a/c temp control

The rheostat sounds and feels normal. There is some kind of diaphragm or bellows noise that I've been hearing lately coming out of the black box up under right dash side. Sounds like some kind of flapper or something cycling rapidly back and forth at random intervals.

Is there some kind of vacuum control unit or bellows/diapragm unit that responds to the rheostat? Or is that an unrelated problem?

The Temperature Control Unit went south on my '97 GMC Sierra. CarMax had to consult with a GM dealer to figure out what was wrong. Total repair was over $600. Sure was glad I had purchased a CarMax extended warranty. My cost was $25, and they paid for the rental car.

Vic

}}} The Temperature Control Unit went south on my '97 GMC Sierra. CarMax had to consult with a GM dealer to figure out what was wrong. Total repair was over $600. Sure was glad I had purchased a CarMax extended warranty. My cost was $25, and they paid for the rental car. {{{

Please let me know what the shop did to fix the problem.

Thanks.

Sorry, I shot my mouth off without digging out my old paperwork on the truck. Turns out I had the entire Temp Control Unit replaced twice within two years. First time under factory warranty. Second time under the extended warranty. Both times I was told the unit had an internal short and was possibly defective. I know, but that is what I was told. Also, looking at the service write up, the total charge, had I not had warranty coverage, would have been, $ 110 for the unit, with a $71 labor charge. Somehow, I had a higher figure in my head, probably confused with another repair somewhere.

VG

OK. Thanks for the info. Good to know.