Hey guys, I had been having some trouble with my car recently. The SES and SYS lights were coming on at the same time and would go away pretty quickly, they would come on about the same times going to work. One day I had trouble starting the car at work so my dad and I replaced the battery that weekend and things were going well but the light started coming on again. Then this weekend we cleaned up some grounds and checked all the connections to the starter, alternator and distributor and all seemed tight and clean. The following evening the light came on again (damn!), and has been getting progressively worse. Today when I got to work the car would idle pretty rough and try to die on me and the voltmeter would go up and down. I tried starting it a few times on my breaks and at lunch and would take a try or two but would turn over ok and then driving home after work the light stayed on continuously until I got off the freeway. The light turned off then started blinking and when I stopped at the first light on the way home the car tried to die on me again. I finally got her home and had to actually push her into the garage and could not get the car to start again. This is my daily driver and need to fix this possibly today. I also found out how to pull CCM codes last night as opposed to ECM codes and pulled a C and H41 in 1.1, could the alternator cause this? The car would read normal voltage when driving but not while stopped/at idle.

Found out it is not the alternator, so is it my CCM or ECM? I'm going to check all connections later and clean them up - hopefully I can get this fix itself. Where is the CCM by chance? I want to check all connections to it as well.

Sounds like a connection problem somewhere. I would wait for bogus to chime in as he could give you a pinpoint starting location. Have you done ANYTHING to it right before the problems started? Any electrical mods could easily short stuff out as I have recently found. So if you have a new radio, or other newly installed electrical trinket that might have something to do with it....just food for thought.

Pull ALL of the codes and follow the trouble shooting charts in your [color=darkblue:306d79576b]FSM[/color:306d79576b], starting with the lowest codes and working your way up. It is unlikely that either the ECM or CCM is the problem. Unless you know these cars, inside and out, backward and forward, any other method is just wasting time and money.

I am CFI-EFI, and I approve of this message.

RACE ON!!!

I have not done anything to the car that would affect the battery and know about the new stereo - sys issues - I still have stock. I do need to buy a Helms manual but don't have access to one right now. The only codes stored in the car are 1.1 H41 and C41. I did e-mail Bogus who said I should post my question here to get more answers. I told him I thought it could be the alternator which I guess it isn't; he informed me about how the ECM, CCM and EBCM all work and when something isn't connected it gets screwy. About to go check all the wires and connections, see if I can't find something out of the normal, any suggestions? I was thinking about unplugging everything from the ECM, seeing if I can clean things up some and putting it all back together.

What year is the car?

Looking at his website, looks like he got a 90.
You want to open your ECM and check if the Calpack/Eprom is seated properly. You might have a Hypertech- or a custom chip on a carrier that is loose? If the Calpack has a tight fit, time to change the ECM.

Yeah, it's a '90. I'm just about to go out, just researching a bit more. Thinking about checking all my fuses, checking the ground to the CCM (above the oil filter??) and checking the ECM.

New update: Took off the ECM off the bracket plate and unplugged everything wearing rubber gloves and disconnected battery not to cause any static electricity. Connected it back together, then checked all the grounds and they all looked good. I then jacked up the car and found the ground for the CCM above the oil filter but have no room to clean it up so if I can get the car to get me to work the next two days I can clean it up this weekend. Part of one of the wires was exposed as well and I could see no damage but I still don't like that. After I finished I started the car up and the SES and SYS light came on IMMEDIATELY and the voltmeter was moving back and forth rapidly and the lights dimmed very noticably - have not seen lights dim prior but that doesn't mean it didn't happen. Is there still anyway it CAN'T be the alternator? I wouldn't ask if the voltmeter wasn't moving up and down SO much.

gee... that's a good one...


In reference to our email (that's how this all started!), you really need to see what the CCM codes are.

If you are concerned about the alternator, I would yank it and have it tested.

I agree, with all the voltage fluctuation, it's worth testing it. Don't just change it out without a test.

Almost a year ago, my 91 did the same thing. I did some research on the other forum, posed a few questions, followed most advice (specfically Bogus, CFI, Casey and a few others) and found I had a 12 volt to ground challenge - dead short. After a month of weekends and nights, testing as I went, I found the wire to the starter solenoid wire along with the knock sensor and temp sensor wires bared to the bell housing behind the right head. The real down side is that it took out the ECM and the CCM.
Check to see of you have excess drain on the battery. If so, start pulling fuses until the drain goes away. The start with that circuit looking for anything out of the ordinary.
BUT, before you do anything, find a copy of the FSM electrical book (the 91 is a red cover - I think the 90 is as well) Without it, you might as well send the car to a good electrical shop - Just my $0.02
Good Luck
Subman

The voltage reg in the alternator can cause that, although your battery voltage would have to be quite low for the lights to dim with voltage fluctuations from the regulator.

However it seems as though you may have a short to ground somewhere in the system, and until you get this sorted out, you should not be driving the vehicle.

Check down behind the battery where all the terminals are, make sure nothing has been damaged by sulfuric acid. Check anywhere the positive or signal cables run that carry any significant current, and make sure they are not shorting to ground.

I dont have a list of CCM codes for a 1990, so im reluctant to go into that without first knowing for sure what it is. Im not happy to assume the CCM codes for a 1990 are the same as a 92-93, cause many of the DTCs from 94-95 are different to 92-93, and i dont want to hand out a bum steer.

Alright well, I had to work at 7am so I decided to take my chances and see what would happen with the car. When I started the car the SES/SYS lights came on and I was scared the car would try to stall while I went to the gas station first. Started right up after getting gas and about 1/3rd the way to work the lights went out and the car drove like normal. I started it up all day at breaks and lunch, no SES/SYS lights and idle would stay consistent. I left work and about 1/3rd the way home the light came on and flickered on/off until about 2/3rds the way home. The light then went off and came on again when I stopped after getting off the freeway. I made it home again and noticed that only the SES light was coming on now, not the SYS light so I pulled a code 33 which I just read for my year is mass air flow sensor high? What exactly does that mean? Was I putting too much strain on the engine trying to get home? I can get around not driving it tomorrow but this will be fixed this weekend. I've deteremind I think it is the ground to the CCM above the oil filter cause when I would go over a bigger bump in the road the light would blink at me like one of the wires was loose. I also didn't mention that the ground for the CCM was filthy but I don't see corrosion and I don't have the room to clean it up so I'm gonna try to get the car on a lift this weekend. I read that the code 33 can come on if the car is below 1/4 throttle? That would make sense if I'm having problems with my ECM then, yes?

I am liking the bad ground problem more and more.

The intermittent nature of this is bothering me... that and the fact that it is getting progressively worse and more predictable.

Don't replace anything. Test the alternator and check the ECU grounds.

The voltage display comes from the ECU, IIRC.

The CCM is joined via harnesses under the passengers side of the dash. Off white 8 pin (2 rows x 4) connectors. That leads into the underhood harnessing...

The way the car drove with no lights would lead me to believe that you are right about it not being the alternator, it was holding normal volts. I also had it tested ON car the other night and it was still showing normal voltage so I would suspect it's a ground or maybe a short to a wire somewhere. I got rides to and from work lined up for tomorrow so saturday I can tackle this ASAP. I also wanted to share that I meant to say the code 33 is for the manifold absolute pressure sensor - voltage high.

Yeah, I was about to say that your car doesn't have a mass aiflow sensor. Until you get a factory service manual for a 90 Corvette, you are just searching for a needle in a hay stack. I highly doubt the ECM or CCM is failing.

Tomorrow morning I'm going straight for that ground, if that isn't the problem its going to an auto electrician.

Hey guys, cleaned up the ground for the CCM by the oil filter and even replaced a connector (corroded), did NOT fix the problem. Upon restart the SYS/SES immediately came on. I'm thinking of rechecking all fuses, checking to see if my prom is 'seated' correctly, and to also check if I have a short somewhere in the system. Any one else have any more advice?

One more question: How can I check the connections to the CCM?