the coolant is full, and the AC is turned off, but when the dash temp gauge reaches normal temps, both cooling fans will start to operate..the problem is, they will not turn off...could the "coolant temp sensor" cause this without the engine "check" light coming on

there is a chance this is the default position for a realy....i would check all the cooling fan relays to see if they are stuck in the closed/energized position.....

Please tell us what year and model car you are working on. Is this for a '01 OBW?

If all else is well, maybe your radiator is clogged up, meaning the temperature is reaching close to 90 and the fans are coming on to keep it there.

 

Your temp needle will stay in once place for the range 70-90 degrees C. This kind of electronic "cheating" is normal in newer cars, it stops the driver getting nervous over the needle moving up and down.

 

My 2000 Impreza does this. Our 1988XT has the the old style, fluctuating needle.

Please tell us what year and model car you are working on. Is this for a '01 OBW?

Sorry, 01,2.5L, Outback,60K Miles...having performed 3 coolant changes..this problem also happens intermittently

Are you sure no air pockets are trapped in the cooling system? There is a different coolant temp sensor for the ECU and a different sensor for the temp gauge, so if the ECU sensor is reading incorrectly you won't see it in the temp gauge, plus the temp gauge is not linear and has a bit of deadband around normal operating temp as someone already pointed out.

I think the temperature sensor for the ECU is in the circuit that controls the fan relays, which are controlled through the ECU. I would first check that CTS is working as it should be and indicating the correct water temperature. The sensor should be within a certain resistance range at normal engine operating temperatures. If the resistance goes lower than normal the ECU will think the engine is too hot and turn on the fans through the relays.

Are you sure no air pockets are trapped in the cooling system? There is a different coolant temp sensor for the ECU and a different sensor for the temp gauge, so if the ECU sensor is reading incorrectly you won't see it in the temp gauge, plus the temp gauge is not linear and has a bit of deadband around normal operating temp as someone already pointed out.

I would say that I am sure no air pockets in the system ...nothing has changed or been touched since last here and I just recently returned from a 5000mi trip

I think the temperature sensor for the ECU is in the circuit that controls the fan relays, which are controlled through the ECU. I would first check that CTS is working as it should be and indicating the correct water temperature. The sensor should be within a certain resistance range at normal engine operating temperatures. If the resistance goes lower than normal the ECU will think the engine is too hot and turn on the fans through the relays.

I will give CTS a check..also they are cheap to just replace it

I wish I could tell you what the voltage should be across the sensor when at normal temperature but I don't know what it should be.

I wish I could tell you what the voltage should be across the sensor when at normal temperature but I don't know what it should be.

I couldn't find any info either...perhaps a new relay and sensor fix the problem

It sounds to me the fan relays are working ok. The question is why they are being turned on early. Assuming the dash gauge is reading correctly.

Looking at some data for a '97 model it shows the CTS resistance should be about 240 ohms at 194 deg.F . Perhaps the newer models may be the same.

Looking at some data for a '97 model it shows the CTS resistance should be about 240 ohms at 194 deg.F . Perhaps the newer models may be the same.

Thanks Cougar, but I think it will become only more confusing since the running temp of the newer cars are 170F...rockauto.com has a sensor for $14..

If the coolant temp sensor was bad it would be open. That means that the moment you turned your car on the fans would come on. If it does not come on till its at temp i would say its not your coolant temp sensor. But, If you can do the work yourself its a 25 dollar part. I got mine from NAPA.

Thanks Cougar, but I think it will become only more confusing since the running temp of the newer cars are 170F...rockauto.com has a sensor for $14..

 

You're welcome. The spec shows the resistance should be 330 ohms at 176 deg. F.

If the ECTS is starting to get flaky, i.e. inaccurate, it could cause the fans to run like you say. When stopped/slow with a/c off the fans will come on above 202F. Suppose the engine coolant is actually around normal operating temperature ~180F, but the ECTS (being inaccurate) indicates the engine coolant temp is say 205F. The fans will come on and stay on. Some scantools can tell you what the ECU thinks the engine coolant temp is.

A new twist and my mistake..I thought the fans were running continuously..But only in the driveway ...But when driving on the road the fans go out and I know this to be because within 2 seconds of stopping at the corner, the fans are back on..it has to be a rather quick stop to determine the fans coming back on

You're welcome. The spec shows the resistance should be 330 ohms at 176 deg. F.

I just attempted to test the unit..unable to do it with my equippment..a tight spot

A new twist and my mistake..I thought the fans were running continuously..But only in the driveway ...But when driving on the road the fans go out and I know this to be because within 2 seconds of stopping at the corner, the fans are back on..it has to be a rather quick stop to determine the fans coming back on

 

Yes, it can be a bit more complicated when you are moving. The ECU has a control chart for the fans. This one should be close to yours:

I had a simular problem with my 97 Legacy 2.2L.  The fans would not stop and the temp gauge would jump up to Hot.  Then the car would not start.  The radiator fluid was not hot.   After burping the car, replacing the engine coolant sensor it still was having problems.  Then I notice a loose ground.  The ground was attached to the top of the intake manifold in obvious sight.  3 days of frustration a few turns of a bolt.  No more problems.