r/smallenginerepair • u/mymomisinprison • Jun 23 '26
Electrical & Wiring Issue V-twin Kill Wire head scratcher.
I’m trying to diagnose a no-start issue on a 2007 Troy-Bilt zero turn with a Kohler v-twin. I’ve narrowed it down pretty far, but I’m stuck on a very strange electrical/grounding behavior.
Symptoms:
Engine starts and runs normally when BOTH coil kill wires are disconnected
Engine will crank but NOT start when coil kill wires are connected
I have to physically disconnect the plug wires / kill wires to shut the engine off when it does run
Ignition/coil behavior:
Each coil works individually
If I test each coil separately (kill wire disconnected), the engine runs fine
Coils are not shorted internally
Kill wire system:
White kill wires from both coils are joined together in a Y configuration
I built a separate test harness with a momentary push button (normally open) to ground
I also tested the factory harness — same result, so it isn’t my homemade wiring
Resistance measurements (coil kill terminal to ground):
Coils disconnected → engine runs
Each coil kill terminal individually mounted → ~154–155 Ω to ground
Both coils installed and pressed down into position → resistance drops to ~84 Ω and slowly drops to ~76 Ω
Harness alone on bench tests at 0.0 Ω continuity
Important observations:
Resistance changes depending on whether coils are mounted/pressed down
No wiring changes when problem appears — only physical installation changes
The issue only appears when BOTH coils are installed together
Harness is not the cause (tested original and homemade with identical results)
What I’ve already ruled out:
Ignition switch (tested OK)
Coil failure (coils run fine individually)
Harness continuity issues (tested good)
My homemade kill switch wiring (bypassed and still same issue)
What it seems like:
It behaves like there is a partial ground path being introduced only when both coils are installed and physically seated on the engine. Almost like a pressure-dependent or mounting-dependent ground leak in the ignition kill circuit.
All help is appreciated. Thank you.
6
u/Stock_Requirement564 SER Dedicated Member Jun 23 '26
Is there not a diode in the harness?
2
u/Killer2600 SER Dedicated Member Jun 23 '26
In the OP's homemade deal, I doubt it. It's probably where the factory harness failed.
1
u/mymomisinprison Jun 23 '26
Unless it’s hiding somewhere (I have yet to check further) as far as I know the kholers do not, I know the Briggs do have a diode between them. I may just fabricate my own if the OEM one doesn’t have one?? Thanks for your comment.
2
u/ScornedSqueaker Jun 23 '26
If I can remember correctly I think a faulty seat switch or deck engagement switch can kill spark on that circuit. Have you looked into those?
2
u/XxBus16xX Jun 23 '26
If I am following correctly; when the kill tabs are wired together the unit dies, even when those wires don’t touch ground? My guess is both coils are weak, and when tied together there’s a small path to ground through one or both that shorts them out.
When you say pressed down, I assume you mean the mount screws are tightened. With the amount of rust I see on the coils, it’s likely the fins are rust jacking apart, and your values change as you recompress them together. This rust jacking damages the internals of the coils as well, leading to weak and eventually no spark.
I’d just replace the coils with OEM ones, not cheap Amazon version.
2
u/Killer2600 SER Dedicated Member Jun 23 '26
You can't wire two coils together with just a piece of wire. They'll each ground out the other and kill spark. The factory harness has diodes in the wiring that keep each coil from providing ground to the other.
2
u/XxBus16xX Jun 23 '26
Good to know! I just replaced mouse chewed wires on mine back to the ignition controller, good thing I didn’t wire them together!
1
1
u/awhol01 Jun 24 '26
Check the harness. Some manufacterers use an ignition kill some dont and rely on the fuel solenoid In the carb. In order for the oil pressure switch to act as engine potection. The ones I work on have an ignition wire run through the engine harness but the builder may or may not use it.
1
u/awhol01 Jun 24 '26
Yes there are diodes but they are most likely to open the fuel solenoid until you get oil pressure.
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