Let me state I make no claims to being an auto mechanic but I am a master certified multi manufacturer boat mechanic. Most of what I've read points to a "bad computer" on my '93 LT1 / ZF6 but it has been my overwhelming experience in marine products that it's usually not the ECM but rather a failed sensor or harness grounding reference voltage or creating an open circuit. Let me tell you what I got and please tell me what you think.

Several weeks ago I let my kid drive the car for a few days. When it came back there was a quarter in the seat belt female side which temporarily trapped me and the ANALOG temp gauge was reading 0. I ordered a new single wire temp sender and drove on.

Several days later while going down the Hwy. at 60 the engine dead missed several times. "System" showed on the dash. Service engine & service ASR showed up on the message screen. When I got to town the engine was running rich and shut down at a traffic light. I got it restarted and limped to work. My security light was now flashing and ASR off was also on messages. After work the car started and ran well on the way home.

The following morning she ran like ****! all the above mentioned items but constant misfires as well as several other weird things. The electric fan came on as soon as the key was turned on and the security system was going nuts.
I tried cleaning the key switch and using another key with no difference. I disconnected the battery , the ECM and the PROM , all connections looked clean. Put it back together and it ran great for 20 minutes then the same problems came back.
Ran codes and found the following:
Module 1 CCM - code H41 and C41
Module4 ECM - code H51 and H69 as well as a momentary "error" message
Module 9 ABS / ASR - code H62 , H65 , H72 and the electric fan kicked on during this display.

Lastly after removing jumper and starting the engine , my speedo said I was going 249 MPH in the driveway.
Everything is pointing to a failed ECM but I really hate doing diagnostics by swapping parts.
Comments and suggestions will be appreciated.

 

You need a factory service manual if you don't have one already. Buy a scanner if you don't have one also. I'm not familiar with OBD 2 but I would check the FSM for what the codes mean and trouble shoot them per the manual. I know that's not a lot of information but it's what I have done and I would do. Anything I had questions about I would ask in this forum.

HTH,

Joe K

 

Module 1 H41: ECM Serial Data Failure (Bad ECM/Wiring)
Module 1 C41: ECM Serial Data Failure at Test Time (Bad ECM/Wiring)
Module 4 H51: EEPROM Programming Error (EEPROM loose?)
Module 4 H69: A/C Clutch Circuit
Module 9 H62: Tach Pulse Fault (Tach filter missing/damaged? Wiring problem?)
Module 9 H65: Adjuster Assembly Fault (ASR motor wiring disconnected??)
Module 9 H72: Serial Data Fail (this is normal, ignore)

Looks like either your ECM is bad or the MEMCAL is loose. Open the panel on the underside and see if the MEMCAL is loose. Also the pre-94 Corvette ECMs are notorious for cold solder failures, so your marine experience is sadly irrelevant in this case. If the MEMCAL being loose isn't your issue, send the ECM to SIA Electronics to have it repaired. Best of luck.

OBD2 started in 1996. OP has a '93.

 

Never noticed those codes in the short time in the early 90 I went back into auto repair. Was always jump the connectors and count the flashes. Thanks.

Joe K.

 

They were introduced when the Corvette got a CCM in 1990. The CCM, when placed into self-diagnostic mode, acts kind of like a Tech 1 and is able to both display DTCs on the digital dash as well as control certain inputs and outputs.

 

Nomake Wan Thanks, Unplugged and reinstalled the "chip" from inside the ECM (clean and tight) is that what you mean by MEMCAL? Also , all those codes came up after battery had been disconnected (something ran it dead overnight) and replaced within a couple minutes of running in the driveway.

 

I second ECU as a suspect, but could be bad ground or batt/alternator. I’d do some troubleshooting first.

also second SIA...had them reflow my 93 ECU last fall. Similar issues (but no engine misses etc). Once engine hot: Service ASR, SYS on center LCD, security light would blink. Rando/frozen Speedo readings etc...

tried cleaning grounds no improvement.

Car finally wouldn’t restart hot. Literally popped hood, hit ECU and then it fired up which confirmed it for me. Been perfect since the reflow.

 

Yes, the "chip" is the MEMCAL for a '93. Earlier 80s C4s had an actual chip, and the 94 and up use an EEPROM that is soldered on rather than a removable chip.

Clearing the codes may fix some of them (such as the Module 9 codes), but it won't do diddly about the Module 1 C41. That code means that at the instant you performed the test (that is, the moment you turned the key to 'on' to read the codes out) there was no valid serial data from the ECM. That points to a problem with with the wiring from the ECM, the MEMCAL inside the ECM, or the ECM itself.

Best of luck, let us know how it goes.

 

You sound like you have some tech experience. How often have you heard of harness issues on 92/93 product? The power leads to the fuse block near the firewall /under the clutch master have been solder/ shrink wrap spliced. there was slight corrosion on the spades of the fuses in that holder but I cleaned / lubed / reinstalled.
I 'm sorry if I sound like "that guy".

 

..... As a certified Marine Tech you know the importance of bonding ... have you checked , cleaned any ground points ? It is my experience with computer controlled vehicles that when you suddenly experience multiple fault codes the cause is almost always due to conductivity issues . After cleaning up the grounds , I generally unplug , inspect , and replug every electrical connector that I can get my hands on . The friction between the contacts in those connectors is usually enough to "clean" them up .....

 

I'm not sure who "that guy" is, so you're safe. I haven't personally heard of such issues being common, but that doesn't necessarily mean they aren't. I only personally own a 94 and a 95. The one thing that is common (by the sheer number of threads on the topic I've seen since joining) is the 93 and earlier ECMs have age-related solder failures which can be corrected by having the ECM reflowed. So if merely reseating the MEMCAL isn't your solution, that'd be the next thing I would look at.

Previous Owner Syndrome, however, is pretty common on these cars. There are all sorts of nonsensical modifications done by people who aren't entirely qualified to make such modifications. But if the car's been driving for a while with no issues and then suddenly this problem crops up, I'm not sure the harness would be an issue (unless, as stated earlier, the other codes don't go away when cleared; since there are codes for open circuit connections to components in the engine bay).

 

Thanks for the advice. sent off the ECM this morning. Will post results when reinstalled.

 

So yes it was the computer. SIA was surprisingly fast and professional. They "re-flowed" the circuit board and now we're back on the road! Thanks for all the advice.

 

That’s great! I had a similar positive experience, glad it’s sorted!

 

Sweet, glad to hear it! Enjoy!