I have been reading posts, using the search engine, jumping and checking other forums, pretty much all in vain for solid answers. On my 91 (L98,auto,134,000), about 3 weeks ago I noticed the LED on the stock Bose radio did not show the time or station. If you look close, the numbers are there, just not lit up.
About a week later, the SYS started flashing on the fuel mileage monitor. I did pull the CCM codes using the C4Guru instructions and got a C41; H41; ---. The challenge with the Guru page is those instructions are for a 92 and newer and not all info relates back to the L98.
Here's what I ended up trying: Pulled the ECM and checked all connections. Checked the battery charge, pulled the battery to check the ground strap and fusible links, cleaned the terminals, added another ground strap from engine to frame. Added a ground wire to CCM case (behind the radio controller) and to the radio controller. The battery was disconnected and trickle charging for over a week while I gutted the dash. The only potential problem related to the CCM I was able to find was a slightly corroded ground connection on the left wall of the drivers well, just below the hood release.
Since the "ECM/CCM communication is lost", there is no data available from the ECM.
Before I start throwing money at the problem, does anybody out there have a way to narrow down the possibilities (low battery, new radio, blown fuse, etc) and confirm it is / is not the CCM or ECM.
TIA
Subman

 

yea, I haven't had a chance to research the 1990-1991 specific instructions/codes. Sorry...

However, if I was you, I would get a Helm Factory Service Manual, www.helminc.com, ebay or here, and let its diagnostics do their thing.

 

Kind of an update - drove the 91 to work and sitting stops it idles like it's running on 6 cylinders. Mash the pedal, it goes like a scalded dog. Two weeks ago (right at the start of the ECM/CCM challenge) I replaced the fuel pressure reg diaphram and put back the stock FPR, pulled dry compression test on all 8 (195 low - 210 - high), put in new plugs, dist cap, rotor and wires. I checked and dbl checked the firing order, re-set the TPS, checked timing, etc. So all that said, if the ECM is good the engine managment should still function, shouldn't it? The ECM is bad, runs bad. If the CCM is bad, parts of the body / enviro system won't work properly, correct? If the above is true, then the ECM is the culprit and is easier to replace.
Bogus - do not apologize for the awesome work you have already done! If the rest of us put that kind of energy into providing info for the public, well - - it would be something.

Thanks
Subman

 

1990 six speed / 39,000 miles

I have the exact same problem ... CCM code 41 (lost comunication to the ECM) ... car runs fine but would like to find the problem. Per the factory manual I also have the flashing HVAC light associated with the CCM code 41 (light flashes for about 3 minutes after start up and then stops). I also have another interesting problem ... the horn only works when the battery is very low and does not work when the battery is fully charged (replaced the relay and still the same problem) ... don't know if that is relatred to the comunication problem or not.

I put in a new ECM (one that I have that I know is good) and still no comunication / same CCM code. I did as you did also and cleaned all of the ground wires ... still no fix.

I now am in the process of taking the dash apart and getting to the CCM so that I can see if there are any issues there (great fun ... ).

I am starting to think that perhaps the previous owner did one of two things either the chip I have in the ECM is the wrong chip and can not communicate with the CCM OR the CCM was replaced with the wrong CCM ...

I will keep working at it and let you know if I every get this fixed ...