New Replacement Ignition, Part1, General
Dave Cave has an article “Buzz Coil-How To” in February/ March issue of Gas Engine Magazine, see
He uses a GM LS2 Ignition Coil, and a simple trigger circuit suitable for the low and constant RPM hit miss type engines.
The spark timing would be fixed and after TDC, but instead of just one spark, it puts out a burst of sparks. The LS2 is very
small, includes the electronics that was in earlier decades in a separate box and costs only $18. I got to thinking about
trying the LS2 Ignition Coil triggered by the induction type of trigger that I have used for many years with the Chrysler
Control Module. See GM LS2.jpg.
Most small engines have only two levels of spark advance; no spark from zero to a few hundreds RPM and then an advance
spark starting at a RPM where the engine won't kick back. This is simple, least expensive, but makes a compromise on
timing. With the induction type trigger we have an added level of spark advance for much easier starting. It provides
a spark starting at about 100 RPM that is timed at Top Dead Center, TDC for no kickback and is easier on the battery
and starter motor. I chose to try the GM PC2 Crank Sensor for the trigger because I had them, but some signal conditioning
is required to have them work with the LS2 Ignition Coil.
GM PC2 Modification:
The LS2 Ignition Coil is triggered by a signal form a computer. This signal is at Zero Volts until a 5 Volt pulse comes along and
triggers the spark. The PC2 Crank Trigger puts out more like an AC type signal with a negative and a positive pulse. The LS2
won't like this negative pulse. This is easily fixed by routing the PC2 output thru a bridge rectifier like those used for converting
the AC from the Stater to DC for charging the battery. This gives us a trigger pulse that is a doublet of two pulses.
The induction coil, PC2, has an output voltage that increases with RPM which we need for RPM control of spark timing. Voltages
much higher than 5 Volts would not compatible with the LS2. So we just clip the top off the trigger signal at 5.1 Volts which is
above the trigger level of the LS2. This requires a 5.1 Volt Zener diode . I am using small components so I can fit them in the PC2
connector, but have an alternative of a small box. See GMPC2Mod.jpg
The result wave form looks like this:
Note that both the Advanced and the TDC trigger pulses are firing sparks at this RPM.
The complete ignition is shown in PC2LS2Sys.jpg.
The wiring is very simple. The Black wires go to ground. The Red wire provides power from the battery and the Green wire from the
PS2 provides the trigger signal to the Green wire on the LS2.
The PC2 needs to be mounted with .024” initial air gap to the tallest trigger pin on the flywheel. This is important because
this air gap sets the voltage output thus the transition points between the three levels of spark advance. A wider air gap will
increase the RPM for the first spark for both the TDC and advanced. A non-magnetic feeler gauge is shown for setting the initial
For all my testing here I used my Tecumseh HH 120 engine simulator (which is driven by a tread mill motor) since it already
has the trigger pins installed. The PC2 trigger is installed on a bracket mounted where the original Solid State Ignition was and
is adjusted to be aligned with the tall trigger pin on the flywheel at an air gap of .024 inches.
The amount of spark advance is determined by the physical location of the trigger pin on the flywheel. To the best of my knowledge,
the Tecumseh HH and OH series of engines were the first to use the Trigger Pin concept for switching between levels of spark advance.
The Onan NB engine uses this concept too, but with only one trigger pin since I believe it has a compression relief .
If you are using a different engine and there is no room for the PC2 , I am working on an alternate bobbin type trigger like
that I used on the Briggs and Stratton ignition replacement. https://youtu.be/VS_HKaXx9JY.
In New Replacement Ignition, Part 2, Details, I plan to be addressing:
- PC2 Modification, fabrication of the connections to the LS2.
- Parts List
- Tecumseh Installation and initial testing
- Onan NB Installation
- Briggs and Stratton Installation
- Kohler Installation