Electric fan fried my Pertronix ignition

[rlpriest]

Started my Roadster build last year and got the engine running last fall. I am now in go cart stage and let the engine run to see if my electric fan would come on when engine got hot. The engine just stopped, I spent three weeks trying to find the problem. Was getting gas but no power through the coil. Replaced the ignition switch and coil but no luck. I have a 347 from the engine factory so I called them. They told me they have seen the electronic ignition in the distributor get fried when the electric fan comes on, some kind of back feed. I disconnected the fan wiring, replaced the ignition and it started . Has anyone had this problem? Obviously I screwed up my wiring.

[mike w]

I had a similar problem with my Mallory electronic module with a Pertronix Flame Thrower II coil. After talking with Mallory I found that they require a ballast resister to be installed between the coil and distributor. I opted to replace the electronic module and coil with a Malloty coil that is internally resisted. I don't know if the fan had anything to do with the failure.

[tirod]

No, not so obviously, or you wouldn't have done that. I am aware that the factories sometime do install a diode in the circuit when a temp sensor and the A/C clutch can both start the fan. It keeps the temp sensor from engaging the clutch.

How the fan could supply power back to the ignition is something else, and would relate to the trigger circuit or how the main fan relay draws power. That brings up the question what harness was used, donor, painless, etc. Some auto electricians like to claim auto wiring is dirt simple, and it can be - starting from a clean slate. Using some other engineers design and modifying, tho, can be a challenging exercise in discovering his logic. He likely had a lot of other institutional requirements put on him by DOT and the liability section of corporate.

We can't even trust water pumps to all be the same - two identical looking pumps can rotate opposite each other. There's nothing to take for granted in auto wiring, the mess of snakes it's typically described is exactly the way to treat it.

[Doutie]

Wiring is simple, at the factory they pack the wires full of smoke, if you let the smoke out they won't work. Seriously thou, your feedback can be caused by by reverse RF. When a coil is excited it creates a magnetic field and if that circuit is shut off the field collapes and causes a very brief high voltage spike back towards the power source. This spike is found in even small coils, such as the winding in relays, solenoids, etc. Jacobs Brakes on truck engines have a diode to prevent this spike from arcing the contacts in the switches and shorting their life. Circuits like low voltage ignition pick-ups need protection from such spikes of high voltage. In old points ignition, the coil is excited when the points are closed, and at the moment at which they open the magnetic field collappses into the core and makes the high voltage to jump the spark plug gap. At the same time it is trying to arc across the opening points but that is absorbed by the condenser and stored until the points close again.

[rlpriest]

Mike, tirod and Doutie - thanks for the feedback, I finally got an answer back from Pertronix and they said a resistor is in the system to protect the ignition module so maybe my wiring did not cause the issue, I am using the FF harness and it does have a separate fuse fuse line for the fan. Is it possible for the ignition module to just go bad??

[mike w]

I also installed a power filter before my distributor to help protect my module from electrical spikes. I am not sure if this would work for your Pertronix system though.
http://www.summitracing.com/parts/MAA-29351/?rtype=1