Hmm, that's interesting. I know that the compressor comes on and off whenever the defroster is on, and it's annoying, so I turn it off once the windshield is defrosted. Same with the AC, I only use it when it's really warm in there. But the previous owners said they usually had one or the other on all the time. Maybe that's good, if it keeps the seals and stuff in good shape. Does seem like sort of a goofy system, though.

Anne
'91 S-10 Blazer
150,300 miles

 

Keeping the seals lubed up and in good shape is not the only reason for running the A/C in defrost mode. A secondary effect of any air conditioning system is the removal of moisture from the air, that's why you have a nice pool of water under your truck/car in the summer time. By running the ac during defrost, you are aiding in the removal of moisture from the in-cabin air which accelerates the defogging of the windshield. So during defrost, you are forcing heated air through the ac core where a portion of the moisture in the air is removed and runs out through the HVAC drain. So the cycling of the A/C does have another purpose which would not work very well if it were only on a timer from key-on or whatever.

I wonder what refridgerant is used in your '91 Anne... If it is R-12, that would explain the high price for a top-off. R134a is much cheaper because it is MUCH more environmentally friendly (won't kill the ozone as fast - lol). They have a new refridgerant that says it can be used in existing R-12 systems, but I cannot for the life of me remember what it is.

*EDIT* - ha yeah... FR-12 is the replacement that is good for both existing R-12 systems and R134a systems.

 

Thanks for the explanation, Swart. I don't know what kind is in mine or if the mechanics I was talking to had even looked it up. I will certainly keep FR-12 in mind and do some more investigation before I let them charge me $151 for a recharge. (I don't know if this price perhaps included other stuff such as diagnostic tests to see if anything else was wrong with the AC system.)

Anne