Overheating with Rear AC ON

[camaro108]

Hey everybody. Our 2002 Yukon XL that we use here in AZ for towing boats has started doing something very strange. We had the whole top end rebuilt, radiator, water pump, thermostat, coolant tank, and fan clutch replaced. So the whole cooling system. Being 105F+ here, the truck actually runs great. It has 258K miles on it and my mechanic is amazed on how well it runs. We originally had all of the above done because it was overheating at idle.

Now, after all the work, it still overheats. But with one strange thing…. only when the rear AC is on. If that is off, and the temp. knob is turned to about half way, the truck runs nice and cool. Any time the rear AC is turned on to cold and the fan is put on, the engine overheats.

As far as the front AC goes, it does NOT overheat when it is on. The AC also doesn't blow very cold from the FRONT when the AC in the REAR IS OFF. Once the rear is on, the front gets ice cold, but then the car overheats.

Very strange situation here and even my mechanic is confused. Anyone else

 

[Wagnasty]

What do you mean by overheat, hotter than normal or?

 

[retiredsparky]

Wow, you have a difficult problem. Is it possible that the gauge temperature sending unit is bad? Sending units frequently fail on high mileage vehicles and then the gauge shows the engine is either too hot or too cold. Have you checked the actual temperature of the coolant with an infrared sensor? If you have a scanner you could plug in to see what the PCM says the coolant temp is.

The AC system may have a totally unrelated problem. If the engine is truly not overheating, you could run both AC units to keep the interior cool. Has the AC system been checked by a refrigeration tech?

 

[DaNoobLi]

I recently had a similar problem with the front/rear AC system and the AC control module had to be replaced. My truck didn't run hotter because of it though. As far as the overheating goes I would make sure that it's actually running hot then check for a faulty sending unit.

 

[retiredsparky]

You could have a faulty (even though it is new) fan clutch. Buy or borrow an infrared temperature scanner to check your true water temp.

I suppose you have checked the belt tension to ensure that the belt is not slipping.