Cliff Harris

The fire in my '86 melted the vacuum lines to/from the HVAC programmer board so I didn't know which was which. I started looking for information on the C68 option and along the way I found this thread (see post #7):

http://forums.corvetteforum.com/c4-t...ble-codes.html

Which apparently comes from this thread:

http://forums.corvetteforum.com/c4-t...c-problem.html

Neither of these methods work on my car. I looked in the shop manual (page 1B1-6) and it says to press and hold the REAR DEFROST button, press and release the manual up button, release the REAR DEFROST button. I thought the "manual up" button was the fan button with the "^" symbol (which is what the original poster thought in the second thread), but it turns out that the WARM button is the correct one.

My display never shows "-00", as mentioned in the thread. It starts out at "-01" and goes up through "-09". My software version is 80.

So I'm curious to know what combination of buttons work for the various years and what software versions they correspond to. Please post with what ACTUALLY WORKS on your car vs. something you read/heard somewhere. There is way too much bad information out there (more on that later).

Steve59

I'm interested also. But I don't think your above posted procedures work on an 86. They don't seem to work on my 87. It may have begun in 89. Others will chime in.

Deepa

for my 91 c68, i push and hold the fan up and fan down buttons unitl -00 is shown

Cliff Harris

I just found out that the C68 controller is different if you don't have the C49 option (electric rear window defroster). I would think this would apply to convertibles (mine's a coupe). I have seen references to different methods to get into diagnostic mode for coupe vs. convertibles, and that apparently is because the non-C49 cars don't have a REAR DEFROST button.

I was looking at the list of system faults in the shop manual. The list looks a little strange to me:

00 - None
01 - Feedback pot open
02 - Outside temp sensor open (it says "ambient")
04 - Outside temp sensor shorted
08 - Inside temp sensor open
16 - Inside temp sensor shorted

The thing I spotted right away is that this appears to be a binary sequence. The cool thing about a binary "bit mapped" list like this is that the numbers just add and you can get multiple faults with only one number. So 03 would be 01 & 02.

Since I'm in the middle of messing with the HVAC programmer and it just happened to be disconnected, I went into the diagnostics and checked the fault value, which was 01. This is valid because the temperature blend door feedback pot gets its supply voltage from the HVAC programmer (I just finished drawing the schematic for this, which I will post in another thread). In the interest of science, I accessed the inside temperature sensor and disconnected it. Then I got a fault number 09 (01 & 08). Then I shorted the connector pins and it read 17 (01 & 16).