Charge Coil Testing Last updated April 20, 2006
In the Capacitance Discharge Ignitions, CDI, which includes the Tecumseh SSI, there is a coil of wire which, when the magnet
on the flywheel flies by, generates a few Alternating Current ( AC ) voltage pulses. These voltage pulses are converted to
Direct Current ( DC) by a couple of diodes which in turn charges up the capacitor to as much as 450 Volts DC. This usually
occurs about 180 to 270 degrees of crank rotation before the actual discharge of the capacitor for the spark.
The three different engine configurations discussed here include:
1. The Early Solid State Ignition where both the charging coil and the electronics module are under the flywheel. These
engines use an external DELCO motor / generator. Part number 610579 Figure 5-2-1, mag.jpg
2. The Later SSI where the charge coil is under the flywheel but the electronics is above the flywheel. These engines also
use an external DELCO motor / generator. Part number 610965 is for the Charge Coil Figure 5-2-2, SSI over.
3. The configuration with a TECUMSEH built in starter and alternator where the Charging Coil is part of the Stator Assembly
used for charging the battery. There are two configurations of these: 10 Amp Stator, Part number 610747 and 20 Amp Part Number
610902,( The 10 and 20 Amp refer to the battery charging portion of the part.) See Figure 5-2-3. OH140Stator
5-3 Testing Early Solid State Ignition with Delco Motor / Generator, 610759
There are two different configurations of the charging coil in the 610759 assembly. One of the configurations, I call configuration
B, has the diodes physically located in with the winding. The other configuration, I call configuration A, has the diodes
physically located on the printed circuit board inside the potted electronics module. For configuration B, I think the part
looks like this: Figure 5-3-1 Chargecoil.jpg
5-4 On the Configuration B, with the diodes in the Charge Coil, if you disconnect the charging coil from the electronics
module and measure the resistance, in one direction you should get infinity or open. Switching the ohm meter leads, and measuring
again, with a good digital meter I get about 7 Meg Ohm. With a inexpensive analog meter, I get 7 K Ohms. (The diodes are nonlinear
so different values at different test currents is not unreasonable.) Using the diode test function on the digital meter, I
get from .863 to .911 Volts. The normal junction voltage for a silicon device is about .5 Volts but in our case, we have two
diodes with the resistance of the winding across one of them, so these numbers are not unreasonable to me. The measurable
junction voltage is very hard evidence that there are diodes in the part.
For the configuration A, that doesn’t have diodes in them, I get from 923 to 958 Ohms in both directions and nothing
on the diode test function.
If it turns out that the charging coil has the diodes in it and is good, then you are lucky to have the configuration where
you can replace the capacitor in the electronics module using the section on the web page Solid State Ignition Repair.
5-5 Testing Later Solid State Ignitions with Delco Motor
/ Generator, 610965
My Objective here is to provide baseline measurements for
a known good part. Unfortunately, I don’t have one. If someone could loan me one, I could test it and return it. I would
also like to try to rewind the coil on mine.
The part, Figure
5-5-1 and schematic, Figure 5-5-2 are shown below:
5-6 First I should note that a similar schematic appears in the Tecumseh manual, 8 to 18 HP Cast Iron engines, but the
diodes are reversed, making the output negative instead of positive. At this point, I don’t know which is correct. A
simple meter test with the actual hardware , if I had a good part, would resolve the issue. This Charge Coil mates with a
SSI, p/n 610906,which is also used on the 20 Amp charging system. The autopsy on the 610906 indicates that this part also
has diodes in it for the 20 Amp Stator and the diodes do indeed provide negative pulses to charge the capacitor. This indicates
that the diodes above are backwards and that the manual is correct. Also, there is another coil called "exciter" which I have
no clue what that is for, so I left it out. It probably pertains to a different configuration.
5-8 Testing Solid State Ignition Where the Charge Coil is part of the Stator,
20 Amp PN 610902 Figures 5- 8-1, 20staror.jpg Figures 5- 8-2, 20statorCt.jpg
The charge coil is wound on two poles with a very fine, 32 gauge
( .008"), wire with a resistance of 137 Olms. The battery charging coils are wound with very heavy, 14 gauge (.0641") wire
on the remaining 16 poles. The connection for the kill switch has a 40.5 olm resister in series with it and goes to the center
pin on the plug. The voltage output of the charge coil disconnected from the SSI varies with RPM from 50 Volts peak to 120
I found it interesting that there are no diodes in the part; thus
AC is provided to the SSI Module, P/N 610906. Other charge coils ,P/N 610965 have diodes to provide a DC to power the some
part. This implies that the SSI has diodes in it to convert the AC to DC and that the SSI can accept either an AC or a DC
input. I think I would like to do more testing before I really believe it. I have a SSI soaking in carb cleaner to soften
the potting compound so I can get a circuit diagram from the physical parts.
5-9 Testing Solid State Ignition Where the Charge Coil is part
of the Stator, 10 Amp PN 610747 Figure 5-9-1
The above is a 20 Amp Stator commonly used on OH 140 and OH 160
engines. Jacques Lacasse has provided us the following on a 10 Amp Stator from an HH 12-120061B engine. It is important to
notice the differences. The 20 Amp Stator has 18 poles while the 10 Amp has only 12. The windings on the 20 Amp Stator are
two sets in parallel, alternating poles while the 10 Amp is a single winding over 11 poles. The Charge Coil on the 20 Amp
Stator occupies two poles while that on the 10 Amp, only one. The SSI for the 20 Amp is a 610906 while that on the 10 Amp
is the 610748. The two also use different Voltage Regulators and different Flywheels; the flywheel is a bit smaller than the
one on the 20 Amp system. Jacques has measured the resistance of the Charge Coil output to ground at 425 Ohms.
This indicates that there are no diodes in the Charge Coil.
5-10 How the Stator works
If one had to repair or rewind a stator it might help if he or
she knew how it worked.
In Figure 5-10-1, you will notice that there are 6 unusual magnets; they have either a NORTH or SOUTH pole in the center
and the opposite pole at each end. Each magnet has 3 poles. Thus the 6 magnets make a ring of 18 magnetic poles, 9 NORTH and
9 SOUTH. The magnets are arranged such that there is always a pole reversal from NORTH to SOUTH or from SOUTH to NORTH as
one moves around the ring. The key here is that it is that this pole reversal produces the CHANGE in magnetic flux that induces
a voltage in a coil .The reversal from SOUTH to NORTH produces a positive voltage and the reversal from NORTH to SOUTH a negative
voltage. Now lets look at the stator, Figure 5-10-2.
Guess what, the stator has 18 poles also. The Charge Coil winding
for the ignition uses 2 poles and the alternator winding for charging the battery uses 16. ( There is no electrical connection
between these two windings.) If you look close at the alternator winding, it has two wires going between each pole, not one
wire. The voltages induced by the reversal of the magnet fields are added in series. As one coil is producing a positive voltage,
the coil next to it would be producing a negative voltage if the coils were wound in the same direction. If the two were added
in series they would cancel each other out. So, every other coil is connected in series up to 8. It appears that the other
8 coils are wound in the opposite direction and connected in parallel to the first set for the output connection.
Since we have a very high current rating, 20 Amps, and 14 gage
wire, I think that the two parts of the stator winding are designed for each to carry 10 Amps. I haven’t taken a stator
apart yet, since the only one I have is a perfectly good one on loan.
3 Amp, One and Two Diode, Rectifier Created April
19, 2006, 3AmpRect.wpd
Some of the older Tecumseh engines had a small stator under the
flywheel and the Rectifier assembly on a board as shown in Figure 5-12-1,H 60A.jpg. Often the diodes fail and are no longer
available, so this section is about how to replace the diodes with ones that are readily available, Figure 5-12-2, H60B.jpg.
The circuit diagram is shown in Figure 5-12-3, H60Dia.jpg.
The original diodes were held in place with a fuse holder like
clip that are riveted in place. I drilled the rivets out and replaced them with 4- 40 screws. The clip was replaced with a
terminal lug shown in figure 5-12 -2. The replacement diodes are 1N5406 soldered to the new lugs. It is important have the
banded end of the diode towards the fuse. Radio Shack has 3 Amp diodes; there 1N5402 has a 200 Volt rating which is a comfortable
margin. If the original clips are held tight, one could solder the diodes directly to them.
Figure 5-12 - 2 Testing:
To test the complete system, disconnect the wire to the BAT + terminal
and run the engine at about 3000 RPM. You should measure about 14 Volts DC at the BAT+ terminal. You can also measure the
AC voltage across the two GEN terminals, about 30 Volts AC. To check the diodes, shut the engine down and disconnect the wires
to the GEN terminals. Most newer multi meters have a diode test function. Using this to check the diodes, you should measure
about .5 Volts in one direction and open in the other direction ( by reversing the red and black meter leads). If you don’t
have a diode test feature, measure the resistance across each diode. You should get open in one direction and with my digital
meter I get about 7 Meg Ohms in the other direction and with an inexpensive analog meter I get about 7 K Ohms.
5-12 -3 Voltage Regulator
It is worthy to note that this circuit is only a rectifier which
converts AC to DC. There is no voltage regulation. I think that to voltage output of the Stator is limited by design so as
to not over charge the battery, that is to not exceed 14.7 Volts.
5-12-4 3 Amp, One Diode, Rectifier
The circuit above, Figure 5-12-3, is of a two phase stator output.
If one erased to lower half of the stator and lower diode, the circuit would be single phase and one diode, very common to
modern engines. The rest applies to these configurations.
One could also erase the lower diode and labeled the lower stator
output , AC for tractor lights.