Ignition Control Module Experience

[chengny]

Here are the pinout values for the ECM (relevant pins traced in red). I forgot to include them this AM - sorry.

Expected voltages are specified for both operational modes - with engine static (ignition in RUN) and with engine idling (and at temp).

Note that the four EST associated leads should be at 0 VDC with the engine off and key in RUN. Knowing whether at least those values are within the acceptable range might help you confirm/eliminate the ECM's as the problem. Also, as a bonus, it may save you from buying another ignition module.

I imagine the harnesses will need to remain connected to the ECM to get proper results. So in order to capture the data, you will probably have to pierce the insulation of the leads with your probe.

You must be registered for see images attach

You must be registered for see images attach

 

[1987 GMC Jimmy]

Update

Alright, so thanks @chegny. Very helpful diagrams there. So I just went ahead and swapped out to another remanufactured computer and my third ignition control module. Fired right up, ran well, and computer did perfect. Also did a new coolant temperature sensor and pigtail because the sensor and pigtail were practically disintegrated. Started right up and ran fine several more times after that. I was lubing the speedo cable and doing light bulbs in the cluster so when I got done, I tried to start it up to go for a test drive, and it chugged a few times and nothing. I was so pissed I just walked off for the night. I came back yesterday and today and tried again (after getting a new battery), and it acted like it was trying to start, but it just wouldn't and then it would give up trying after a few moments. It did this in the exact same manner twice. I've probed the four wire distributor connector per the diagram, and I'm getting 0 volts with the key on except at the white EST wire. It'll flash between zero and 0.06V a few times and settle back at zero as long as I leave it there. It's done this two or three times, as I've repeated the test. I tested the HEI coil, and it's resistance numbers are up to spec. Everything's tight outside of the distributor, but I have noticed that the connectors are cracked and brittle. Actually, everything that's plastic in there is all cracked up. It's a 247,000 mile distributor. I'm considering just replacing it. Unless there's something funky happening with the EST circuit, everything looks good. The bitch of it is that this time the car worked for several days before doing something slightly different than before.

 

[chengny]

Maybe this isn't even an EST related issue. From what can gather, the truck reacts in two different ways:

1. The key is turned, the engine cranks, fires right up and runs fine - and stays that way
until it is intentionally shut down.

2. The key is turned, the engine cranks, doesn't fire (or kind tries to fire but only sputters).

What I haven't seen described is: a normal crank, fire, run scenario - but then - while idling smoothly, the engine just suddenly dies.

I'm sure you've checked for sufficient voltage available to the coil primary (10+ VDC at the pink lead to the B+ terminal). But I'm assuming that this check was only done in the "static" condition (IOW, just with the key in the RUN position).

Check voltage at the pink wire with the key IN RUN. Then, while a helper turns the key to START and the cranks the engine over, check that voltage again.

If a problem (e.g. ground/short) exists in the starter/solenoid assembly and the engine is cranked over, it can cause a huge drop in total system voltage. Voltage to the coil primary falls below the required 7-10 volts.

If the problem is intermittent in nature, the engine will sometimes fire right up and run fine. Other times, it won't fire - it just coughs - and at the plug gaps the spark will either be non-existent or a weak yellow trickle.

Very rare, but something to check.

 

[1987 GMC Jimmy]

Maybe this isn't even an EST related issue. From what can gather, the truck reacts in two different ways:

1. The key is turned, the engine cranks, fires right up and runs fine - and stays that way
until it is intentionally shut down.

2. The key is turned, the engine cranks, doesn't fire (or kind tries to fire but only sputters).

What I haven't seen described is: a normal crank, fire, run scenario - but then - while idling smoothly, the engine just suddenly dies.

I'm sure you've checked for sufficient voltage available to the coil primary (10+ VDC at the pink lead to the B+ terminal). But I'm assuming that this check was only done in the "static" condition (IOW, just with the key in the RUN position).

Check voltage at the pink wire with the key IN RUN. Then, while a helper turns the key to START and the cranks the engine over, check that voltage again.

If a problem (e.g. ground/short) exists in the starter/solenoid assembly and the engine is cranked over, it can cause a huge drop in total system voltage. Voltage to the coil primary falls below the required 7-10 volts.

If the problem is intermittent in nature, the engine will sometimes fire right up and run fine. Other times, it won't fire - it just coughs - and at the plug gaps the spark will either be non-existent or a weak yellow trickle.

Very rare, but something to check.

Will check the pink 12V wire here in a little bit for a voltage drop when starting, but it has nominal voltage at key on. Went and had this third ICM retested, and it passed.

 

[chengny]

I'm familiar with this scenario - it happened to me 3-4 years ago, and figuring it out just about killed me.

New complete distributors (3), gear engagement with the cam verified, new plugs, new wires, new rotor and cap, timing checked again and again, valve binding and preload checked. Always got 12.4 volts at the B+/pink wire terminal. This was all checked by many people (other than me), all of whom know stuff about stuff. Never got more than the weakest little yellow spark that could barely jump across the electrodes.

Finally, I went so far as to clamp a distributor by the shaft housing in a vise, attached one plug wire and a plug to a post, applied 12VDC to the B+ and spun the shaft with a 1/2" drill motor.

That made a spark - OH BOY did it make a spark! I had my hand near enough to the posts - that didn't have wires attached - so that when I pulled the trigger on the drill - I lit up like a Christmas tree. Took about a week for my heart to return to its normal rhythm.

But, you what - if I put that same distributor back in the truck and cranked it over - NOTHING!


One night after about 50 tries I started to notice that the only time the engine seemed to fire (and only for a brief moment) was at that instant when I would give up and let the key snap back to the run position from start.


Long story short: Went to the corner of the garage found a beat up old starter. Wired it up, bolted it on and turned the key. Not only did the test plug that I had laying on the manifold make a big, bright blue spark , but the engine fired up before it had made 2 revolutions!

 

[1987 GMC Jimmy]

I'm familiar with this scenario - it happened to me 3-4 years ago, and figuring it out just about killed me.

New complete distributors (3), gear engagement with the cam verified, new plugs, new wires, new rotor and cap, timing checked again and again, valve binding and preload checked. Always got 12.4 volts at the B+/pink wire terminal. This was all checked by many people (other than me), all of whom know stuff about stuff. Never got more than the weakest little yellow spark that could barely jump across the electrodes.

Finally, I went so far as to clamp a distributor by the shaft housing in a vise, attached one plug wire and a plug to a post, applied 12VDC to the B+ and spun the shaft with a 1/2" drill motor.

That made a spark - OH BOY did it make a spark! I had my hand near enough to the posts - that didn't have wires attached - so that when I pulled the trigger on the drill - I lit up like a Christmas tree. Took about a week for my heart to return to its normal rhythm.

But, you what - if I put that same distributor back in the truck and cranked it over - NOTHING!


One night after about 50 tries I started to notice that the only time the engine seemed to fire (and only for a brief moment) was at that instant when I would give up and let the key snap back to the run position from start.


Long story short: Went to the corner of the garage found a beat up old starter. Wired it up, bolted it on and turned the key. Not only did the test plug that I had laying on the manifold make a big, bright blue spark , but the engine fired up before it had made 2 revolutions!

Hey, finally got the test done on the HEI coil 12V supply (pink wire). Voltage is 12+ with key on. With starter engaged, it drops to a healthy 9.3-9.6 volts. I also tested the pickup coil for ***** and giggles, and it was within spec for the resistance number. I was wondering. Even though I had the ICM tested, and it passed on all parameters, could it still pass on the bench and be bad in the vehicle?

 

[1987 GMC Jimmy]

@chegny: I'm starting to think what happened to you is happening to me. I've got an intermittent no turning over condition now. Like I turn the key and nothing. Plus, it's got a brand new battery, and it's really sluggish to turn over. Can it be a starter grounding issue, internally or externally, a chewed wire, etc.? I've also got a new ignition cylinder if that's what I need to try.

 

[1987 GMC Jimmy]

Finally got it figured out. Sorta. Went through everything, and I guess it was flooded so bad that it had inundated the plugs and cylinders with gasoline. We were trying to look at the spark, and we pulled the number one plug, and it just magically started on seven cylinders and shocked the **** out of my little brother. We put the plug back in, started it, and I adjusted the mixture on it with my vacuum gauge. It's running now. I had to replace the remanned ECM I got because it was defective, and it should be here soon. Nothing was really done to get it to start, but it works great now so I'll just go with it. I did ruin the oil with all that gas, and it did leak fuel out of the plug holes so I know it was pretty well saturated. It should be fine here soon.