LT1 Running lean
Instructor
Joined: Nov 2013
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The icm is mounted with the coil on the front of the pass side cyl head. That harness runs to the opti plug which you can unclip with an angled pick right between the water pump and block. Once removed take a multimeter set to ohms and check end to end of the same colored wires in the harness make sure there no more that .5ohms if anyone check ol, (open) or mega ohms then the wire is bad. Some opti come with this harness new some dont. You can get just the harness if it's bad.
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From: Dallas Texas
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I am following the diagnostic for code 42. In the FSM, code 41 is EST. So, I probed the terminal B on ICM connector, and the ground. It was at 2.8-3. But I doubt that test was accurate because I did it with the engine cold. The first step is to clear codes, to see if they are intermittent or not. How do I clear codes? Couldn't find it in the manual.
Instructor
Joined: Nov 2013
Posts: 154
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I am following the diagnostic for code 42. In the FSM, code 41 is EST. So, I probed the terminal B on ICM connector, and the ground. It was at 2.8-3. But I doubt that test was accurate because I did it with the engine cold. The first step is to clear codes, to see if they are intermittent or not. How do I clear codes? Couldn't find it in the manual.
Drifting
Joined: May 2007
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From: Motor City
Pulling the battery cable won't clear history codes. There's a specific procedure to clear them though the DIC. Sounds like you have an FSM, a quick read will show this procedure in the CCM section.
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From: Dallas Texas
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I'll give it a second look. Yes, I do have the 93 FSM. I couldn't find anything for code 41 in module 1. I found a list of codes, and 41 is electric spark timing. So for now, I'm using the diagnostic for code 42.
Drifting
Joined: May 2007
Posts: 1,426
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From: Motor City
- code 41 in module 1 is ECM serial data circuit. Page 8D-54.
- information on clearing dtc's from the CCM and ECM can be found starting on page 8D-9
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Code 41 in module 1 went away?? Started the car this morning and you can see how bad it's running. It's also making a weird sound when I turn it off.
And the sound on turn off:
OK, followed the diagnostic chart for code 42. While cranking, it was at 2.8-3 volts.
Step 3 says"Igniton "OFF"
With test light to B+, probe ignition coil module harness connector terminal "C".
Light should be on is it?"
In my case the test light was not on. But tell me if I did step 3 correctly. I put test light into "B" and put the red wire(positive) into terminal "C". Doing that, the test light did not turn on.
And the sound on turn off:
OK, followed the diagnostic chart for code 42. While cranking, it was at 2.8-3 volts.
Step 3 says"Igniton "OFF"
With test light to B+, probe ignition coil module harness connector terminal "C".
Light should be on is it?"
In my case the test light was not on. But tell me if I did step 3 correctly. I put test light into "B" and put the red wire(positive) into terminal "C". Doing that, the test light did not turn on.
Last edited by 1993C4LT1; Nov 11, 2015 at 12:43 PM.
Safety Car
Joined: Sep 2010
Posts: 3,752
Likes: 190
From: Indiana PA
Are you sure you don't have a vacuum leak? Booster?
Just a random thought.
No idea what that whine is when off! Odd.
Can you feel anything moving like fuel in the rail or anything while its doing that?
Just a random thought.
No idea what that whine is when off! Odd.
Can you feel anything moving like fuel in the rail or anything while its doing that?
Last edited by 93Rubie; Nov 11, 2015 at 07:21 PM.
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No leaks at the booster. I'll have to check tomorrow if I can feel something moving when the whine is audible. So code 41 in module 1 went away. Now I have a code 41 in module 4, which is pretty identical to code 42. It tells me to disconnect ECM connector A in the diagnostic, do I have to disconnect the battery as well?
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From: Dallas Texas
2025 C4 of the Year Finalist - Modified
Did the diagnostic for code 41, and same result as code 42. It tells me "Open ignition coil module ground circuit". I hate to sound like a beginner, but what does that mean?
Advanced
Joined: Feb 2014
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Also verify that the coil harness and connectors are in good shape. They are know to cause issues.
Also, I don't think you did step 3 right. I think by 'B+' they mean battery positive. Connecting the test light to the battery and pin C of the ICM harness would verify that the ground is good.
As far as what the ECM code means, the ECM is looking for something like 5Kohm to 7Kohm of resistance between ICM pin B and ground. If it doesn't see it, the code is set. If the resistance is high enough, I've seen the ICM cause 'Start, then dies' condition on LT1 engines.
Also, I don't think you did step 3 right. I think by 'B+' they mean battery positive. Connecting the test light to the battery and pin C of the ICM harness would verify that the ground is good.
As far as what the ECM code means, the ECM is looking for something like 5Kohm to 7Kohm of resistance between ICM pin B and ground. If it doesn't see it, the code is set. If the resistance is high enough, I've seen the ICM cause 'Start, then dies' condition on LT1 engines.
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From: Dallas Texas
2025 C4 of the Year Finalist - Modified
Yea, I looked up the code and found a helpful thread on digital corvettes. Can't believe I didn't get it
Did the test correctly. Engine was running so bad on Wednesday, I didn't even want to start it. Does it matter if I performed the diagnostic with a cold engine? Anyways, with diagnostic done correctly, I got "Faulty ignition coil module connection or faulty ignition coil module". I doubt the ICM is bad, I put the thermal grease and installed a new AC Delco ICM about 2 weeks ago. How do I test the connection?
Did the test correctly. Engine was running so bad on Wednesday, I didn't even want to start it. Does it matter if I performed the diagnostic with a cold engine? Anyways, with diagnostic done correctly, I got "Faulty ignition coil module connection or faulty ignition coil module". I doubt the ICM is bad, I put the thermal grease and installed a new AC Delco ICM about 2 weeks ago. How do I test the connection?
Last edited by 1993C4LT1; Nov 13, 2015 at 04:50 PM.
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Yea, I looked up the code and found a helpful thread on digital corvettes. Can't believe I didn't get it Did the test correctly. Engine was running so bad on Wednesday, I didn't even want to start it. Does it matter if I performed the diagnostic with a cold engine? Anyways, with diagnostic done correctly, I got "Faulty ignition coil module connection or faulty ignition coil module". I doubt the ICM is bad, I put the thermal grease and installed a new AC Delco ICM about 2 weeks ago. How do I test the connection?
Did the test correctly. Engine was running so bad on Wednesday, I didn't even want to start it. Does it matter if I performed the diagnostic with a cold engine? Anyways, with diagnostic done correctly, I got "Faulty ignition coil module connection or faulty ignition coil module". I doubt the ICM is bad, I put the thermal grease and installed a new AC Delco ICM about 2 weeks ago. How do I test the connection?
Thread Starter
Race Director
Joined: Oct 2011
Posts: 16,089
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From: Dallas Texas
2025 C4 of the Year Finalist - Modified
Thread Starter
Race Director
Joined: Oct 2011
Posts: 16,089
Likes: 819
From: Dallas Texas
2025 C4 of the Year Finalist - Modified
Hey guys, I want to diagnose code 26 which is "quad-driver module #1 circuit", but the FSM says I need a Tech1. Is there anything I can do without the use of a Tech1?
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From: Dallas Texas
2025 C4 of the Year Finalist - Modified
Found two things
1. That whining sound is coming from the 2 way check valve I replaced. If I disconnect the vacuum lines on passenger side of the intake manifold, the whine goes away.
2. The auto parts supposedly gave me thermal grease, I took their word for it. Looking behind the ICM, there is a clear liquid. Thermal grease is actually white, as I learned today.
1. That whining sound is coming from the 2 way check valve I replaced. If I disconnect the vacuum lines on passenger side of the intake manifold, the whine goes away.
2. The auto parts supposedly gave me thermal grease, I took their word for it. Looking behind the ICM, there is a clear liquid. Thermal grease is actually white, as I learned today.