94 Impreza overheating

[RemcoW]

I have a 94 Impreza that is overheating.

I've burped and added coolant. The fan does not appear to ever turn on.

 

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 still 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 to normal. Can that gauge be believed?

 

 

I am wondering what the ECM uses to determine when to turn the fan on and at what speed. I take it, it must have a way to read the temperature and when a set limit has been exceeded a fan turns on (so it is not a thermo switch but truly a sensor of sorts).

On the schematic, the thermometer used for the dash has a wife going to the ECM as well. From experience I do know that the thermo sensor (a sensor mounted right next to the thermometer on the manifold) is also wired to the ECM. I know it can screw up fuel mixture if the thing is bad and can cause a hard starting condition (btdt). Can a bad sensor also not cause the fans to turn on?

 

If the thermo sensor is used to determine what fans to turn on, I am wondering why the thermometer indicates a normal temperature condition. It would be hard to believe both are bad

 

I will over the next couple of days measure from the ECM to the temp sensor to make sure it isn't open. Was curious if I could fake the system out if I just put a potentiometer (or possibly short the connections?) in its place to see if the fans can be turned on.

 

Sorry for the long post -- anyone else have any suggestions?

 

Tia

[idosubaru]

those fans need to be turning on. if the temp is going up...if the gauge is to be believed (which it probably is)...then the fan should be coming on. not sure where your sensor is for the fans though. i think that's the one in the radiator though right? test that one.

[RemcoW]

those fans need to be turning on. if the temp is going up...if the gauge is to be believed (which it probably is)...then the fan should be coming on. not sure where your sensor is for the fans though. i think that's the one in the radiator though right? test that one.

 

The fan works fine when I short the relay's contacts. I strongly suspect the wiring to the thermosensor or the sensor itself.

I've had the radiator out when I had the engine out and don't remember seeing any wiring to a sensor. The sensor I am talking about is sitting on the manifold.

[lmdew]

I think the temp sensor and fan switch are different. Look at the R/H intake manifold. You should see two sensors. You can put the ECM into D-check green connectors if my memory is correct and the fans should cycle.

 

If they are doing that the wires should be good. Have you burpped the system? If there is air in the system it will cause problems.

 

Good luck. Larry

[RemcoW]

I think the temp sensor and fan switch are different. Look at the R/H intake manifold. You should see two sensors. You can put the ECM into D-check green connectors if my memory is correct and the fans should cycle.

 

If they are doing that the wires should be good. Have you burpped the system? If there is air in the system it will cause problems.

 

Good luck. Larry

 

In D mode they are supposed to cycle? I didn't know that - thanks!

 

Yes, it has been burped.