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-mo...ps-dieing.html