Blowing ICM's

[jshow816]

Hey guys, I'm having some problems with my Bonneville.. When I purchased the car it had a aftermarket ICM installed already, and after about 5 months the ICM failed. I installed another aftermarket replacement (Borg Warner) which lasted another 6 months before it failed, and was replaced under warranty. Every time it seems to happen the same way.. The new ICM's will have the car running smooth as a kitten for a while, then every once in a while it will misfire at start up and store code p0300, but clear up after about 30 seconds and run fine. Eventually I'll lose spark on at least one coil and will have a major misfire or the car will cease to run altogether until I replace the ICM.

Any ideas on what I should check or what is causing these ICM failures?

Edit: Also may be important to note that 2/3 coils were replaced with aftermarket coils prior to purchase. All coils ohm out in spec and appear to not have cracks.

[MKMike]

According to this very enlightening article (read Page 2 columns 2 & 3) http://www.wellsve.com/sft503/counterp_v7_i2_20031.pdf , there are 3 types of things which cause repeated ICM failures.
The third reason was very surprising to me.

1) Excessive resistance in the secondary ignition circuit, which includes spark plugs, spark plug wires and coils, as well as poor compression.
Wires often appear OK while having excessive resistance.

2) Poor grounds

3) Excessive AC voltage from a defective alternator. You will not see this on your dashboard gauge or DIC.
Your alternator is supposed to send DC voltage to your electrical system.
A defective alternator may show no obvious symptoms while a diode is failing and sending damaging AC voltage into the system.
Easy test is to hook up a DVOM to the battery, set it to read AC voltage and make sure there are fewer than 0.5 AC volts.

IMHO, there is actually a 4th possible cause, which is poor quality aftermarket parts.

[00Beast]

I'd check all your connections leading to the ICM, as well as at the battery, and grounds in the engine bay. Do you know if it's killing the same coil every time? I'd also check and make sure that the mounting plate where the ICM goes is clean and coated with some di-electric grease, and that the bolts are clean and helping make a good ground connection.

[jshow816]

Thanks for the input. I have a DMM at work so I'll check for AC voltage on Monday and report back. I don't think I have a compression problem because when the ICM is functioning the car runs like a champ.

The first time the ICM failed it lost spark on the middle coil, and the second failure I lost spark on all coils. I have not coated the mounting plate with dielectric grease, and the car has some cheapo spark plugs and wires from the previous owner. I think I'll coat the plate with dielectric grease and and replace my plugs and wires and check the grounds. I know the battery ground is connected tight, and the battery to chassis ground is good as well. Where is the block to chassis ground located?

[MKMike]

I have not coated the mounting plate with dielectric grease, and the car has some cheapo spark plugs and wires from the previous owner. I think I'll coat the plate with dielectric grease and and replace my plugs and wires and check the grounds.

Good starting point.

I know the battery ground is connected tight, and the battery to chassis ground is good as well. Where is the block to chassis ground located?

The block to chassis ground spans from the block near the starter (towards pass. side) to the passenger side in front of the underhood fuse block--both ends are ring terminals.

There are 19 different ground points on our cars and if your car is like mine, they could all use a good cleaning.

IDK if there's a ground diagram/schematic on this site but you can probably find schematics on AutoZone's website.

[jshow816]

Sorry I took so long to get back to you guys, I've been busy with work and stuff. Haven't had a chance to do the tune up or add the dielectric grease yet..

Anyways, I checked for AC voltage between alternator output and case/engine ground = .541 AC volts at idle with high beams on

AC Voltage between alternator output and body chassis ground = .522 AC volts at idle with high beams on

AC Voltage between battery terminals = .341 AC Volts at idle with high beams on

AC Voltage between Batter + and chassis ground = .324 AC Volts at idle with high beams on.

Fluke 77 meter was calibrated in January and shorted leads yields .001 Volts AC, so I trust the readings.. Do you think this is cause enough to replace the alternator? I'm going to have a friend rev the engine for me while I run this test again to see what kind of voltages I get with higher RPMs.

[MKMike]

Given the high cost of ICMs and coils, I'd play it safe and replace the alternator, which does fail the .5 volts test albeit only slightly. Maybe it gets worse when the car warms up, as electronics often do . A second article that I found (written in 2008) http://www.underhoodservice.com/diagnos ... e-testing/ , states
"The past rule of thumb had generally been 0.250 AC volts. However, equipment manufacturers now recommend 0.500 AC volts as a real-world limit. "

In addition, since the plugs and wires are "cheapo spark plugs and wires from the previous owner" definitely replace the plugs and wires with good quality ones---avoiding Bosch because there have been numerous reports by Bonneville owners of troubles with them.
ACDelco OE wires and OE plugs aren't terribly expensive and there have been no reported issues with those.

Clean up the underhood grounds, especially the ones beneath the ICM; a black plastic box with wires and a ring with a bolt through it has the ECM ground in it and the ring connector ground on the stud is an ICM ground; then make sure the battery connections are clean and tight.
I'd get a used but good OE ICM and Coils from a reliable junkyard or buy a decent set from from Harofreak00.

That would cover the bases and should eliminate the problem.