I've got a '97 and the HVAC won't work. There arent any trouble codes. The head unit seems fine, when you press the different buttons the display changes correctly but the ventilation doesn't change. Generally it blows about medium speed with ~50% or more of the air from the dash vents. Sometimes it will stop, sometimes it will blow full-blast AC for several seconds.

I took out the head uinit and the system kept running about medium speed, ~50% out the dash vents like it usually does. The manual says to do the 'powertrain ODB check', is that just reading codes from the DIC?

Thanks for any help on this!!

 

I can only share waht happened with my '99 FRC. The battery leaked for a LONG time. Pulled battery, battery tray, and saw the extensive amounts of battery acid damage below the tray. The HVAC vacumm lines were literally disintegrated. Previous owner ignored it. It's a known problem that the leaking battery acid drips down into the compartment below which conatins major wiring harnesses, the PCM nad yes, your HVAC 1/8" vacuum lines. Basically the Mass Air Plenum (MAP) sensor, located at the back of the intake manifold is the vacuum source. A 1/8" black plastic vacuum line is 'buried' in the wiring harness and runs to a three input/output check valve attached to a vacuum storage tank located under the passenger fender by the door. This is your supply line from the engine. The return line goes back up in the battery area from the check valve/vacuum tank area and through the firewall to the HVAC control head. This line is also buried in a wiring harness. That's your HVAC vacuum circuit: MAP Sensor to check vale on vacuum storage tank back to the HVAC control head in the consol. The lines get eaten away by the leaking battery acid thus losing the vacuum source for the HVAC contol head to do its thing. NO CODES!

Here's how I fixed it on my '99:

1) Pulled the battery
2) Removed the battery tray
3) Unwrapped the wiring harnesses
4) Used CRC Battery Cleaner Spray to nuetralize the acid. You can get this at most auto parts store or use baking soda and water to nuetralize the acid. It's a bit more messy though than the spray.
5) Washed the area completely (Will have to go back and paint/repair the damage)
6) Jack up the car enough to get the passenger front wheel off and remove the Acess panel (several 9/32" bolts). Use the CRC stuff to clean any areas you missed from above. Asses all and any damage. Check your PCM for caes damage fro acid.
7) Getting to the MAP sensor on the back of the intake manifold is hard so I decided to splice this line if I could find enough undamged line
8) I searched the wiring harness and found the remainder (and destroyed) vacuum source line, cut it back a bit to ensure a solid piece with no cracks, holes, or damage.
9) Located the control head vacuum line (also 1/8" black plastic)as it entered the firewall near the evaporator and was able to cut it back to a clean solid piece.
10) Used 1/8" Black plastic emission hose from Auto Zone and 1/8" splices
11) Installed a new check valve at the vacuum tank in the fender
12) connected the Source vacuum line from the MAP to the BOTTOM of the check valve and the HVAC control vacuum lne to the TOP of the checkvalve and secured the two runs. Test it before you wrap it up.13) Rewrapped the wiring harnesses
14) Bought an Optima Red Top and reinstalled everthing I removed.
14) All's well. Not a hard job. Harderst part is finding any usable vacuum line after the damage
15) If you're source line from the MAP sensor is totally gone you'll have to run a new one which requires sliding the intake manifold forward. Search the forum on how to do this. Same thing with the HVAC control head, since you've already got it out, I'd just run new vacuum line rather than splice it like I did.

 

WOW... Thank you so much! I am a little confused though. My 'head unit' which has the ***** and sits under the radio in the car has no vacuum line attached to it, only a single connector with about a dozen or more wires. Looking at my shop manual it looks like some other piece has all the vacuum lines and works the actuator doors.

Is there supposed to be a vacuum line to the head unit? Where is the 'other' controller that does the vacuum routing and how can I get to it?

Thank you again, I REALLY appreciate the help!

 

You're probably correct. There's an electronic part and a vacuum didtribution part. I didn't pull my HVAC control head or the console, I just found the vacuum supply line going through the firewall inside the battery compartment. So I don't know what goes on behind the console. If you have the console plate off, the vacuum distribution box is handy and you can get to the vacuum supply line, I'd run a new line to the check valve through the firewall grommet. Otherewise, splice the line inside the battery compartment, It'll hold vacuum.

Just a side note, if it is indeed battery acid damage, get a Optima Red Top (Glass Mat battery, sealed, no electrolyte, no leakage). I bought an Exide regular battery to replace the leaking one that came with the car after I first fixed it. That leaked too and ruined my original fix so I had to replace the vacuum lines again. This time I bought the Optima (after cleaning everything all over again) and rewrapped the wiring harnesses and new vacuum lines with electrical tape and enclosed them with corregated electrical plastic tubing available at Pep Boys or any other Auto Parts store. Hopefully that'll be the last time.

 

The vaccum is used to control/move the HVAC doors and does not go to the head unit but to a router that gets the vaccum from a small reserve tank that is mounted on A pillar on passenger side.
The tank is what has the small vaccum hose that comes from the back end of intake manifold where the MAP sensor and vaccum hose for brakes is.