I've been looking at various threads related to this issue, but haven't found what I need yet.

I have a 1991 Vette, L98 automatic.

Digital dash displays "SYS" every few seconds, speedometer displays "0" at any speed, and no codes in ECM.

CCM displays C41 - comm error to ECM and H41 - error in history.

I can disconnect my battery for say 5 mins, reconnect and SYS goes away and speedometer works again. Car runs great and is driveable.

After approx. 5-10 mins car will stall if at idle, and SYS displays again.

Engine cooling fans will also run at ignition on when SYS error comes up.

Should I be looking at my ECM as the culprit? The E-Prom?

Codes cannot be retrieved from ECM as long as SYS error is present, but after batter disconnect/reconnect I can communicate with ECM thru diagnostic port under dash.

Thoughts anyone? This is my first Vette so not much experience with this kind of issue. I've driven Chevys all my adult life, but never run into anything like this.

Thanks.

 

Have you pulled any ECM codes when the SYS light is not on?

 

Yes, no codes stored in ECM

 

any luck , my vette is doing the same ....

 

No luck yet. Hoping someone on here has the answers I need, or can at least point me in the right direction.

The only thing I know to do is start changing out parts and hope the problem goes away.

 

From the symptoms it points to the ECM. The fans coming on with the ignition on indicates a failure in the ECM. No comm with the CCM and you can't pull codes from the ECM by shorting A to B also points to the ECM.

The ECM processes the Vehicle Speed Sensor signal and converts it to 4K pulses per mile. The CCM uses that information to display the speed in the LCD. Since zero is displayed in the speedometer that also points to the ECM.

Not likely the Eprom itself is bad since it's read only after
its been programmed.

Ground for the ECM is above the oil filter and bolted to the engine block. One of the negative battery cables also goes there. The other negative battery cable bolts to the frame.

Power for the ECM comes direct from the positive battery cable thru a fusible link to the ECM Green connector pin 1 (Orange wire) and also to the ECM Brown connector pin 17 (Orange wire). The same Orange wire also goes to the oil pressure switch.

The switched power for the ECM comes from the Battery
jumper block via a cable from the positive battery cable.

Water from the cowl is dumped onto the battery jumper block which is located behind and below the battery. Clean all of the fusible link terminal lugs. (Disconnect the battery if you do this).

Reseat the four ECM connectors, the MemCal and if the problem still occurs, replace the ECM.

If you do replace the ECM you need to reuse your
MemCal. To remove it from the ECM, turn the
ECM upside down and remove the small cover plate.

This will expose the MemCal. (Small circuit board
containing the Eprom). Press down on the two ejector tabs. The Memcal should pop out of its socket. Install it in the replacement ECM. The connector is keyed.

 

Thanks. That's more information than I've been able to find. I am familiar with the e-prom removal procedure as I've replaced a couple of ECMs on other GM vehicles. I will verify the connections you've suggested and if they are all intact and clean, then I will proceed with the ECM replacement.
Thanks again.

 

Ghopper141.....TONIGHT I DROVE CAR IN THE RAIN.....LOOK WHAT HAPPENED, IT FIXED IT SELF LOL.......

HOOKED ON VETTS MENTIONED A LOT OF GOOD STUFF, AND WHAT I SEE IS WHERE I AM GOING TO LOOK AT NEXT.....

THIS IS WHAT HAPPENED TONIGHT IN THE RAIN.......HERE WE GO !!!!!!!!!!!

>>>>>>Ok drove car home tonight,,,, after watching it carefully, I noticed a week ago while driving it she began to miss and the speedometer read 220 km on the digital display.....then shortly thereafter, it shut down.....I pull to side of road, I tried to start up again it would not.......I disconnected battery,, reconnected it, and it started...therefore by disconnecting battery something was reset......TONIGHT I took car out in the RAIN.....after about 15 minutes of driving everything started to work......SO something got wet and completed a ground ?????? before jumping A and G i had codes in module 1, module 4 err, module 9 codes as I mentioned in my past post.....tonight I have codes in all three modules, module 1 had h41, h54, h72......module 4 --- was clean and I was able to get into the ECM HURRAY LOL, module 9 had code h72,,,,,, but still the service ASR light was on.......when I got home I checked the codes again but this time using the dash function I cleared codes in each module, and the SERVICE ASR light went off....and now the car in NO LONGER in LIMP MODE, and I also have full power and the AC is working again......SO MY QUESTION IS "WHAT THE HECK DID THE RAIN DO TO COMPLETE WHAT CIRCUIT" to me it seems, as crazy as this will sound, when car was dry it was missing some connection, and all the electrical issues developed, the car got wet and the rain bridged something....... with electrical its the opposite lol......so let me calm down and ask the question.....if the RAIN fixed the car, what can I look for when its dry.....WOW, what a question...If anyone of my customers told me what I wrote....I am sure I would think this person was crazy.......I now have see it all......THINKING out loud, again driving in the rain completed a GROUND ??? connection, something and all the functions mentioned cleared up.......HELP>>>>> your friend JOE..... ps am I crazy lol.....KK

ONE LAST NOTE.... before everything started to work again, the chimes started for about 20 seconds, after driving in the rain for 10 minutes, thereafter everything started to come back and work after the chimes went off...????

 

Connection problems , both ground and live are common cause of the ECM - CCM Comms drop out.
Smacking the ECM has worked for some , cooling it down has worked for others
Identifying the root of the problem is invariably frustrating which is why you will find most Code 41 problem posts prefaced with " pulling my hair out " or similar
For some replacement of the ECM and CCM has made no change

Post #25 in this thread is interesting
http://forums.corvetteforum.com/c4-s...cm-or-ccm.html
http://forums.corvetteforum.com/c4-t...wes-is-me.html

You may get some help here
http://forums.corvetteforum.com/1554492041-post15.html
http://www.digitalcorvettes.com/foru...5&postcount=17