Afriend of mine just purchased an 89 Rs305 and I have been helping him work on it..So far the only real problem has been That it overheats unless you have the A/C on(So the fan comes on) pretty much narrowed it down to what I believe isthis Sensor(should have been an easy fix) but every time I change it out to a New onethe car runs really rough and throws a check engine light...Any ideas?

Thanks in advance for any help

 

Are you sure you're changing the correct one, with a correct one? There is a coolant temp. sending unit mounted in the front of the left cylinder head which is for the dash gauge.
There is also a coolant temp. sensor mounted near the front of the intake manifold, near the water neck. It's actually a thermistor (a resistor that changes with engine temperature), and has a direct effect on the engine's emissions and other sensors. If that sensor, or it's circuit/wiring isbad, it should be throwing a code 14 or 15.
And I thought the cooling fan was controlled through the ECM? I could be mistaken.
Is it possible you're messing with the wrong one?

 

Here is a Pic of the sensor I have been changing(Not actual car just a google pic)...Keep in mind the car also just had a new thermostat put in before car was purchased