Our beater 94 Impreza is overheating. The fan does not appear to ever come on so that would be a obvious cause.
This is what I've found:
Shorting the main fan relay contacts (the one in the under dash fusebox) turns the main fan on - the fan and associated wiring is good.
Tested the relay on the bench and it is good.
The relay coil has 12V on one side so one would assume that the ECM switches the other side to ground when cooling is deemed necessary. I put a meter across those two points and never see this relay be driven on.
When the AC is turned on, neither fan turns on. The AC is broken so I am not sure if it is supposed to behave this way.
When it overheats, the temperature gauge stays in the normal range. It seems to behave normally in that when the car is cold, it is indeed indicating cold and it slowly creeps up.
I am wondering what the ECM uses to determine when to turn the fan on. On the schematic, the thermometer used for the dash has a wife going to the ECM as well.
right next to the thermometer on the manifold) is also wired to the ECM. It would be hard to believe that the thermo sensor is bad, because that would mean the thermometer is bad as well.
Currently, I am leaning towards replacing the thermometer because clearly the indication seems to be bad. I might just see what it would do with a potentiometer to determine whether the ECM truly uses that signal.
Does anyone have any other suggestions?
Remco