Yes, I think it's the ECM or the ECM's serial data wire. Remember, the CCM gets the values it displays on the dash from the ECM. The "Normal Mode" message contains things like Engine temperture, etc. If the ECM link is not working then the CCM has nothing to display and will throw the Serial Comms code.

 

Thanks for the input. I was leary to start with about buying a reman ecm from the local parts store. There is always a possibility of the "new" ecm being just as fubared as the orig. Although, when I first put it in, it did make a difference, however I think my problem now with the car not wanting to fire may be the module (anyone???). That is the only thing ignition related that I didn't replace.

If I do end up having to replace the CCM, where would be a good place to find one? It sucks that I'll have to have a dealership reprogram it, because it has been my experience with them that they usually don't know what they are doing regardless of what they say and usually tear something else up. At least the couple of them around here.

As always, any suggestions, complaints, or general places I can go are welcome!

I'm tearing into it again bright and early in the morning and will let everyone know who is watching this post what I find.

KC-Shadetree Performance

 

I went through this same issue a year ago. I could kick myself with everything I changed out, CCM, 2 ECM's, tune up parts, etc...

I had 2 chips and never thought they could both be bad (inadvertantly corrupted), well they were. I had a friend show up in my old corvette. we popped my ecm and chip in his car and it acted up, swapped in his chip and ran perfectly. so, my ecm had not been the problem, nor was the ccm, soooo, I erased one of my memcals and burned a fresh file to it and she fired right up.

 

there are no "new" ECMs... they are all remans. I got mine from Ecklers... a little pricy, but I know theirs are GM.

I still say confirm the wiring before you go any farther. It will require a helm manual and time... but if you take it all apart, confirm the wiring is ok, then you can start testing components.

GET A HELM MANUAL and USE IT!!!! www.helminc.com

 

well stated.........validate ccm funtionality before throwing in unneeded parts...

 

I know there are no "new" ecms I just meant new as in new to the car. I'm gonna order a Helm's and I'm getting ready to take apart the dash and chase some wires. First I'll try a few other chips and see if that helps, my dad has been saying all along that maybe its the chips I'm using but I'll certainly try it.

Thanks,
KC

 

Logically, I agree with the above.

However, I should point out, that the serial link has two parallel wires -- in case one opens, the other will carry the data.

For my similar problem, I disconnected every computer from the serial-link but the ECM and CCM and the problem was still there -- this way, I proved it could not be the ASR/ABS computer or the C68 climate control computer.
And, after talking to Chris Petris (who writes articles for Corvette Fever Magazine and owns his own Corvette shop) on the phone late one night (almost midnight), he told me that when there is a serial-link error, on pre-'94 cars, most of the time the problem is the ECM bus-driver loads down the serial-link. And, in my case, he was correct. Replacing the ECM solved my problem. My problem was intermittent and only seemed to happen after the engine had warmed up enough to cause the oil temp to reach about 160 degrees. I think this was the time it took for the under hood temperatures to "heat soak" the ECM.

Chris also told me this problem does not seem to happen after the new style ECM in '94.

Tom Piper

 

"Tom Piper" thanks for the input. I don't get any codes in the CCM by unplugging any of the other items and the gauges still say LO with the SYS flashing. I've even removed the stereo again and still all that comes up is the C41 for the CCM and the H72 for the ABS/ASR. No flashing light on the A/C panel either. I just find it odd that I can't go in and clear the DTCs in the diagnostic mode. Maybe your onto something about the ECM loading up the serial input, although it just shows ERR for the ECM. I've gotten the CCM out (through the driver's side console) and nothing appears to be wrong, but I'm going to start checking grounds and making sure I have continuity between all associated items on the serial bus.

The car did start earlier so that I could move it in the shade, but it ran horribly rich, sputtered and backfired, wouldn't even idle. I think that this new problem may be module related. After I shut it off, it was pretty well dead. When it did backfire however, the tach would jump to around 4000 instantly and fall right back so that's gotta be an ignition problem. The only thing ignition related that I haven't replaced is the module. It's hard to fix or diagnose a problem when others keep popping up.

Back to the shadetree

KC

 

Had the problem with my 91 were the digital readout on the dash was scrambling. I was not having any other problems though. I found that one of the plugs going into the CCM was dirty. When I moved it around the dash cleared up. I removed it and cleaned it with some contact cleaner and the problem was solved.

Try that and see what happens.
Hope this helps.

 

With my problem, even though the ECM was definitely defective, there were never any codes that related to an ECM problem directly.
My codes were for the serial-link failure, one for the ASR/ABS, and the green LED on the C68 climate control would blink.
The CCM controls the communications on the serial-link; it derives the rpm and (I think) vehicle speed from the ECM and passes it on to the ASR/ABS and C68 climate control. All of this is done over the serial-link. So, if the serial-link goes down, it affects all these computers.

In your case, it wouldn't be a bad idea to disconnect all the computers (ECM, CCM, ABS/ASR, and C68 climate control) from the serial-link and measure the link to ground to make sure the wire isn't grounded somewhere. Since my problem was intermittent and didn't happen until the engine warmed up, I was fairly sure the wires weren't grounded.

I know of very few scan-tools that have software to function with the CCM -- most scan-tools only function with the ECM.
The Vetronix Tech-1A ( and, I think the Tech-2) have software cartridges for:
Powertrain -- ECM/PCM
Body -- CCM, radio, climate control, Air bag system
chassis -- ASR/ABS, LTPWS
Mass Storage -- all the above in one cartridge

If I were you, I would find someone with a Vetronix Tech-1A and the software ("body" or "Mass Storage" cartridge) to communicate directly with the CCM (not the ECM). Then, I would check for CCM trouble-codes and run diagnostics on the CCM. This communications with the CCM is over the same serial-link. So, if you can run diagnostics on the CCM, the serial-link isn't grounded.

KC
Another thing to keep in mind, normally (there are always exceptions) after the engine is started, the ECM can run the engine "standalone" without the CCM functioning or the serial-link functioning.


Tom Piper

 

I had had ECM problems similar to Tom’s. It took the Craig Moates <sp> software to figure out the problem.
The car was running rich, real rich and I couldn’t figure out the problem all the sensors tested within specs with the multi meter, so last ditch effort hook up the laptop and monitor the ECM real time.
Turned out that the ECM was misinterpreting the TPS sensor as full throttle all the time, well no wonder it was running rich.

Bought a new ECM from Napa in Hawaii ($110) w/exchange and it cured the problem. The vette was back to running perfectly.
About two weeks later the same exact problem arises. The new ECM failed in the same exact manner. What are the odds… luckily Napa gave me a one year warranty on the ECM, so I went down and traded it for another ECM and the car has ran fine ever since.
That was about 3 years ago.

Shortly after the second ECM failed I bought a spare ECM off Ebay for 40 dollars and a new stock PROM from GM for around 50 dollars or so. I keep them in the back of the vette at all time just in case they decide to die again.

 

I'm headed out right now to Advance to swap out the ECM. It will probably be another 2 or 3 days before I get another one but in the meantime I'll double-check all the wiring. Thanks for all the replies.

KC

 

Don't mean to highjack the thread, but you are the first person that I have come across that has dealt with a problem similar to what I have now. Nice to know that cleaning and reseating the CCM connectors worked for you, I'm hoping to have the same luck on my 92.

 

I just checked with ZIP 'cause I read someone bought an ECM through them and they've got them listed for $95. I'm gonna try and get ahold of someone on the phone to verify that and order one from them and get my $318 back from Advance. Just so anyone looking for an ECM knows.

KC

 

Update. And remember this post is a continuation from my post "ECM Fried?". Since I've ordered an ECM from ZIP and I still waiting for the one from Advance, although I'm gonna get my money back for it, I decided to throw the old ECM in to see if it would start. It was a little rough getting it started but the plugs were probably fouled out but it started and ran fine. The gauges don't say "LO" anymore the coolant just toggles between 107-108 and the oil temp stays at 170. No flashng "SYS", no service ASR light. Still ran horriby rich, but I think that is because the ECM is interpreting the signal coming from the CTS at a constant 107-108 and dumping in way too much fuel for a warm engine. Seeings how the gauges were working, even though they were wrong I shut it off and went through the diag mode in the CCM with the paper clip. C41 changed to H41, got the normal "---" for the ECM and the same H72 for the ASR/ABS. Cycled over to 1.7, held eng/met to clear the codes, back to 1.1 and finally got a C12. The ECM has apparently been the problem all along. I'm just glad that I don't have to replace the CCM and mess around with a dealership.

KC

 

$318 from Advance… Those guys are on crack.

I haven’t seen to many people here have problems with the CCM. They seem to be mostly trouble free. I think it’s because GM decided to move the ECM into the engine compartment.
Real smart GM lets put a computer in one of the most hostile environments on the car.
Geniuses

 

Yeah, lets move it from inside the car where it's nice and dry and somewhat cool and put it under the hood where it's about 230* and is prone to getting wet. Must have been the same "engineer" who thought it was a good idea to utilize the Opti-spark. "See, we could put it right here underneath the water pump, because they (and the 3 or 4 hoses that attach to it) rarely leak". Some times you have to wonder why the $$$$ GM does some of the stuff they do.

KC

 

Yeah, lets move it from inside the car where it's nice and dry and somewhat cool and put it under the hood where it's about 230* and is prone to getting wet. Must have been the same "engineer" who thought it was a good idea to utilize the Opti-spark. "See, we could put it right here underneath the water pump, because they (and the 3 or 4 hoses that attach to it) rarely leak". Some times you have to wonder where the $$$$ GM gets some of the they come up with.

KC

 

interesting that you comment on the location of the ECU... I have put my hand on it, and where it is located, it does not get that hot...

the other thing, ECUs were failing in the dash, as located in 1984-1989 C4s... I don't think the failure rate is any different.

 

Yeah, well, it's still a sh@tty place to put it. Got datamaster working with my old ECM and the coolant temp on the scanner matches that of the gauges (still about 107-108) so I think there's nothing wrong with the CCM, with the old ECM I can now clear the codes stored in the CCM. I'm just gonna wait on my new ECM to get here before I mess with any thing else.

KC