounting bracket.

The winding has two ends, the START end and the FINISH end. The START end will be connected to diode later. After 400 Turns, put a piece of tape to hold the wire and a narrow thin strip of E6000 adhesive to keep the wire in place while you finish it.

After the adhesive sets, solder the START end to the BANDED end of the 1N4005 or equivalent diode. Solder the YELLOW 22 gage stranded lead wire to the other end of the diode. See Figure 1-25 , Option2B.jpg and Figure 1- 26, Trigger.jpg.

Figure 1-25

Figure 1-26

Solder the BLACK stranded led wire to the FINISH end for the ground connection. Bring the YELLOW and BLACK wires around the bobbin in opposite directions and braid them tightly. Make sure the solder connections don’t touch. When you finished testing, put a coat of E6000 over the windings to fix and protect them.

The Bolt and Magnet

A soft iron 1/4 - 20, NC bolt, 1 1/4 inches long is used for the pole piece. Stainless steel or brass won’t work since they are non-magnetic. A 1/4" diameter rare earth magnet is mounted on the head of the bolt with JB Weld epoxy. Use a compass to make sure the right pole of the magnet faces the trigger pin, see Figure 1- 26, Trigger.jpg. Be careful to not get the compass any closer to the magnet than necessary since the magnet can ruin the compass.

Mount the bobbin on the bolt using a nut or E6000 adhesive after testing, making sure that the side of the bobbin where you started winding faces away from the magnet. The trigger coil is then mounted on a bracket.

The Mounting Bracket

Some care is required in making the mounting bracket because the center of the bobbin trigger magnet needs to be centered with the tall trigger pin when the piston is at TDC,( the key way in the crank shaft is straight up).Use steel because of it’s magnetic properties. I found a length 1 1/4" X 1 1/4" folded steel at Lowes. Figure 1-27 , Bracket.jpg shows the lay out. Tractor Supply has a similar material. Years ago I found a Bottom Retainer from a series 426 Overhead ( garage ) Door made a great bracket. On the Onan NB, a different bracket would have to be provided for the Trigger Coil.

Figure 1-27

The Trigger Pins

I assume that the interest here is in Tecumseh engines which already have the trigger pins installed. If this is not the case, trigger pins are available as a kit, Tecumseh P/N730201.

Testing

Before actually running the engine, the timing must be checked with a timing light. Figure 1-28, TDCAdv.jpg shows the timing as revealed by an inductive timing light. The left image shows that the spark occurred just as the tall trigger pin was leaving the trigger at low or cranking RPM. The image on the right, a higher or running RPM, shows the short, orange trigger pin just leaving the trigger as the spark occurred. ( Actually OEM ignitions show the trigger pin just to the left of the or before the trigger pin, which we should follow. ) There is difference of about 4 Degrees and is the result of a difference in the polarization of the trigger as explained below.

Figure 1-28

We need to check the timing at three RPMs.

1. Cranking RPM with compression - definitely a TDC spark

2. Cranking RPM with no compression,( no spark plug installed).

3. Running RPM.- definitely an Advanced spark. ( Run a cold engine for 30 seconds or less with the blower housing removed.)

If one needs to increase the RPM of the transition form TDC to Advanced, increase the air gap by .005 inches. If the engine is hard to start, decrease the air gap by .005 inches. This lowers the RPM where the ignition first puts out a spark.

Polarization

If the trigger pins are as shown in Figure 1-27, TDCAdv.jpg the polarization is reversed. If the trigger pins are to the left of the trigger coil instead of the right, the polarity is as OEM. The trigger coil will output either a negative going pulse followed by a positive going pulse or a positive going pulse followed by a negative going pulse depending on the direction the wire is wound, the placement of the bobbin on the bolt or the direction of the poles on the magnet. Reversal of any one of these will reverse the polarity and induce a 4 degree spark advance in the timing. In this case, two wrongs do make a right and the easiest change is to flip the bobbin on the bolt if you haven’t glued it in place yet.

Conclusion

You can replace the Bobbin Trigger with the Third Generation Trigger and follow the instructions above.

List of Material for Bobbin Trigger

Bobbins, Plastic : Micheals, Sewing Basket, or Wife’s sewing basket.

Singer class15, Brother / Kenmore 2518P

Magnet Wire, 30 Gage: Radio Shack, Three pack, 278-1345, $7 or www.All Electronics.com (800-826-5432), MW-30-4, 1/4 Lb $8.43

Yellow Lead Wire: All Electronics, 22YL-25, $2.10 ( For black use a magic marker ) or Radio Shack has 22 gage stranded wire but not yello

Magnets: All Electronics, MAG-76, 1/4" Dia x .2" , $1. or www.wondermagnet.com .0065, NdFeB Disc 1/4 Dia. X 1/8, $.20 + S&H

Terminal Strip: Radio Shack, 274-688, four for $1.50

Diode: Radio Shack, 1N4005, $1 for two, or All Electronics, 1N4005 8 for $1

E6000 Adhesive: Michaels, $4

Bracket: Steel WorkS (1305), PlateStl-Angle, 1 1/4" X 1 1/4" - 3FT ( 14GA), Lowes or Tractor Supply

Bolt: 1/4 NC, 1 1/4 Long and nuts