I have a 2001 Trailblazer (Blazer) that the heater/defrost seems to cycle back and forth between hot and cold every 5 to 10 seconds. I have the Heater/AC Control that has the Auto Temp control. I can hear the vent flaps move as I rotate the controls between heat and defrost. The fan also works on both the Auto setting and the manual settings. I'm not so I will check this out tomorrow but I think I hear the A/C compressor kick in when it changes to the cold cycle. Any ideas?

 

Check that your coolant level is full in the radiator. Your a/c compressor will cycle when it's in the defrost mode, that's normal.

 

Coolant level is fine. What else should I check?

 

Got any codes?
Check your TPS.

 

No codes.

With the heat and A/C off you can hear what I think is one of the duct flaps moving from one position to another. It sounds like it is coming from under the glovebox area. The air flow blows hot but then sounds like the fan is getting starved for air, then the cold air blows and then turns warm again. Could this be the motor that controls one of the doors in the vents?

 

i have a similar problem, i lost heat a while back and recently it hasnt been getting above freezing until well after lunch at the the earliest. about a week ago i had heat suddenly for one day. it was not there the next. it made a weak comeback the day after and hasnt come back at all since. i flushed the heater core and nothing happened. im changing the thermostat this weekend but i was wondering if that doesnt work if it might be my compressor. i ask because my compressor is messed up, i can hear it come on for about 2 seconds, then off for 2 seconds, and on and off... i have charged it with coolant last summer and it still shows charged. it actually shows a little high when the compressor works, when it doesnt you cant trust the reading.
all help appreciated.
(sorry didnt mean to steal ur thunder man)

 

OK, I checked on four of the actuators that I could find. 2 each on the passenger side and drivers sides under the dash. They both are working. Vaccume is good. I can hear what sounds like a small electric motor opening and closing a flap within the semi round housing just to the left of the glovebox. Is there a small motor that controls either A/C and Heater core air flow. Any ideas?

 

Also, when you turn on the A/C it will turn itself off during this cycling for both recirculating and fresh air.

 

The auto temp control has to have a temp sensor somewhere. Mine doesn't have that option but it sounds like a bad sensor. Not sure where it would be located.

 

If you have Autotemp control they use an outside ambient air sensor like the overhead console temp sensor located out by the front of the radiator...