94 LT1 sudden no-start solved -- Ignition Control Module (Coil Control Module) - CorvetteForum - Chevrolet Corvette Forum Discussion

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94 LT1 sudden no-start solved -- Ignition Control Module (Coil Control Module)

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Old 10-20-2012, 10:37 PM   #1
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Default 94 LT1 sudden no-start solved -- Ignition Control Module (Coil Control Module)

All fixed -- YEA! -- but thought I would share what I learned...

Story goes as follows:

Friday, drove car, ran perfect, no problems what-so-ever, pulled into garage and there it sat for a little more than a week.

Over that following week, replaced both horns (NAPA Fiamm USA made $16 each), but never started car.

Monday, go to start and would crank, but not catch (not start not fire).

Start diagnosis --
1. no codes
2. can smell gas from tail pipe
3. can hear fuel pump
4. fuel in fuel rail (rags below pressure relief/connector and press, fuel escapes every time (didn't have a pressure gauge at the time).
6. no spark at spark plugs
7. checked all fuses (passenger side, and 2 in engine compartment)
8a. Use FSM book 2 6E3-C4-1 - Chart C-4.
8. OPTI power feeds ok -- tested OPTI connector -- used FSM book 2, diagnostic chart which includes pull OPTI plug, check for power and ground
9. ICM feeds OK - tested ICM connector (requires disconnecting Tach Feed by disconnecting passenger side gray connector and jumpering the lower (+b) power feed from connector to coil) IMPORTANT NOTE: part of the ICM tests include verifying that one of the ICM connector's connection is receiving between 0.5 vAC and 4.5 vAC while cranking. This simple test verifies the ECM is working **AND** that the OPTI is working. THIS IS A GREAT TEST TO PREVENT UNNECESSARY OPTI REPLACEMENTS.
10. Diagnostic chart then indicated Ignition Control Module bad (chart tests are not exact enough to rule out a bad coil, albeit suggests a bad coil would set a DTC).

Autozone has ICM tester, which confirmed ICM was bad.

ICM and Coil appeared to be original, so 18 years is pretty good service.

Ordered ACDelco ICM (D1986A gm# 10483139) and Coil (D573 gm# 10477208 - installed just in case) and had to educate parts guys on why DO NOT USE dielectric silicone grease behind ICM.

Purchased silicone based heat transfer grease from Radio Shack (silicone base heat sink compound 2-pack 2760255).

Disconnect battery (negative cable).

Removed bolts holding coil (removed power steering bracket lower bolt and swing unit to passenger side out of way), drilled out aluminum rivet. super cleaned brackets (coil bracket is actually 2 brackets) and aluminum heat sink.

Utilized corvette forum member's suggestion and applied heat transfer grease with a putty knife to get even thin coat (thin is better than thick as thick insulates and reduces heat transfer).

Apply heat transfer grease between coil brackets, assembled coil to bracket with new screws/nuts, applied heat transfer grease to back side of coil bracket (note none was between or behind before but was supose to be per FSM) and reinstalled bracket/coil.

Applied heat transfer grease to front side of coil bracket, both sides of the aluminum heat sink and back side of ICM (again with narrow putty knife to get thin even coating) and carefully installed and torqued to spec (14 in lb -- NOTE: manual step by step instructions area is wrong (indictes ft lbs), but end of section specs are correct).

Reconnected battery -- and voila -- car started right up.

BTW, at idle over 15 minutes, with my infared heat gun, saw NO noticable heat difference between the ICM and head/heat sink (ambient temp was 65, ICM only got to 78 in 15 minutes). So either I did a really great job, or the good quality ICMs do not heat up as much as less expensive aftermarket brands.

lessons learned:

I really thought it was the OPTI, but following the FSM diagnostics and discovering the ICM test which verifies OPTI is sending high and low resolution pulses to the ECM and the ECM is sending the vAC feed to the ICM -- was a great to find -- phew, what a relief as I really thought it was going to be the OPTI.

2. to All, inspect your ICM heat sink for dirt/oil and screws for being loose or grease dried-out. I suspect mine either the old grease dried out or oil intrusion reduced the heat sink's effectiveness. As preventative maintenance -- consider removing the ICM and aluminum heat sink (heat sink is only held on by ICM screws) and clean and reapply heat transfer grease and reinstall (30 minutes of preventative maintenance might be well worth not being stranded).

Last edited by theadmiral94; 10-21-2012 at 12:01 PM. Reason: added FSM reference and part numbers
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Old 10-21-2012, 09:18 PM   #2
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Default test drive update - wow and thoughts about ICM

Test drive complete. WOW, improvements I didn't expect.

Whereas the car ran great before, there was a little hessitation at different RPM levels I just chalked up to age.

Well all the hessitation is gone and seems like engine has more power throughout its rpm range.

So, if a LT1 has hessitation in/around the 2k area, consider the ICM and/or Coil -- might be why.

Also, regarding my and everyone's concern about the temperature of the ICM --- consider this....

After the test drive, the ICM's temperature (178 degrees F) was just a few degrees less than the head (180 degrees F) to which it is bolted via the heat-sink and coil bracket. So the heat transfer compound I used is keeping everything at about the same temperature, which 'may' be what is desired, but...

The FSM stipulates 'silicone grease for cooling' to be applied between head and coil bracket, and between the ICM and the 'mounting bracket where the module seats' (presumed to be the aluminum heat sink but based on FSM pictures, it is possible the aluminum heat sink was added to the car but NOT the FSM)..

Oddly, the FSM does not indicate 'silicone grease' to be applied between the two parts of the coil bracket -nor- between the coil bracket and the aluminum heat sink (whereas I applied it to both these areas -- thinking I was doing good, hum...).

If these other two areas were purposely intended to not have 'silicone grease for cooling', then it may have been intended to temperature isolate these parts from each other and provide a greater temperature difference. I am going to give this more thought...

Also, considering where the ICM is bolted, versus where it could have been bolted, I must wonder if the ICM might be intentionly positioned so as to raised its temperature to the temperature of the head. This could be desired if it has temperature compensating components so that its results vary and are in concert with the temperature of the head...

Last edited by theadmiral94; 10-21-2012 at 10:12 PM. Reason: corrections to spelling
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Old 10-22-2012, 02:14 AM   #3
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Recently I removed the coil bracket so I had better access to the water pump bolts on the passenger side (they were loose). When I removed the bracket, there was heat sink paste between the bracket and the head. I decided to clean it and reapply new paste (I bought the Radio Shack paste also). Since I had it off I also took the heat sink and ICM off. I also found paste between the bracket and the heat sink; and also the heat sink and ICM. I'm the second owner but after all the delayed maintenance I have done, I doubt the PO did or had this done. So I'm thinking it may have been done at the factory.
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Old 10-22-2012, 03:02 AM   #4
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There are many varieties of thermal grease. The most common in my experience is the white stuff - zinc oxide.

The purpose of the grease is to eliminate the AIR between the parts. Air is an insulator. It's better to be generous to eliminate all air pockets. The excess will get squeezed out when the parts are bolted together.

The ignition module gets very hot. The heat is generated by the switching transistor that turns on the ignition coil every time a cylinder fires. That's why it needs to be solidly bolted to a metal part to conduct the heat away. If the ignition module is not well connected thermally to a heat sink it will overheat and usually cause a miss at higher RPMs (when it's working hardest).
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Old 10-22-2012, 08:45 PM   #5
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This thread is proof that good diagnostics and NOT guessing fixes cars. Nice job. Saved time and money.

The Opti-Spark gets blamed for all kinds of things are NOT its fault.

I had a 94 LT1 I was working on recently. Had a miss under load and actually had detonation at higher RPM.

I found two things:
A. Bad plug wire to #7, all wires miss routed and zip tied together in bunches, all holders broken. Fixed this first, still had a miss. #*#&$

B. Found still had miss on #7, no spark. All others ok, coil ok.
Pulled Opti-Spark suspecting bad cap. Upon inspestion found bad cap indeed. Caused by plastic rivet failure on rotor. Rotor came up(forward) under RPM and dug into the cap causing the spark to not get to the contacts. Worst of it was on #7.

Installed new cap and rotor, runs perfect as before.
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Old 08-30-2015, 10:52 AM   #6
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Default 94 LT1 sudden no-start solved -- Ignition Control Module (Coil Control Module)

Wow! This forum is awesome. 😎 this has got to be the best place on earth for corvette advise. Love it, keep posting all your trials and successes. You all just helped me with a 3 year old post.
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