I just bought the truck it has a 4 cylinder 2.3l. The previous owner told me about the problem. I figured it was just the thermostat or possibly the radiator. I replaced the thermostat and removed the radiator and flushed it and checked for blockage. Everything is ok, but it still overheats. If i turn off the ac it will go back to normal. The ac clutch is not frozen. Any ideas would be appreciated.

 

I would check to make sure that the fan clutch is in good working order. If it's weak or not engaging properly, it can't draw enough air flow thru the condensor/radiator to remove the excess heat when the A/C is running.

 

I took the fan clutch off and took it to autozone to compare to a new one.They both had the same amount of drag while cold. Should there be a signinficant amount of difference when the car is heated, because i only felt a very little amount of change.

 

A new and used fan with a viscous coupled clutch will normally have the same amount of drag. The drag is caused by the initial viscosity of the silicone fluid in the viscous clutch assembly. However, if the clutch plates are worn, then the mechanical engagement fails to occur properly causing the fan to turn at a slower speed then it should (internal slippage).

How a viscous coupling works (cooling fan or differential).

A viscous coupling is made up of alternating circular plates. The plates have tabs or perforations in them. The plates are mounted in a sealed drum, and are located very close to each other. The drum is filled with silicone, or some dilatant fluid. When the two sets of plates are rotating in unison, the fluid stays cool and remains in a liquid state. When the plates start rotating at two different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat up and solidify (Silicone when heated will turn into a near solid; the viscosity of dilatant fluids rapidly increases with shear). The fluid in this state will essentially glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations on the plates, along with the number of plates and fluid used will determine the strength and onset of when this mechanical transfer will happen.

Hope this helps.