this seems to happen sporadically, but a couple times, when first starting my truck in the morning, when it is colder, the a/c clutch makes this loud clicking as it goes on and off, every 20 - 30 secs or so. is this normal?

 

my ac compressr will engauge when the defrost is on.

 

Are you sure its not the grid heaters cycling on and off? they make a clicking sound when the relay picks up and your lights will also dim when it kicks on.

 

i had thought that about the grid heaters but today i opened the hood while it was doing it and actually watched the clutch on the a/c go on and off.

how often do the grid heaters cycle? how loud do they click cause this ac clutch is a very loud click.

does anyone know why the a/c goes does that with the defroster on?

 

it helps keeps the windows from fogging up.

 

Yup, most of the domestic car companies started hard wiring the A/C to the defrost on your window. I guess they started doing this 20 or 30 years ago (my 83 El Camino works the same). Some of the new stuff gives you a seperate button for the A/C that I believe allows you to override the A/C to off on any of the HVAC functions. I could go into more details about why it cycles when its cold but the short story is it's supposed to.

 

Blowing warm "conditioned" air keeps the windshield and side windows from fogging up.
As a side benefit, it keeps the seals in the AC system from drying out by engaging the system in the non-AC months.