does anyone know if the heater grids can go bad or are there any solenoids that go bad, mine come on when its 90 degrees out, I can tell because when I start the truck my batteries drain down and I can see my rpm's go up and down . I disconnected them and i have no problem, but it's gonna get cold soon .

 

Check your Intake Air Temperature sensor.

 

A IAT sensor reporting cold temp will make cycle on. Follow the wiring back from manifold and you will find two heavy duty relays that control current to heaters. The relays are controlled by PCM, based on ambient, coolant and IAT.

 

They will come on when first started at any temp, as long as they quit cycling after the engine runs for a few minutes.

 

not true for my truck as they only cycle when its cold out, warm temps like now they do not come on...period.

 

I had the same problem a while back. New iat and no more grid cycling

 

sounds like that is what it is the iat. will try it this weekend.

thanks