How to damage a GM HEI Module

[NewZed]

I picked up a spare oil pressure sender and decided to try it out to make sure it was good. After swapping, I forgot to put the 1 and 2 plug wires back on and started the engine. I let it run for a few seconds then remembered the plug wires. After stopping the engine and re-installing the plug wires the engine wouldn't start anymore, unless I shot some starter fluid in to the manifold. After starting with fluid (by a single detonation I assume based on the noise), it ran normally. I started it several times using the starting fluid, but it wouldn't even pop without it.

 

I tested everything I could think of and finally swapped modules, and it started right up.

 

Voltage was dropping to ~9 volts at the coil and (bad) module wire while cranking so maybe the module just wouldn't work off of the low voltage anymore or was actually drawing too much current. I didn't measure cranking voltage with the new one because it starts so fast, there's no visible voltage drawdown. I'm not sure how the module got just damaged enough to allow the engine to run, but not to start.

 

For the record, I'm using a Z31 coil with a BWD brand HEI module. Replaced it with the same brand. It had worked great for over a year, probably 20,000 miles.

 

Just a little story for anyone having starting problems with an HEI module, or doing tuning work. Pulling plug wires on a running engine might not be a good idea. And the HEI modules might be a little bit sensitive to electrical disturbances.