C68 HVAC Programmer Compatibility for 1990

[Axial]

Greetings, all!

My car has been exhibiting the symptoms of a failed C68 module:

• 100% Unresponsive head unit
• Triple dashes in the temperature display
• Blower on full blast, hot, through the defogger vent

I have just removed the module from the car. While doing so, I noticed the somebody had been in here previously because the vacuum lines had been taped together and the washer+unidirectional keeper that holds the vacuum line block into the unit were missing.

Hmm, ok.

After removing the module, I was surprised to see the following part number on its sticker:

16145131 LT5

I thought this part number was for 1994-1995 ZR-1s and that the 1990-1991 cars required Part No. 16160831 LT5 and that the two were not interchangeable. Am I mistaken? The HVAC had been working fine for three years before suddenly failing one day.

I have the Batee rebuild kit and will be rebuilding the unit, but now I'm wondering if it didn't fail because it's the wrong part.

Thoughts?

[S.hafsmo]

They are indeed different, but you'd might be able to use a later programmer on a 90 with some modifications. If I recall correctly, the early cars have a different AC request signal, which can be rectified with a relay.

[Axial]

Originally Posted by S.hafsmo

They are indeed different, but you'd might be able to use a later programmer on a 90 with some modifications. If I recall correctly, the early cars have a different AC request signal, which can be rectified with a relay.

Interesting, thanks for the info. Since this programmer was working when I got the car, I suppose somebody else already did the work to bridge the compatibility gap. Any pointers on where I might look for said relay? I'd hate to rebuild the box, only to find out it's the doodad intercepting and corner-turning the signals that is responsible for breaking my HVAC.

[S.hafsmo]

You could ask Batee. I got the info from them a couple years ago.

[Corvette95]

Batee is your best bet. I understand that the 1990 ZR-1 programmer is unique and has TWO part numbers due to an issue with some original programmers would not reset correctly after battery disconnect. I know there is a TSB on that somewhere. So if you get a new module get the updated part for 1990 (which are extremely hard to find).

[Axial]

Update:

Exchanged a few emails with Batee, who was super helpful (thank you!).

So far, it looks like the big diode smack in the middle of my C68 programmer module's main board is toast; you can see the the charred bits through the conformal coating. In-process of yanking it off to do a proper test on it (had to go acquire some soldering braid) but it's not a diode that's included in the rebuild kit as it's not one of the ones that typically goes poof. Batee says any of the kit diodes ought to work in its stead, but I'm hoping it has a number on it so I can track down a form-fit-function replacement.

Batee did say he has never seen the part number I mentioned my unit shows in the OP for the ZR-1 before, but it is definitely there and printed on the sticker. Given how it was missing its single-use retainer on the vacuum block, missing its cardboard backing, and held to the firewall with entirely the wrong bolt, I'm going to go out on a limb and say this unit is a rebuilt mutt featuring the outer shell from a later car with the innards of the appropriate vintage (it is an "old-style" board, as it should be). The cork sound dampers on the vacuum solenoids are also in fantastic shape, which I wouldn't expect on an almost 32-year-old example with 76k miles.

[Axial]

Update 2:

The diode turned to powder during extraction, making it impossible to identify via testing or inspection. Oof. Swapped it for one of the larger diodes in the kit, replaced the transistors, and skipped on the caps since the ones on there were not giving off any bad vibes. After installing it back into the car, it worked fine for about 20 minutes and then it began steadily degrading until it went back to the triple dashes.

Since I would've needed a new kit anyway, I sent the unit to Batee for a rebuild; he replaced the caps and the transistors, tested it, and signed off on it. I put it in my car and I get weird failure modes. At first it was doing weird mode changes all by itself, sometimes giving me a blinking light and sometimes not. I thought there might be a vacuum leak triggering states, so I took a dive under the plenum last week and fixed a bunch of loose connections. Now, after having done that, it's still failing but it's more consistent; it will give me a grace period when I start the car to select a mode, temperature, and fan speed and then it will stop responding to commands. I can still turn on the rear defroster and I can enter diagnostic mode from this state, but I can't change the HVAC settings until I turn the car off and then turn it back on.

Anybody encounter this before, where the HVAC effectively "freezes" its settings while driving the car and you can't alter then without shutting off ignition and restarting?

[GOLDCYLON]

Originally Posted by Axial

Update 2:
Anybody encounter this before, where the HVAC effectively "freezes" its settings while driving the car and you can't alter then without shutting off ignition and restarting?

After 20 years of driving C4s two of them Zs short answer No. I would try another unit.

[Corvette95]

Sounds like ( and truest no offense) but a shotgun approach to this. Maybe former owner, maybe a little bit of current owner, but a FSM and patient will go a long way. Now, sounds like you are past repairing current unit, you say C68 but do you really mean problems with C68 control head or the programmer? I have quite a bit of experience with both. Corvette did change and update the programmer in 90. It was due to the controller losing its position or calibration when battery power was removed or the unit was removed and reinstalled and the programmer could not find 75 degrees setting (middle temp). PSA ALWAYS set your temp switch to 75 when parking your car for any time, I do it every time , to prevent the rubber gasket surrounding the vent shaft not to sit and get stuck in either hot or cold, setting it to 75 cures that issue . Also, it is the default setting when battery power is removed from car and reconnected. The old programmer if not shot, you can remove all battery power from car. Remove the resistor panel on engine side of heater box, pull out the resistor housing and GENTLY move the blend door to mid way. ALSO spray liberty the doors runner flap with standard WD-40 (recommended maintenance-this problem or not) after reassembly (you can leave the resistor cover off outside blower housing for a visual check) cut a/c reconnect everything . Controller will move back and forth, sometimes for a long time (this is what the new design fixes) and sooner or later (15 minutes) your blend door hopefully self aligns, if not repeat this procedure. Once (and unfortunately if) it calibrates itself you are good to go but it will happen again if these directions are not followed. It?s a 90 only thing. I know this works most of the time but can be time consuming. It?s buried in the 90 FSM supplement but was shown amd proven to my by Gordon Killebrew. Moral of the story in absence of other issues , give the programmer as much time as it needs to find its zero (75 degrees center). Good luck hope this helps.

[Axial]

Update: the issue was a multi-part failure, none of which was a blend door or a vacuum line.

The HVAC Programmer unit was faulty; I caved and replaced with another unit that solved the electronic controls locking up but did not solve the frequent, random mode switching.

I discovered that the compressor had a cracked shaft seal; I replaced the compressor, converted to R134a, and the mode switching was dramatically reduced.

Replaced all of the coil packs and plug wires due to the car running extremely poorly and smelling of gasoline; found one coil with a nice crack going up the side. Car now runs strongly and the HVAC behaves as it should, with the exception of a new refrigerant leak that I need to trace. I will laugh if it's the R134 adapters.

To any who provided some pointers, thanks! Even if it wasn't identifying the issue, springboarding helps think through the problem. My guess is there were some pretty wild voltage or current spikes due to the bad coils and seizing compressor causing giving bad signals to the HVAC electronics, but that's all speculation.