Dreaded 96 stuck on heat and defrost.
Decided to tackle this today. All was going well had the programmer ready to go back in after about 1.5 hours.
I decided to forgo using the manifold and just hook vacuum lines directly to each other.

Get it back together go to check it, I secretly know it wont work because that's just how it goes with this POS, the fan is on full speed and there are no green lights on the buttons. The system won't respond. Yes, I have cleaned buttons and they were all working before hand. Now none are lit up and I have this weird thing on the LCD. I held down the fan buttons and it responded and went into diagnostic mode and it pulled no code.

I did try unplugging and plugging it back in. Same result. I tried disconnecting my battery for 30 seconds and reconnecting it. Same thing.
What is this? What do I have to do now??







This is all it will do and it comes on full fan speed at Key On. Still blowing out the defrost and floor.

 

Your programmer's capacitors are shot. Super common issue, I had the same thing happen on my dad's '94 and the same thing will happen on my '95 eventually.

Remove the programmer again, bust out your solder tools of choice, and either grab the rebuild kit from BATEE or order the following parts from Mouser or Digikey:

UPV1H390MGD1TD x2
ULD1H220MDD x1
UHE1H101MPD x1

Thread about the repair here: https://www.corvetteforum.com/forums...-repaired.html

Enjoy!

 

The system defaults to heater and defrost if anything is wrong or not working correctly.

You also don't have all the circuitry hooked up if you bypassed the unit. So how can you tell what it wrong ?

Before you do anything else you need to fix the issue, which for '96's are those little rubber **** that close up and cause it to not work right. So either buy a manifold from a '95 (or before) or insert brass tubes to cure the issue, put it back together and then see if you still have a problem.

Then, there is a video out on youtube on how to take the controller apart and clean the contacts behind the buttons.

You need to do this first.

 

1. He removed the manifold and connected the vacuum lines directly to each of their respective vacuum connections. You can debate whether this is a good or bad idea in terms of vibration resistance, but it's not 'wrong'. He bypassed the '****' you're referring to. So that's not it.
2. He didn't bypass the unit and did have all the circuitry hooked up.
3. He stated plainly that he already cleaned the contacts in the head unit.
4. The dashes on the head unit are a clear indication that the HVAC programmer has had its capacitors fail, as most will by now.

 

Not everything is the capacitors.

I have a '96. And yes I am fighting the same issues. Do you have a '96 ? I have read you don't.

Now I can agree maybe I read it wrong and interpreted it as he simply hooked up vacuum lines bypassing the box. If so, that part I was wrong.

However, I have had my unit out of the dash and cleaned the connectors when I put a radio in. Sometimes it wouldn't try to come on, initiate the system. Of course it can't change the blend doors, the little rubber **** cause that issue, which we are going to fix when we put the new analog dash in.

Yes I understand about the capacitors, I was building computers for people back when the problem happened. It was caused by industrial espionage in China and cost the major PC companies millions replacing machines. I had systems that I built for people that lasted through the board's warranties and then went **** up so to speak.

It is pretty easy to spot the bad capacitors, they look like they are erupting. Absent this, it costs nothing to pull the head unit out, clean the contacts and see if it will initialize.

There were times my less than (at the time 20,000 mile car) wouldn't "light up the green lights". It was just time and oxidation on the contacts.

Here is how to clean the contacts, which can be the simple fix.


Here is the batee video

 

My point is it is much less expensive and risky to put the system back together the way it should be, all hooked up, and clean the contacts before getting into trying to solder up a system or send it away and spend money when it could simply be time and the cost of a pencil erasor, a q-tip and some alcohol (maybe).

After taking the head unit apart, ours does what it is supposed to do, except switch blend doors, it turns on, it turns on the fan, controls the fan speed, shifts between heat and A/C. changes temperature correctly.

There were times it wouldn't even turn on until we cleaned the contacts, or it would work half the time.

 

Not everything is the capacitors, but the three dashes on the head unit is.

No I don't have a '96, but I have a '94 and a '95 which use the exact same C68 components as the '96. The C68 components didn't change for that one year.

 

Hello,

When I did this "fix" I had already done the dash unit and cleaned the contacts. I had good green lights across the entire unit. Everything in the head unit was responding as it should. Before the contact cleaning it was not. Afterward it worked flawlessly. Very simple fix. But still stuck on heat and defrost.

So I knew the issue was the programmer under the dash behind the pedal. Corvette yoga was going to be the exercise of the day. Upon hook up the dashes at the head unit appeared. I think my main mistake was not disconnecting the battery from the car when I did this. I could be wrong. It is always a good idea to disconnect the battery when doing anything electrical on a car especially an older one. I didn't think it would be of concern because I figured with the key off the AC system was void of power. But with the key out of the ignition when I plugged it in I heard it power up in the box.

I called Batee.com today and he said as has been said here this is a common problem in these cars. Something gets loose on the board or a resistor goes out. He said he can go through the dash unit and the programmer and repair them at a very reasonable price. I will send them to him next week and hopefully I can get this resolved. He said going back with the rubber manifold or bypass with direct vacuum lines works either way with no issues. I went the bypass route to remove any possible issue with that funky rubber thing. Mine was totally clogged.

I am also going to get a new gauge lens from him for my main gauge cluster. Mine is clouded. This will be a nice repair for sure.