4th Gen 93 LT1 Camaro ICM Trouble

[Sook]

So I had the Red Shift Orange car at MSR Houston in an HPDE this past weekend for a shake down. The car was recently re-wired to try and chase down electrical gremlins that have been plaguing it for years. I think all the issues have been sorted and the new harness has exposed the root of the problem.

In the 30 minutes the car ran on track (for the whole weekend, pretty pathetic) she munched 2.5 Ignition Coil Modules (ICMs). One was a GM ICM, the other two were Auto zone specials. Short of swapping the ICM in the middle of each session I'm at a loss of how to fix the issue. When she was running, she was the fastest thing on the track (if you exclude the two Z06 vetts).

I put a cheap-O oscilloscope on the 12V bus of the car this evening, and noticed that with the car running or off, the alternator seems to generating some high frequency noise (roughly +/- 1V @ 1.5kHz). There is always some measurement uncertainty, especially since a good scope costs more than the whole car. Verified the electrical noise is coming from the alternator by disconnecting the coil exciter line on the back of the alternator.

I swapped the alternator with a new reman unit, no change in measurements.

Other symptoms:
- There is a low hum coming from the alternator when the engine is not running. I suspect it's there when the engine is running, but no way to hear it.
- The tach needle is erratic at idle and fine above idle. I'm thinking this is indicative of electrical noise.

I have a 470 ohm 1 watt resistor on the exciter line (I think it's L on the plug). Alternator is charging beautifully @ 14.5V

Not sure where to go from here. I'll probably try swapping a third alternator into the car out of desperation. Hoping to run TWS comp school, not sure how well it'll go unless I bring ~20 ICMs to the track.

Thoughts?
- Josh

[Supercharged111]

Charge the battery between sessions? Or throw down on an AC Delco alternator because that Autozone garbage is nothing but a ticking time bomb. Or is there a local shop that does legit alternator rebuilds? I'm not sure what could smoke an ICM. Maybe try a new PCM or opti? Which ICM are you using? There's the 93 only, 94-95, and 96-97 OBDII versions from GM. I bet they're all the same from the parts stores selling that Chinese garbage and even if not, their curves are still not the same as GM's. I have a special place in my heart for parts store electrical parts hatred. Funny how the original parts last like 20 years and 200,000 miles but the junk you buy from Blow Reilly's lasts a year tops.

[Pranav]

Did you check your 470 ohm resistor to make sure it is still functioning like a 470ohm resistor?

I do not use a resistor in mine at all. In fact my alternator is wired completely differently than most everyone else that runs an LT1 in this group.

The traditional way to do it is to wire a 470ohm resistor inline with the L wire on the plug, and run that off your ignition switch.

Instead I just run my ignition on signal directly to the F wire. I then loop back the S wire to the main positive post of the alternator. My L and P wires are not hooked up to anything.

I member seeing a post a long time ago about how the 470ohm method is prone to failure somehow and doesn't match exactly how GM has the warning light wired in the cluster of something.

I've been on the same old, used GM alternator with this wiring method since early 2012 when I first built the car, which makes it the only rotating assembly on the car I haven't replaced at least once, well except for the A/C delete pulley and steering wheel/column...

I don't know if making this change is the magic bullet you're looking for but it's something worth trying.

I have no clue as to how ICMs can keep frying though, and if it is alternator related at all.

I think I've heard the same noise emitting from my alternator with the engine off (and ignition on?) too. I will hopefully be working on the car tonight and see if mine does it too.

[marshall_mosty]

When I read this post, all I heard with Charlie Brown's teacher talking... "Wah, wah, wah... Wah, wah..."
Electrical stuff is greek to me..

[Sook]

I do not use a resistor in mine at all. In fact my alternator is wired completely differently than most everyone else that runs an LT1 in this group.

The traditional way to do it is to wire a 470ohm resistor inline with the L wire on the plug, and run that off your ignition switch.

Instead I just run my ignition on signal directly to the F wire. I then loop back the S wire to the main positive post of the alternator. My L and P wires are not hooked up to anything.


I think I'll try to get a new pig tail and give your wiring method a try.

[Fbody383]

http://shbox.com/1/93_ign_system_schematic.jpg

I'm in the "not necessarily an alternator" camp. Can you get an O-scope on the PCM ignition control output?

If it's not a ground or heat issue at the ICM, I don't see how it's not the PCM.

What do you smart guys think?

[RichardP]

I don't see how it's not the PCM.

We replaced the computer and the chip in the computer. Still thinking we should have let it burn...


Richard P.

[Sook]

http://shbox.com/1/93_ign_system_schematic.jpg
If it's not a ground or heat issue at the ICM, I don't see how it's not the PCM.

As Richard said, tested two computers and three mem-cal chips (6 combinations) over the past few weeks. They all seem to run the same.

[Pranav]

You mention having a new harness; can you elaborate?

Also have you tried checking/swapping the coil? Grabbing at straws here, don't know...

[Pranav]

http://ls1tech.com/forums/lt1-lt4-modifications/1484262-icm-keeps-dieing.html

[Pranav]

http://ls1tech.com/forums/lt1-lt4-modifications/1793867-icm-s-literally-burning-up-need-help.html

[Fbody383]

We replaced the computer and the chip in the computer. Still thinking we should have let it burn... The ICM triggers the coil - is the primary coil winding bad under heat/load so that it's grounding out the ICM?

Are you comfortable that you know what's killing the ICM? Heat? Current? Other?

[Sook]

Factory wiring harness was replaced with a Howell EFI engine harness.
http://howellefi.com/general-motors/gm-lt1-products/wiring-harnesses/lt1-harness-1992-93-corvette-1993-camaro/

Swapped the alternator wiring around to Pranav style, car is charging, DC bus is still a bit noisy, and I'm still getting erratic tach. No idea if it's better or worse. One thing I do notice is that it looks like the alternator is kicking on and off, going between 12 and 14 volts occasionally. Usually always 14, but it dips for seconds at a time. I guess S is a sense line, looping back to the alternator output. Not having to futz with a resistor is nice.

The coil has been swapped twice, the one that was on the car when I initially tried to start it seemed to have a damaged connector. The second on was swapped when trying to diagnose the initial ICM failure. Third one still on the car.

I'm not comfortable saying the sky is blue, much less what's killing the ICM. I'll probably do a cooling mod as well - still no smoking gun on what's killing them.

- Josh

[GlennCMC70]

Make sure you are using thermal paste under the icm and not dielectric grease.

[Pranav]

Is it possible that one of the ICM wires are reversed? I've heard of painless harnesses with a random wire being swapped by mistake...

[Supercharged111]

Factory wiring harness was replaced with a Howell EFI engine harness.
http://howellefi.com/general-motors/gm-lt1-products/wiring-harnesses/lt1-harness-1992-93-corvette-1993-camaro/

Swapped the alternator wiring around to Pranav style, car is charging, DC bus is still a bit noisy, and I'm still getting erratic tach. No idea if it's better or worse. One thing I do notice is that it looks like the alternator is kicking on and off, going between 12 and 14 volts occasionally. Usually always 14, but it dips for seconds at a time. I guess S is a sense line, looping back to the alternator output. Not having to futz with a resistor is nice.

The coil has been swapped twice, the one that was on the car when I initially tried to start it seemed to have a damaged connector. The second on was swapped when trying to diagnose the initial ICM failure. Third one still on the car.

I'm not comfortable saying the sky is blue, much less what's killing the ICM. I'll probably do a cooling mod as well - still no smoking gun on what's killing them.

- Josh

Cooling mod schmooling mod. I've been doing this since 2013 and haven't smoked an ICM though I did deal with extensive knock sensor issues. I have smoked an ICM on an older truck so I'm not invincible. Point is, the ICM itself is not such a POS. The factory puts out parts that work, you just need to figure out what's screwing you over and don't rule out those garbage ass Autozone parts.

[Sook]

Running thermal grease, even got some fancy stuff by the time I got to the 3rd ICM.

I checked the ICM wires per shbox's diagrams, but who knows - won't hurt to check them again.

Here's a video of the tach:

https://drive.google.com/open?id=0B_ab7vummuyKNkpNbTdsTWc3N1k

I let the car idle in the garage for about an hour with a new ICM and alternator. Over that time the tach 'jumpiness' got worse. Alternator voltage dropped from 14.5 to 13.5 on average. Now I'm suspecting the opti/plug wires. Maybe high voltage leakage is escaping to ground some where and polluting the system and damaging solid state electronics. I'll try to get a hold of a set of wires (I'm on a first name basis with all the local car part shops lately...) and see if that makes a difference.

Thanks for all the responses, hopefully I'll get to the bottom of this soon. I'll report back with results!

- Josh

[Fbody383]

Is it possible that one of the ICM wires are reversed? I've heard of painless harnesses with a random wire being swapped by mistake... Like power/ground on the windshield wiper motor on a 69 Camaro harness... Good news is if you take the entire harness back out they'll warranty the harness.

[Storm Trooper]

Running thermal grease, even got some fancy stuff by the time I got to the 3rd ICM.

I checked the ICM wires per shbox's diagrams, but who knows - won't hurt to check them again.

Here's a video of the tach:

https://drive.google.com/open?id=0B_ab7vummuyKNkpNbTdsTWc3N1k

I let the car idle in the garage for about an hour with a new ICM and alternator. Over that time the tach 'jumpiness' got worse. Alternator voltage dropped from 14.5 to 13.5 on average. Now I'm suspecting the opti/plug wires. Maybe high voltage leakage is escaping to ground some where and polluting the system and damaging solid state electronics. I'll try to get a hold of a set of wires (I'm on a first name basis with all the local car part shops lately...) and see if that makes a difference.

Thanks for all the responses, hopefully I'll get to the bottom of this soon. I'll report back with results!

- Josh

It looks like a ground issue to me. I bet you're pulling the ground through your ICM. Make sure your batterie to the chassis ground your chassis ground to the block. Also check your harness from Howell, I use the same harness. make sure you have a good ground attached to the back of the aluminum head. Measure the resistance from ground to the ground wire on the ICM. Make sure you have a good ground. Did I say check your ground? :-)

[Storm Trooper]

Factory wiring harness was replaced with a Howell EFI engine harness.
http://howellefi.com/general-motors/gm-lt1-products/wiring-harnesses/lt1-harness-1992-93-corvette-1993-camaro/

Swapped the alternator wiring around to Pranav style, car is charging, DC bus is still a bit noisy, and I'm still getting erratic tach. No idea if it's better or worse. One thing I do notice is that it looks like the alternator is kicking on and off, going between 12 and 14 volts occasionally. Usually always 14, but it dips for seconds at a time. I guess S is a sense line, looping back to the alternator output. Not having to futz with a resistor is nice.

The coil has been swapped twice, the one that was on the car when I initially tried to start it seemed to have a damaged connector. The second on was swapped when trying to diagnose the initial ICM failure. Third one still on the car.

I'm not comfortable saying the sky is blue, much less what's killing the ICM. I'll probably do a cooling mod as well - still no smoking gun on what's killing them.

- Josh

With the 470 ohm resister between your sense and your sorce. Your alternator should put out steady it should not fluctuate. Also if you have a bad ground your alternator will not put out correctly. Did I say check your ground? :-)

[Sook]

With the 470 ohm resister between your sense and your sorce. Your alternator should put out steady it should not fluctuate. Also if you have a bad ground your alternator will not put out correctly. Did I say check your ground? :-)

On your advice, removed the harness ground from the block, sanded it down, cleaned it with goo, and refastened.

Swapped the MSD plug wires with autozone specials, since that's all I could get today for diagnosis purposes. Got the old wires off, ohm'd them out... suspense... 8 out of 9 were bad (two orders of magnitude low on resistance), and the 9th is questionable at best.

Two of the wires looked like this!
1685

Unfortunately the set I got from autozone was missing the coil wire, so I used the questionable MSD wire for testing on the coil. New coil wire will be here tomorrow for further testing.

Fired up the car and the tach isn't bouncing around any more, getting an occasional wobble that I'm hoping will be fixed by swapping the coil wire. Alternator isn't getting stupidly hot like it used to, ICM is getting hot but I can actually touch it (not at all possible before). I didn't think it was possible for these wires to be bad, could have sworn we changed them this year.

I'll probably try to get a better set of wires for TWS, swap the ICM just in case and bring a spare. I'm not 100% sure this solved my problem, but fingers are crossed. Seems like we're always finding more things broken on the car, and fixing them just creates more problems.

- Josh

[GlennCMC70]

While the harness ground is important, you also need a block to chassis ground.
I also recall a ground strap from the ICM to the chassis.

Also still have the finned heat sink under the ICM?

[Supercharged111]

Aren't MSD wires lower resistance by nature anyway? That totally screws with your non-adjustable dwell which is why I run factory wires. That and factory wires aren't all the same length like that Autozone garbage. I also want to footsomp Glenn's input. Someone else asked about thermal compound and I may have skimmed too quickly but don't recall an answer.

[Pranav]

FYI I checked yesterday, and my alternator does make a tiny bit of noise with the engine off and the ignition switch on. Has been that way for a long time but it's quiet enough for me to have to try listen for it.

Yeah I found a bad plug wire last weekend; stuff happens.

Look into the heat protective fiber wraps if you're burning them up. I made all of my own MSD wires with these ends:

https://www.summitracing.com/parts/msd-3325

The opti and coil uses these:
https://www.summitracing.com/parts/msd-3304

I run the #7 wire under/behind the motor mount bracket (with a zip tie holding it there) and have it come towards the plug from the back. It actually is the only one of my plug wires that gets a wrap as it is still close to the exhaust:
https://www.summitracing.com/parts/the-14262

If you're interested I have a box full used plug wires, mostly MSD.

[BryanL]

On your advice, removed the harness ground from the block, sanded it down, cleaned it with goo, and refastened.

Swapped the MSD plug wires with autozone specials, since that's all I could get today for diagnosis purposes. Got the old wires off, ohm'd them out... suspense... 8 out of 9 were bad (two orders of magnitude low on resistance), and the 9th is questionable at best.

Two of the wires looked like this!
1685

Unfortunately the set I got from autozone was missing the coil wire, so I used the questionable MSD wire for testing on the coil. New coil wire will be here tomorrow for further testing.

Fired up the car and the tach isn't bouncing around any more, getting an occasional wobble that I'm hoping will be fixed by swapping the coil wire. Alternator isn't getting stupidly hot like it used to, ICM is getting hot but I can actually touch it (not at all possible before). I didn't think it was possible for these wires to be bad, could have sworn we changed them this year.

I'll probably try to get a better set of wires for TWS, swap the ICM just in case and bring a spare. I'm not 100% sure this solved my problem, but fingers are crossed. Seems like we're always finding more things broken on the car, and fixing them just creates more problems.

- Josh

Another guy missing a pinky??? What's going on in CMC. Al says those wires look great you just need to cross them.

[Sook]

While the harness ground is important, you also need a block to chassis ground.
I also recall a ground strap from the ICM to the chassis.

Also still have the finned heat sink under the ICM?

Ground strap is present, heat sink is still there with thermal compound. The ICM is bolted directly to the head without a ground strap.

I fired up the car from cold today and it seems to be dropping spark and surging until it got some heat in it. Still was running like crap and O2s are all over the place. Revved the engine a bit and it smoothed out mostly. Tach is still slightly noisy, but much better than before.

I don't have a high confidence that the car is going to make it through TWS weekend... Getting pretty f'n sick of LT1s