Ignition control module (ICM)

[FastWoman]

Wow, what a flurry of activity on this topic! I've been busy building our dock -- dead tired -- so I've been away.

I think Zed Head covered most of it. The tach/ecu wire seems to branch somewhere in the wiring tree. All you need is the one wire -- yes, to the (-) of the coil.

The ballast-type coil might be a bit too much of a load on the HEI module without a ballast resistor -- might have fried it. A non-ballast type coil is probably a better choice. I use the Blaster II coil with good results.

Tomo, I see no reason why you would have to disconnect any of the original IM wiring if you didn't want to do so. You could simply unplug the old IM and do as you will. I just hate stray wiring floating around the car. It seems like asking for trouble. You'll have ignition noise traveling around through dead-end wires (acting like antennas), and if anyone ever decides to plug back in an OEM IM, you'll have fighting ignition modules. I would simply isolate the input (reluctor from distributor -- red and green wires) and output (the ground wire to the neg of the ignition coil), and everything else can stay in place pretty harmlessly. My thought is that you still have the wiring in place, but it's not connected to anything and doesn't carry any voltages or noise. My entire approach to retrofitting is to make everything as painlessly reversible (to OEM) as possible.

[Zed Head]

Check the wiring diagram. It's in the FSM. Engine Electrical chapter, Ignition section.

[FastWoman]

z-ya, Pin #1 always goes to the negative, with any configuration, as far as I'm aware. The +12 IGN goes to the + side of the coil, and the ignition module grounds the other side of the coil to create the spark. Same story with a points ignition system. When the engine is running, an overly simplistic explanation is that the neg side of the coil goes back and forth between +12 and 0 volts, while the pos side of the coil stays at a constant +12V. In practice it really isn't this way. Rather, both sides gyrate wildly in voltage. You could probably trigger both your tach and ECU off of the pos side of the coil, but the neg side is going to give both devices a more appropriate signal that they are designed to read.

FAIW, my tach jumps a tiny bit too, during hard acceleration. That's because there is sometimes slightly less efficient combustion, and this results in a recoil signal (not exact terminology here!) at the end of the ignition pulse, hence a "double" pulse. A condenser sounds like a reasonable way to fix the issue. I just haven't gotten around to working on mine yet.

[TomoHawk]

Does it matter which ground hole you use on the module?

[z_ya]

Alright success!!!!!!!!!! (again) I bought an accel coil (like 20 some dollars) and now it will start multiple times so i'm just gonna watch it and if worse comes to worse i'll switch back to OEM. The way I mounted it was where the old "clamshell" was, and to double ground it just in case I also ran a wire from the screw I used to an AFM ground that I am using just s a fail safe. So far so good but in a few days we will see I guess. Thanks for all the help guys I wish I would of thought about this before buying the ICM but it will be sent back because it was still causing issues.

[z_ya]

And after one day I am switched back to OEM control module. So SOMETHING is not right. And I really don't know what.

[FastWoman]

Tomo, I think both mounting holes are the same electrically. Regardless, I'd bolt them both down tightly. Better two grounds than one!

Z-ya, my HEI retrofit has been in use now for over 3 years, and my Z is a daily driver. (OK, I don't drive much, but still...) So there's nothing wrong with the concept. There has to be something wrong with your wiring.

I can only guess as to what might run your engine successfully for a while and then fry the module. Here are a few guesses:

1. Heat dissipation issues: These modules are designed for pretty high temps, because they're ordinarily stuffed inside hot distributors. I've been told the modules don't need any special heat sinking and that they are fine mounted directly to the fender in the clamshell location. (The person who told me this had a '78 daily driver.) However, I always overengineer everything, and my module has a heat sink on it. Anyway, just check your module to see if there's any reason it might be overheating, which I can't imagine. FAIW, these modules don't generate a lot of heat. Mine barely gets warm enough to tell that it's warm. Of course that's with the heat sink doing its work.

2. Too much load. The only real load on the module is the grounding of the coil. One possible problem, previously discussed, is that you're trying to drive a ballast-type coil without a ballast resistor. I don't really know the accel coil, but be certain it's not supposed to be driven in series with a ballast resistor. The second possible problem might be that you're grounding the + side of the coil, where you're also supplying your +12VDC IGN. If you ground out your supply voltage, I suppose you can cause enough electromagnetic fluctuations in the coil to create a spark and run your engine, but the HEI module is really going to take a beating.

This is all just speculation, but if I had to guess, I'd say the most likely possibility is that you're grounding your supply voltage and not the opposite side of the coil from your supply voltage. (For instance, maybe you've got your +12VDC IGN running to the neg post and are also grounding the neg post with the HEI module.) So make certain your +12VDC IGN and your HEI are wired to OPPOSITE sides of the coil.

[z_ya]

I had the ballast removed once I changed the coil. It still made the HEI fail at high rpm. I'm very certain that going off of the diagrams posted my wiring was correct. It is entirely possible the P.O. had done some "rigging" of her own..... Going off of the other wacky stuff done to this thing I wouldn't put it out of my mind. Either way I'm back to OEM. With a new ICM and coil an it seems to be a bit better.

[TomoHawk]

It's quite possible that the PO tried to wire the ignition like the points-style engines had back then, not knowing the car was totally different with the points-less electronic system. From what I remember, we could do that stuff without looking at the wiring diagrams (at least on the Fords) and when the points-less distributors cameout, it was a headache to figure out. Even the parts guys seemed confused because they wanted to sell you points, condenser, AND a ballast resistor, as it was just the usual thing to do.