shane99

I'm hoping someone could offer some experience with the above problem. The maual says that it may be a short on the Serial Data circuit. Where should someone check to find the short? As its not very clear where the wores disappear to. Tommorow Ill be pulling the stereo out and looking at the CCM connection. This is an intermitent problem on the car that first shwowed up when i had been driving it hard then hit some wet snow. Possibly short from that..i dont know?
Any info would be helpful.
Thanks.

Tom Piper

If your car is a '93 or earlier, my guess is the ECM. I had a very similar problem that I thought was the serial link, but it turned out to be an intermittent problem with the ECM. The '94 and later C4 does not seem to have the problems with the ECM that the '93 and earlier do.

Since the CCM (the CCM is really the master computer and the rest are slaves), ECM, ASR/ABS (also called the EBCM), and the C68 climate control (automatic climate control) are connected together by the same serial link, you can disonnect the ABS/ASR or C68 climate control and the vehicle will still run -- if the problem goes away after disconnecting one of these, you've found the culprit. However, it is difficult to differentiate the problem between the ECM and CCM because they have to both be working for the security system to work.

The serial link is simply two parallel wires that run from one computer to the other -- the wires are in parallel for redundancy in case one gets cut, but if either one shorts to ground they are both shorted to ground.

The ECMs in the C4 before '94, seem to have more problems with the heat from the engine under the hood than the '94 and later years do.

Tom Piper


[Modified by Tom Piper, 7:54 AM 4/21/2003]

shane99

Thanks Tom.
The car is a 92/LT1. After the first few faults it wouldn't give the ERR code when running the diagnostics using pins A and G of the ADSL. It would show no faults when run on the ECM.
The weird thing is just as it would fail simaltaneously I would hear a 'thunk' noise from the negine bay in front of me. Could tell if that was a source or related to going into limp home mode. Did your do that by any chance?
When it would fault out and go into limp home mode. The Speedo and digital readout for anything but voltage would not show. Was yours doing the same? Just trying to narrow it down some more before I make the purchase.
Interestingly enough the security light would flash as well as the the dash read 'SYS' at the time the security light would flash.
Brian


[Modified by shane99, 4:42 PM 4/21/2003]

Tom Piper

My car is a '92.

If you think you are experiencing voltage problems, always find out why the voltage is not right first. Computers are very unstable without the proper voltage.

Yes, all the failures happened at the same time. The "SYS", the Security light sometimes, the ASR light, and the green LED on the C68 climate control system would start flashing. However, my speedometer and gauges still functioned.

What helped me narrow mine down was the problem would usually happen just after the engine warmed completely up (my problem was intermittent). Because I had already disconnected the ASR/ABS computer and the C68 Climate Control and the problem was still there, I knew the problem was the CCM or ECM. But, since the ECM is under the hood where it picks up heat from the engine I figured (correctly) that the problem was the ECM. Like yours, I got no ECM faults.

I have a Tech-1A scan-tool with all the GM cartridges and still had a hard time pinning this one down. Other than CCM, ASR, and C68 Climate Control trouble-codes, all it would tell me was there was a serial-link failure.

Later, I sent an email to Chris Petris of the Corvette Clinic in Sanford, FL. He writes techinical articles for Corvette Fever magazine. He called me late one night and said this is a fairly common problem with the pre-'94 C4s. However, since there were other things that were of more interest to Corvette owners, this problem would probably not make it into the magazine because of space. By that time, I had already pinned the problem down to the ECM. But, I thought it was a nice of him to make a special call to assist me.

In addition, after a lot of investigation, I found out why there were so many trouble-codes from many computers. The ECM is responsible for sending (over the serial-link) the speed of the vehicle to the other computers. The ECM, as a slave computer, sends the vehicle speed to the CCM (the CCM is a master computer). The CCM is then responsible for sending this information to the ASR/ABS and C68 Climate Control computers. If the ECM does not send the information in a given period of time, the CCM requests it again. After several failures in a row, the CCM considers this to be a serial-link communications failure. The CCM, ASR part of the ASR/ABS computer, and the C68 Climate Control need to know the vehicle speed in order to function correctly.

Tom Piper


[Modified by Tom Piper, 12:46 PM 4/21/2003]


[Modified by Tom Piper, 12:53 PM 4/21/2003]

shane99

Tom:
First of all thanks again for the explanation(s). I did lose the speedo when the error came up. The car had also just warmed up when this problem had occurred(4 occasions). So some part of the computer must be(incorrectly) changing its value when the heat changes its: resistance, etc.
I was able to order one for $200CDN this morning. I was amazed as usually we get such a hit on prices for 'vette parts (ie $875 for one FX3 actuator!!).
Have you found any way to insulate it from the heat? I was thinking of making a heat shield and possibly lining it with water tank wrap. That is if it doesnt look that ungoddly. I did notice that the sides of the connector were curled up a bit. A sign of plastic heat shrinkage(?).
Hopefully the part will be in by friday and I'll be 'vetting again.
Thanks again.

shane99

The EMC arrived early and I just finished installing it and going for a test ride. No more codes and she's running just fine.
Tom:
Thanks for the help you saved me countless hours of time. Very very much appreciated.
Brian.