My 1991 Corvette (L98) just started losing memory on the A/C unit. Everything holds memory in the accessories position (even overnight), but as soon as I crank the car, the memory is lost, & resets to 75 degrees. All manual controls function correctly, but the A/C fan speed controls must be manually pressed. If I press the Auto Fan button, the fan just quits. I've been trouble shooting this thing for weeks, but have gotten no where fast. Both, the A/C control head & A/C programmer are new. I am showing the Sun Load Sensor value at 199 (which is out of range on an L98 (Not on a ZR1, though). 183 is Max Dark, & 38 is Max Light. My A/C control head is not giving me any trouble codes of "07" or "08", though. I'm not showing any trouble codes from the ECM, either. The UART has 2 serial data lines leading from the CCM to the heater & A/C programmer. I have removed the factory radio, which has a blue wire that connects to the UART or "E" & "C" Bus. "E" & "C" Bus is the Entertainment & Comfort Data Line. It is supposed to be disconnected if there is a Code "10" or "---", but my blue wire is not connected, & I have no trouble codes from the A/C control head at all. Is this control head also going bad, or is there perhaps a problem from the ECM or CCM? I'm not sure if the memory circuit is in the control head, programmer, ECM, CCM, or what. Has anyone else had this problem? Does anyone have any ideas on how to fix it?

Thanks.

 

You might try the Audio Section. There have been other posts about problems once the radio is removed.

 

I removed the radio when I bought the car used, since it didn't work anyway. Any ideas on how to check it without having to buy a new BOSE unit? The rear plastic is melted from where the BOSE amp went bad before I ever got the car. However, all A/C was working until last year. Everything was replaced as new, & now (out of the blue), the memory just quit with no trouble codes. Do you know where I can find some of the other radio problem posts? I've done a search under radio, but just found where some people swapped their radios. The car came with the 1991 Bose Gold with CD system.

 

you really need the Helm Factory Service Manual to help track this problem down.

I am guessing the problem is with the constant power circuit... something is killing it on start up. Hence, it's no longer constant.

 

My L98 was doing something quite the same.

Turns out the starter brushes were wearing out and dragging the battery down during each start. Shortly after, the starter would no longer turn over.

It was a good starter and gave its entire life to starting this Vette for over 20 years. I kept it rather than return it for the core deposit. It now rests in peace on the garage shelf........

 

I think the '92 C68 climate control works like your '91.

The best way to trouble-shoot the problem you are having is with the Vetronix Tech-1A or MasterTech scan-tool using the "body" cartridge (software) or a dealer Mass-Storage cartridge.

With these scan-tools and the appropriate software, you can diagnose problems with various modules in your car, including the CCM, C68 climate contol, and the radio.

However, unfortunately, there is a procedure that has to be followed that is in the shop manual that requires running a jumper wire from the E&C bus to the data link to the CCM before you can access it from the ALDL connector under the dash. And, this jumper has to be removed when done.
In my case, on my '92, I ran the jumper through a switch so I could simply use the switch to enable it.

After the jumper is in place, you can use the Vetronix scan-tool to monitor the commands on the E&C bus to see what commands (in English) the C68 control head sends to the programmer. In addition, you can simulate the control head with the scan-tool and send commands to the programmer to see if it responds.

The Tech-1A is now obsolete and is replaced with the MasterTech.
http://www.vetronix.com/aftermarket/

These scan-tools are usually available at a GM dealer.
They are expensive.

Tom Piper

 

Last night, I remeber in the service manual that the E & C bus is to be by-passed from the radio if there is a short or open noted by a 10 or --- HVAC code. If the HVAC starts, then replace the radio. Therefore, perhaps I don't have to have the radio data link connected, just the HVAC. So, I'm going to try connecting the power, acc, & ground wires from the radio receiver module. Unfortunately, I won't be able to do that until Thursday. At least it won't take long to do.
Also my alternator (new Powermaster) & Optima battery are working great. My aftermarket radio holds memory. Everything works, except the HVAC memory (only when the car gets cranked).
I get a discount on GM parts, so I'll look into the tools. I live next door to an ex-manager of a garage. He probably has the tool. He's done work for me in the past.
I absolutely hate the BOSE system. Every one of the 3 C4 Corvettes I've bought had a BOSE system that did not work! If I can't figure this out, I may just live without the AUTO fan buttons & endure the memory reset. It'll cost a small fortune to put new amps, speakers, control head, & whatever else that BOSE system may need. I'm hoping that activating the radio receiver module will do the trick. I'll keep everyone posted. If you get any more ideas, then let me know.
Thanks Again!

 

I've been sick the past 2 days, so I haven't had a chance to connect the radio receiver module, yet; However, I did receive a code of 01 tonight confirming previous suspicions I had about the heater door not working correctly.

 

The code went away, & the heater door started working again. Weird. The fan still doesn't work in Auto mode, though. I have an aftermarket radio wiring harness on its way, so I'm waiting for it before I rewire the radio to the receiver.

 

Great news. Today I had the chance to work on the car. I took the dash off, removed every control in the front, & even removed the programmer. I reconnected everything. I found that the glove box light connector just doesn't connect very well, so I made a direct connection that is removable to it. That light has never worked, since I bought the car. I discovered the tan vacuum line had a small split in it right at the program module. The radio is still disconnected from the radio receiver module, and I did find an open in the light blue with with stripe wire that connects to the receiver module. That is the data link line if I remember correctly. I fixed the open, & everything works great. Therefore, I can still run my aftermarket radio without all of that BOSE junk!
If anyone else has memory loss problems, please, check that light blue with black stripe wire. The radio receiver module might not be getting a good signal, or could be going bad. If something else starts to happen, I'll start a new post. For now, all is well.

THANKS TO ALL THAT RESPONDED HERE!!!!!!!