I noticed yesterday that the A/C compressor on my '88 F-150 will come on randomly when the heater is on. I don't remember it doing this before. At low RPMs, this will cause the engine to jump a bit when the AC comes on and goes off. I turned on the AC (fairly warm day) and it worked great and the compressor clutch stayed engauged as it was supposted to. When the heater is on, the clutch hits randomly. Is this normal?

 

Im guessing its normal as you want to keep the humidity from fogging up the windows and guess how its done...

With the A/C?

Many U.S. cars run the air-conditioner in the defrost position to remove moisture from the air, so your air-conditioner (a/c) compressor may be running even in the winter.

http://www.inct.net/~autotips/musty.htm

 

it is normal as long as it is above 32 degrees F out side. depending on what you have the selector set to.