94 Corvette heat/ac controls don't work. The blower is on all the time and ca't shut it off. None of the buttoms do anything. The LED readout screen has 3 lines across it. The whole system seems to be dead accept the fan runs. Any ideas ?

 

I'd start with the Display. It needs Power (ignition and Memory) and Ground to light up and show you anything. If it has that much, you probably need a new or rebuilt one.

Display communicates with the Programmer over serial data lines which then sends low voltage signals to a Blower Module which amplifies those voltages to control Blower speed. It also sends voltages to a temp door motor that controls air flow to the heater core and evaporator; directs vacuum to solenoids that open or close doors so that the air blows out of the vents/floor or both and another door that let's in outside air. And the Programmer also sends the compressor request to the PCM which controls clutch engagement through a relay. I'd guess that the Programmer isn't getting any serial data from the display so tackling that first will lead to a diagnosis of why the blower is stuck on high. Your other option is to get ahold a bidirectional scanner which with the right card can emulate the Display. Then using the CCM link, you can send all the Display stuff to the Programmer. If everything works, you only need a Display.

 

I have a 96 HVAC head unit. $150
and a programmer too, $150

 

I don't know about the Dash, but the Programmer is unique to the '96 Vette and is over $500 new.

 

The 1996 programmer is now over $1000. from chevy !!

 

This is a 94, not a 96. Same answers to my problem ?

 

Same answers for the '94 - everything from '90 to '96 works the same. In '90, GM re-engineered HVAC giving the ECM (PCM) control over the compressor. Why the '96 Vetted ended up with specialized processor (the Programmer) eludes me (my guess is that some of the other models - maybe Caddy - switched to dual air zones so it needs a different harness). Prior to that, alot of the parts were interchangeable with everything they built; ie if it worked on Buick, it worked on your Vette. If I had a '96, I'd buy a spare Programmer - even if it was out of a wreck. Seriously doubt they'll continue to manufacture it much longer, though somebody with knowledge might figure out how to take one apart and fix it. For now and from postings around here, the '96 seems to have problems with the vacuum harness connection and there have been a couple of tips posted for fixing that. That doesn't seem to be a frequent problem with the '94 which has the usual penchant for a shorted Blower Module and more commonly, an open in the wiring or connector at the Low Pressure switch (which is exemplified by no compressor engagement even though the charge is good). Your '94 may need a Blower Module, but you won't know until that you get the Dash communicating with the Programmer again. One other thing you might do is check the fuse protecting the circuit that carries the actual blower voltage back to the Programmer. It's 5 amps, and if it opens, you can get high blower at all times. It's usually labled Blower Module or something like that. Check your Owner's Manual for specific labelling and location or Shop Manual schematic.