A/C kicking on and off with defrost

[Beefcake35]

Hi All! Have a question for you gurus. My defrost wasnt working so I located the problem with vacuum. Now I am noticing when I am in defrost the A/C compressor kicks on and off about every 5 seconds. I dont remember it doing that before. It runs constant when I am ac mode and is off constant on everything else except defrost when it kicks on and off. Thanks in advance!

[MIKE S]

Running on defrost is normal. It dehumidifies the air. It should cycle the same amount of time on a/c or defrost

[bd]

Expanding on Mike's post, ambient temperature will affect compressor cycling frequency - cycling faster as the outside air temperature drops, especially if the A/C system is low on charge.  But a properly charged system at a given ambient temperature will cycle the compressor the same irrespective of whether A/C or defrost is selected.

[Beefcake35]

ok. Was worried! Thank you for your help! understand a bit more about it now... you guys rock!

[MIKE S]

Back in the day when GM first implemented compressor activation with defrost a lot of people would disconnect the compressor clutch in the winter because they thought it was wasting fuel. They did not understand the dehumidifying action of the design. It also helps keep the shaft seal lubricated in the winter to prevent leaks.