Vaccuum drops with AC on...?

[Matt Weiser]

I lose about 4lbs. of vacuum according to my gauge when I switch on the air conditioning. And it's only with the AC, not the vents or heater. Is this normal or do i have a leak somewhere? If it's a leak, where's it likely to be?

Thx....!

 

[6=8]

If your engine RPMs are dropping significantly when you turn on the AC, that might account for it.

The other item is all the ducts are vacuum actuated. I recommend you try this to tell what is going on.

1. Duplicate the issue - Start the car, AC off... Note the "Good" vacuum reading
2. Turn AC on to "Norm"
3. Did the Vacuum drop? If yes, move the selector to "Vent" to turn compressor off.
4. Did the Vacuum stay the same or raise back up?

If the vacuum reading drops on AC and not on vent, then the AC compressor may be loading the engine down, reducing the vac. (Alternator too... cooling fan runs on high too) If this is a big change in RPM, than the computer may not be adjusting for this, or the compressor is faulty (Bad bearings or internals)

If the vacuum is the same for Norm and Vent settings regardless of the AC compressor on or off, then you may have a dash vacuum leak that only appears when the dash vents are selected/open.

When I turn my AC on, there is a momentary drop in RPM (and vacuum) until the ECM changes the idle settings to compensate. If I were to guess, the IAC (Idle Air CTRL valve) and TPS may need adjusting. If these are improperly set, the ability to adjust for loads at idle may already be maxed out and can't do it.

Hope this helps!

 

[Matt Weiser]

Thanks for the feedback. I tried your test already and saw the vac drop in the Norm AC setting and not Vent. But it didn't occur to me that this could occur simply from the AC loading the engine and dropping the rpm a bit. I'll try it again and watch the rpms.

And by the way, the compressor is brand new. Also, when I built the car I swapped in a whole TR dash and climate control unit and fixed a bunch of vacuum leaks in the process. But I could have a vacuum leak where the vac hoses attach to the control unit behind the dash; I didn't really pay a lot of attention to that at the time.

 

[Matt Weiser]

I ran into another odd issue yesterday. Maybe related...?

I drove up into the mountains for the day, and when I hit about 4,000' elevation, all of a sudden the AC just quit, started blowing warm. Coming back down a few hours later, all of a sudden the AC kicked back on and started blowing cold again when I passed through about the same elevation point heading downhill. Really weird. After that it cycled cold and warm until I was nearly back in the flatlands.

As far as I could tell, the compressor was working just fine, so I'm guessing it's a problem in the dash controls somewhere.

Anybody seen this problem before? Any idea which component could be altitude sensitive like this? Is there any kind of valve in the dash system that could be suspect? Could a vacuum leak cause this?

 

[Matt Weiser]

TTT...anyone...?