View Full Version : AC changes from front vents to defrost?

06-08-2005, 04:13 PM
Here is a strange one that I could not find any threads about. My 93 5-speed runs fine and the AC works fine UNTIL I get up to a decent speed, or am pulling fairly high boost, but normally only at highway speeds. Running the MAX AC usually blows cold air from the from vents. At speed (under load) the air changes to the defroster vents and it sounds like the internal damper that recirculates air in the cabin also opens up. It is still blowing cold from the defroster vents but because the air is coming off the front windsheild, the air feels warm when it hits you in the face. Being in Arizona, the windshield gets freakin HOT!
What is up with that? The other not so critical problem that may or may not be related is that my boost gage does not move smoothly through it entire range. It tends to "jump" a little bit between -15 and -5 psi even though the engine is running fine.

06-08-2005, 04:55 PM
Sounds like a vacuum leak/low vacuum condition. The ac/htr uses vacuum to operate the diverter doors for the various modes. When you transition from vac to boost sounds like it is being felt by the ac sys and the door is slammin g shut. Make sure the vac lines for heater and ac controls are connected properly and that any check valves in the vac sys are still good. If the check valves are bad the vac side of the sys is not being isolated during transition to boost. MO mikeH

06-08-2005, 08:37 PM
I'm almost afraid to ask where the check valve(s) are located. I know there is one near the drivers side firewall connected to a line. Is there any connections under the dash or behind the instrument panel I need to be concerned with? The funky reading on the boost gage may indicate a related cause of the problem or is it a separate problem? It does not seem to be just under boost, it must also be at highway speeds? It is a weird problem.

06-08-2005, 09:23 PM
I had this same problem with my 93 a few years back. Check for a red colored hard vacuum line that runs to the tan colored vacuum tree that mounts under the wiper cowl. I believe you'll find it broken. Repair it using a soft vacuum tubing with an ID the same as the hard line OD. Seal both ends with a dab of RTV sealent.

06-08-2005, 11:23 PM
Good shot Cobra85! It was NOT the hard plastic line but you did point me to the problem. There is a small valve on the bottom of the vacuum tree under the plastic cowl. The valve has two 1/8" connection ports, a white tubing (goes inside cabin) and a black one (disappears into passenger side fender). Both are 1/8" tubing with rubber end fittings. They seem to need to be a the correct port. The white tubing goes on the bottom port, I had them reversed.
When I reinstalled the engine, I must have swapped them. :p