Defroster Button + Squeeling = ??

[livewirevoodoo]

Hi everybody. Haven't been around these parts lately as the ol slushbox has been runnin solid. Hope everyone is well.

Gotta little problem I've been lax on following up on, hopefully I can get some pointers:

A/C + Heat work like a charm, no problems there, but when I press the button for the front windshield defroster I get a cyclic (on and off) squeeling from up front somewhere. In addition to the squeeling sound the car runs a bit rougher until the squeeling stops. Only when i press the front windshield vent button (right below the recirculation button to the left of the fan knob).

Any help, as always, is greatly appreciated.

Thanks!

[Porsche-O-Phile]

Defrost engages the a/c compressor. If the belt is a little loose, it could be slipping on the compressor. Try checking the belt tension or you can run just the a/c to see if the symptoms can be produced that way.

Does it sound like it's coming from the engine bay or the fan motor area?

[Razorback1980]

I agree with Jeff...has to be the a/c compressor (because the engine runs rough) but what is strange is that you said the a/c works ok by itself. I'm not sure why the defroster would change the way the compressor works.

[Zero10]

Perhaps the combination of the additional electrical load (from high fan speed) and the A/C compressor is enough to make the multi-rib belt slip?...
That or the A/C compressor clutch is going, but since it works normally on it's own I suspect not.

[Razorback1980]

Good point Zero...didn't think about the extra electrical load. You may want to check the tension on your a/c alt belt.

[jcommin]

Might be a low battery. Sometimes the alternator howls, initially, when the demand for amps spikes.