Volvo: volvo 240 ac, electric cooling fan, fan clutch

Question
The AC runs great on the 1991 240.  When the AC runs the engine guage says that it is overheating.  I turn the AC off and the guage says it is normal again.  We replaced the electric cooling fan pressure senso
(part 3540227)thinking the fan was not running keeping the engine cool.  
It is still doing the same thing.
Can you suggest anything?

Thanks!

Answer
Vicki,
First verify that the car is in fact overheating. I have the tempature compinsating board in the instrutment cause the gauge to read high, or even low. They are replaceable.
Things that cause overheating: bad thermostat, slipping fan belt, clogged radiator, bad fan clutch, bad water pump. You may need the help of pro to do a complete test.