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C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

1996 LT4 ICM Issues and Schematic

 
Old 04-25-2019, 07:47 AM
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grandspt
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Default 1996 LT4 ICM Issues and Schematic


Heatsink and Module relocated with female end of wiring harness.

The male end of the wiring harness.

Another view.

Temperature of module relocated when car was hot.

Temperature of Cylinder head location of ICM from factory.
Hey guys, this may help somebody someday.
So I have been chasing a high RPM miss since my water pump took a **** two years ago (the car has about 40,000 miles, sits garaged in the winter and is a weekend car).
The miss occurred at 5900RPM and higher but only when the car got hot (closed loop of course but at coolant temps of 205-220) Also I could feel a sag in power under load in 6th gear.
Background when I changed the water pump; I naturally pulled the OEM optispark checked for any damage from antifreeze and it was bone dry inside. I did find that one rotor screw fell out and was lying inside the cap and the other rotor screw was loose. But overall the opti looked good, bearing felt smooth etc. So I replaced the cap, rotor, seals screws. Loctited the rotor screws used OEM AC Delco replacement parts and reassembled it. I replaced the original spark plug wires with Taylor red wires while I was in there because the wires were original. I also replaced the vacuum hoses going to the opti at the same time. None of this fixed the high RPM miss when hot. The car throws no codes. I ran the Tech2 scanner misfire graph mode and found random misfires when hot even at idle.
So what I have done and where I am now; I relocated the ICM and made a wiring harness so I did not have to cut the original harness up. I purchased a female weather proof connector from Amazon (
Amazon Amazon
). I milled out the heatsink for the ICM to fit properly and so that I could remove the connector, drilled and tapped two mounting holes and affixed the heatsink with thermal paste to my radiator shroud on the passenger side. I made a wiring harness extension to mate to the ICM. Amazingly it fixed the high RPM miss, at least I cannot feel it anymore. I hooked up the Tech2 scanner and I do not get the misfires at idle anymore. When driving it does show some random misfires just driving at some low RPM speeds not sure if this is normal. The sag in 6th gear has gotten better butt is not eliminated.
I attached pictures to what I have done so far.
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Old 04-25-2019, 08:11 AM
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Code thrown with NEW ICM's


Now that I have gone through the explanation of what I have done. My next step was to replace the ICM in case the driver has leakage from the heat it has been exposed to. Much to my surprise when I originally pulled this off my cylinder head I found the thermal grease dried up on the back of the ICM to the stock heatsink, also the thermal grease between the two zinc coated mounting plates(which the heatsink and coil mount to) had almost no trace of the thermal grease but still had red Dexcool on it (from two years ago) and mixed with whatever thermal grease was left.
I purchased an AC Delco 10482803 ICM from Amazon: (
Amazon Amazon
).
Then I purchased a Delphi: (
Amazon Amazon
).
After installing both I immediately got a code P1351. The car was running fine but every time I cleared it these two ICM's would throw this code. I sent them back to Amazon for a refund.
My original never throws any codes. Could I have gotten two defective ICM's? I do have another ICM coming from Rock Auto (it is a United Motor M1599) but I don't have much faith in that one.
Does the LT4 use a special ICM? Those were the replacement part numbers according to GM and Delphi catalogs.
I have reverse engineered the cheap ICM I bought from Amazon (I never tried installing it before hacking off the connector, which I should have) and made a schematic diagram of it. Please excuse the hand drafted schematic it is crude. I have googled the schematic of the GM ICM and always come up short. No information out there. So if there are any electrical design engineers out there please have at it. Maybe we can design our own (I am in the process but extra help is always appreciated), I am only a manufacturing engineer but I have a lot of experience in electronic circuits.
Attached Images
File Type: pdf
GM ICM Schem.pdf (59.9 KB, 9 views)

Last edited by grandspt; 04-25-2019 at 08:19 AM. Reason: Amazon links didn't show correctly.
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Old 04-25-2019, 08:48 AM
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Originally Posted by grandspt View Post
I purchased an AC Delco 10482803 ICM from Amazon: (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1).
Then I purchased a Delphi: (https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1).
After installing both I immediately got a code P1351. The car was running fine but every time I cleared it these two ICM's would throw this code. I sent them back to Amazon for a refund.
My original never throws any codes. Could I have gotten two defective ICM's? I do have another ICM coming from Rock Auto (it is a United Motor M1599) but I don't have much faith in that one.
Does the LT4 use a special ICM?
No, the ICM is the same as for an LT1 in 1996. It's probably the same for all LT engines in any vehicle that year. It is different from earlier year Corvettes, though. So make sure the ones you bought are for a 1996. FWIW, I got one cheap ICM from Rockauto that was DOA a couple years ago. I replaced it with something that cost twice as much at a local parts store (maybe Borg Warner? I can't remember), and that worked fine.

Honestly, my guess is that the ICM wasn't really the original problem. It sounds more like you might have a coil on its way out, or a dying O2 sensor to me. You should never be getting random misfires in a stock LT4 - GM was pretty careful to detect those, and by 1996 they installed a crank position sensor just to pick those up. So they obviously didn't intend for that to be normal.
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Old 04-25-2019, 02:13 PM
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I received my Rock Auto ICM it is a United Automotive. It immediately threw a P1351 code.
Damn, I couldn't get 3 different defective ICMs from 3 different manufacturers could I?
I went and found a new coil from the local Chevy dealer. I installed it still no difference.
Using my Tech 2 I did a snapshot of my misfire and low and behold it starts at 1350 RPM continues through about 2500 RPM then stops misfiring after that RPM range.
I can now reproduce this parked in neutral no load bringimg it up to those RPMs and watch the misfire graph. So number one cylinder is the one that is missing the most.
I sprayed water on the #1 wire and it made no difference. I can't see any arcing but I can hear a slight ticking at that RPM and I am not sure if it is under the valve cover a lifter, or rocker?
This doesn't explain the ICM replacements I have been getting.
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Old 04-25-2019, 08:22 PM
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I checked resistance values between pins on all of the ICMs that I purchased. I am getting different values compared to my stock ICM.
Having said that the purchased ones are very close to each other.
This confirms my theory that the ICM I have is a different breed.
Are there any LT4 owners that have recently changed the ICM with a new one? Have you had success?
MatthewMiller I may try the Borgwarner do you have an LT4?
Also I believe that this is not the cause of the misfire from 1350 to 2500 RPM.

Last edited by grandspt; 04-25-2019 at 08:26 PM.
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Old 04-26-2019, 08:42 AM
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grandspt, yesterday was busy and I'm just now getting back to this forum. Yep, I have a 96 LT4 also. Again, though, the ICM for the LT4 and LT1 are the same within that year. They are different for 95s, and I wonder if this is the issue? Do you have the FSM? It discusses this trouble code, which is caused by voltage in excess of 4.6v (i.e., too high) on the IC circuit. It's detected at cranking. When this happens, it says the injector will be disable for the entire ignition cycle. So it sounds like the makings of a misfire to me. It sounds to me like the ICMs you're buying are all functioning correctly, and there's an issue with high voltage on the signal lines going in and out. I what else besides a faulty (or incorrect) ICM could cause this, unless the voltage going INto the ICM is too high.

With any of the new ICMs on the car, when they throw the code how does the car actually run? Is the misfire still present? To me, the most likely source of the too-high voltage has to be related to the changes in the location and harness that you made. Maybe there's something goofy with the ground that you established? I'd return it all to stock and see if the code still remains.

Here are the two pages in the FSM for this code:



Last edited by MatthewMiller; 04-26-2019 at 08:46 AM.
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Old 04-26-2019, 01:48 PM
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MathewMiller, I do have the FSM but I do appreciate the diagram. Still trying to figure out how the PCM knows that the voltage is high on the ICM. I did try one of the new modules eliminating the harness that I made and it still gave me the P1351 code. I even added a nice 14 AWG wire from the Cylinder head bolt to the new heatsink to make sure I had a good ground (even though my stock OEM ICM doesn't apparently need it). Interestingly all of the new ICM's start and run fine, I notice no driveability issues other than the code it throws (the light stays on continuously until I reset the DTC). Relocation and a larger heatsink has definitely solved the high RPM miss from 5800 RPM to 6300 RPM when hot.
I would like to purchase a spare ICM just to have. But I can't find one that won't throw a code (P1351).
I measured the resistance from the pins A,B,C,D using my OEM ICM as a baseline. There is quite a difference on the new ones compared to my old OEM one.
Were you sure that your replacement was a Borg Warner? If so I will try one of those. I have been very lucky that I have been able to return all of these new ICM's.
I still have to chase down my misfires at lower RPM that I apparently have. The car is actually running real strong even with the miss. I have been noticing that my charging voltage this season after taking the car out of storage for the winter has been varying from 14 (high operating temps) to 14.8 volts (cold temps.), I need to check the battery because it is a 12 year old Optima Red Top ( I use a Battery Tender on it for the winter). This was going to be my next step.
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Old 04-26-2019, 03:03 PM
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Originally Posted by grandspt View Post
Still trying to figure out how the PCM knows that the voltage is high on the ICM.
Yeah, that's a good question now that you mention it. I don't know!

Were you sure that your replacement was a Borg Warner? If so I will try one of those. I have been very lucky that I have been able to return all of these new ICM's.
No, I'm not sure. That was two years ago and I can barely remember what happened earlier this week! If I can find the box or order history, I'll let you know.
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Old 04-26-2019, 03:37 PM
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Thank you that would be great if you can find the box or brand/part number.
I appreciate it.
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Old 04-26-2019, 04:02 PM
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I have my original icm I just removed from my GS that was working fine if you want to borrow it to test. I went with a Torqhead setup so I don't need it anytime soon.
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Old 04-26-2019, 05:58 PM
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Originally Posted by RichS View Post
I have my original icm I just removed from my GS that was working fine if you want to borrow it to test. I went with a Torqhead setup so I don't need it anytime soon.
Hi RichS, I really appreciate that. I am going to try a few more things first but I may take you up on that offer.
How is your Torqhead setup? Do you have a write up on it?

Last edited by grandspt; 04-26-2019 at 05:58 PM.
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Old 04-27-2019, 02:04 PM
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Originally Posted by grandspt View Post
MathewMiller, I do have the FSM but I do appreciate the diagram. Still trying to figure out how the PCM knows that the voltage is high on the ICM. I did try one of the new modules eliminating the harness that I made and it still gave me the P1351 code. I even added a nice 14 AWG wire from the Cylinder head bolt to the new heatsink to make sure I had a good ground (even though my stock OEM ICM doesn't apparently need it). Interestingly all of the new ICM's start and run fine, I notice no driveability issues other than the code it throws (the light stays on continuously until I reset the DTC). Relocation and a larger heatsink has definitely solved the high RPM miss from 5800 RPM to 6300 RPM when hot.
I would like to purchase a spare ICM just to have. But I can't find one that won't throw a code (P1351).
I measured the resistance from the pins A,B,C,D using my OEM ICM as a baseline. There is quite a difference on the new ones compared to my old OEM one.
Were you sure that your replacement was a Borg Warner? If so I will try one of those. I have been very lucky that I have been able to return all of these new ICM's.
I still have to chase down my misfires at lower RPM that I apparently have. The car is actually running real strong even with the miss. I have been noticing that my charging voltage this season after taking the car out of storage for the winter has been varying from 14 (high operating temps) to 14.8 volts (cold temps.), I need to check the battery because it is a 12 year old Optima Red Top ( I use a Battery Tender on it for the winter). This was going to be my next step.
I'm thinking it might be easier to find a known working ICM to try on your car or take one of your new ICM's and verify it on another car that way you would have a known good ICM to work with. But the problem could well be in the PCM itself also. No I don't know the actual circuit that senses the ICM for fault but using a known good ICM should be a short cut for that. So you are using a homemade ICM harness? That would be the first item I would have to test. But measuring the resistance on the ICM pins is going to be measuring resistance of micro circuitry and really not a good idea to test these for resistance or would I expect any usable information for anything other than a resistance circuit.

Do you have tuning software? Pretty easy to copy your tune and load it onto a boneyard PCM. I really don't know which PCM's swap with the corvette but it's a good tool to have a spare PCM for troubleshooting. For example I still have my original PCM with the original stock tune for my camaro on the shelf and driving with a PCM from a firebird - picked up another spare from a Roadmaster recently as it was so easy to remove I couldn't walkaway from it.

Your battery is a problem whether your ICM is bad or not. I recently discovered how much better my LT1 runs with a fully charged new battery. I know we expect the alternator to handle the load once the motor is running but there is more to it than that.

Just my guess is you may have a wire broken inside it's insulation or a loose pin in a connector. If it's inside the PCM harness it's gonna be tough to fix but not to difficult to troubleshoot. Focus on the ICM to PCM wiring/connections.
Or you might be surprised to find a bad sparkplug or 2 that were dropped before installing though I admit I don't know how that would cause P1351 code.

Good luck.
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Old 04-27-2019, 02:35 PM
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Originally Posted by cardo0 View Post
I'm thinking it might be easier to find a known working ICM to try on your car or take one of your new ICM's and verify it on another car that way you would have a known good ICM to work with. But the problem could well be in the PCM itself also. No I don't know the actual circuit that senses the ICM for fault but using a known good ICM should be a short cut for that. So you are using a homemade ICM harness? That would be the first item I would have to test. But measuring the resistance on the ICM pins is going to be measuring resistance of micro circuitry and really not a good idea to test these for resistance or would I expect any usable information for anything other than a resistance circuit.

Do you have tuning software? Pretty easy to copy your tune and load it onto a boneyard PCM. I really don't know which PCM's swap with the corvette but it's a good tool to have a spare PCM for troubleshooting. For example I still have my original PCM with the original stock tune for my camaro on the shelf and driving with a PCM from a firebird - picked up another spare from a Roadmaster recently as it was so easy to remove I couldn't walkaway from it.

Your battery is a problem whether your ICM is bad or not. I recently discovered how much better my LT1 runs with a fully charged new battery. I know we expect the alternator to handle the load once the motor is running but there is more to it than that.

Just my guess is you may have a wire broken inside it's insulation or a loose pin in a connector. If it's inside the PCM harness it's gonna be tough to fix but not to difficult to troubleshoot. Focus on the ICM to PCM wiring/connections.
Or you might be surprised to find a bad sparkplug or 2 that were dropped before installing though I admit I don't know how that would cause P1351 code.

Good luck.
Hi cardo0, I have eliminated my wiring harness extension being at fault. One of the new ICMs got hooked up to the stock harness and it still threw the P1351.
I don't think it is a problem with my PCM, otherwise my stock ICM would also set the P1351.
The battery may be an issue for sure. I am working on that.
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Old 04-27-2019, 05:03 PM
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Originally Posted by grandspt View Post
Thank you that would be great if you can find the box or brand/part number.
I appreciate it.
Damn, sorry that took so long. Yes, it's Borg Warner CBE122. I must have gotten it at Advance Auto Parts, but other online places have it too.
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Old 04-27-2019, 07:52 PM
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Originally Posted by MatthewMiller View Post
Damn, sorry that took so long. Yes, it's Borg Warner CBE122. I must have gotten it at Advance Auto Parts, but other online places have it too.
No worries, I really appreciate your help and every one else's help too. I have not tried the Borg Warner. I will order one tonight online.
I did pull the battery and tested it out. It is 100% charged but bad. Low current output. So I have a new one coming out next week. That Optima red top lasted over 13 years. At least it didn't leave me stranded!
I will report back on my findings.
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Old 04-27-2019, 11:50 PM
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i pretty much did the same thing you did relocating my coil and icm, but i moved mine to the top of the crossmember behind the fans. i was also getting a 1531 code and purchased a borg warner icm from advanced auto. i also ran a ground wire from a ground point to the icm. has worked for the last couple months with no issues.



x



x
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Old 04-28-2019, 07:57 AM
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Originally Posted by Sleazy Rider View Post
i pretty much did the same thing you did relocating my coil and icm, but i moved mine to the top of the crossmember behind the fans. i was also getting a 1531 code and purchased a borg warner icm from advanced auto. i also ran a ground wire from a ground point to the icm. has worked for the last couple months with no issues.



x



x
SleazyRider, thanks for your input that is good to hear that you used the Borg Warner and had success too.
I just ordered mine online and I am going to pick it up at Advanced Auto today.
That also looks like a good spot to relocate the ICM. I can't believe how long they survive mounted on the cylinder head.
Relocation should allow the circuit to operate more reliably.
Unfortunately my battery won't be here until Wednesday from Summit Racing (I went with another Optima since this one lasted over 13 years).
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Old 04-28-2019, 09:17 AM
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I would load that new battery up before installing they are having issues with that brand!
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Old 04-28-2019, 04:36 PM
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Originally Posted by grandspt View Post
Hi RichS, I really appreciate that. I am going to try a few more things first but I may take you up on that offer.
How is your Torqhead setup? Do you have a write up on it?

The car was running good with the base tune but had 2 issues. I sent the computer back to have them corrected so I should be able to start breaking in the new motor when I get back from vacation next weekend. Then the dyno tune.
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Old 04-28-2019, 06:52 PM
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Well I canceled my Summit racing Optima battery order. Advanced auto had the Optima battery in stock and since I was picking up the Borg Warner ICM I figured I might as well buy the battery too.
I installed the new Borg Warner ICM (the box showed made in the USA but I don't know which one was; the box or the ICM). Anyway it works no codes!!!!
Thanks guys for your recommendation on the BW CBE122 this makes me feel better that it is not my car. It looks like I must have come across either defective ICMs or they really were not made for the 96 year LTX engines regardless of what the catalogs showed.
I also installed the new Optima battery, I drove the car for a while and got the oil temp up to 208 degrees and ran it through the 1350-2500RPM range, so far no misfires on any of the cylinders. But it was a cool day outside. I am going to leave the Tech 2 hooked up and try it again on a warm day that is if we ever get one here in Boston.
**Question: Could the battery being bad cause too much ripple (The battery does act like a large capacitor) from the rectifier/regulator in the alternator, not delivering a clean supply voltage to the PCM, ICM, Injectors, maybe even the fuel pump? And maybe this was causing my random miss?
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