prescott

Turned my AC on for the first time today.

Good news is I have cold air!

When I slide the position slider to the far left, the system turns off as it should.

Bad news. All air flow positions between the left position and defrost are blowing cold, but through the defrost position. It wasn't doing this a few days ago when I cycled through the positions for defrost when we had a colder day.

Is this an electrical fault, or is this a vacuum fault?

Is there a write up on this? I'm learning this car....one subsystem at a time.

Thanks!
Brian

Alan

Vacuum defaults (no vacuum) are defroster vent open, footwell vent closed, water valve on.

In Phoenix in the summertime it would be too warm and not enough flow to the face...

Possible you have a vacuum source failure.. or a bad leak ? I'd check there first. It could also be electrical - but I'd say less likely.

Alan

SeanR

Brian, if you are able to test your HVAC vacuum system your self, follow Wally's tips at www.928gt.com. Find out if it's working properly. Your AC should be able to blow your hair back with cold air from the center vents when your temp slider is all the way to the left. If you are getting defrost air while in the "square" or "Down" position, you have a leak.

OKB

vacuum starts at the master cylinder, the T rubber distributor with a loop tubing to a grey or blue plastic cylinder (check valve) then black tube goes all the way to the dash vacuum strip.
Pull the loop tube and feel if the vac is starting at the master cyl, then put the loop on and see if the vac is starting after the check valve. If its bad, there wont be any vac there, so they are commonly bad, after that you will have to go in the dash strip and see if the black is attached and if there is vac at its end

prescott

OK thanks guys. The AC was working great last fall...something failed.

Where can a guy get the mighty vac tester? Is it something Napa or Autozone has? Roger?

prescott

Is this it?

http://www.harborfreight.com/mityvac...ump-39522.html

Randy V

Do the easy thing first -

Inspect the one-way valve off the brake booster for failure.

Air should be able to flow only one way through the valve.

Pretty sure it's supposed to flow AWAY from the booster.

Alan?

OKB

you can feel it with your finger, it should be a nice little SUCK

dr bob

I use the equivalent of their 69328 or 60770. Cheaper, all metal, and you get the fittings for brake bleeding included.

Alan

The check valve affects fluctuations in vacuum - it isn't likely to be a cause of no flap action here (usually causes WOT vacuum dropouts).

You can just test if a vacuum will hold on the line that feeds the HVAC solenoid valve manifold (enters firewall below wiper motor, from brake booster T) - There should be no leakage through the HVAC vacuum supply with all power off. You can then try while running - working through all flap positions. As the HVAC actuators are selected the vacuum may dip, but if you can pump it back up and see it hold - you are still OK.

The Harbor Freight vacuum testers are not really as good as a mighty vac - (won't pump down as low) but they do the job for 95% of all vacuum tasks, and the price is right.

Alan