I have a 1994 850 (not a Turbo) and unfortunately the radiator just failed on it. According to my mechanic the plastic reservoir overheated and melted. I've replaced the radiator and had the mechanic check on my AC while he was in there, while doing that he discovered that the radiator fan and AC fan were not turning on as they should be. We tried popping in a different (used) ECU ECM Computer and while the car was sitting in the garage both fans started up fine and ran for the several minutes we had it in the garage, however after driving the car around a few blocks both of the fans cut out and the AC no longer cools. Popped my original ECU ECM computer back in and the same thing happened. We're at a loss, seems like it's something outside of the computer since two different computers have the same effect and we've popped open one of them and there are no obvious faults in soldering, etc.


I've done research in this forum and online and it mostly seems like people have issues getting their fans to stop running, not issues with the fan cutting out after a short drive and not starting up again.... Any ideas? I'm at a loss and can't really drive the car at the moment since it will overheat pretty quickly and likely melt the new radiator.


Thanks!

 

Hi and welcome to the forum.

I'm not really following you here. The car has 2 fans, a climate control fan and 1 fan at the radiator (not 2). When I drive my car, the fan at the radiator rarely come on, unless I am sitting parked or stuck in some bad stop and go traffic. When the AC is on, the fan should run all the time.

So, if the AC is not on or the car's coolant temp is not above 216 degrees F, the fan at the radiator should NOT be on and running.

 

To add, the fan operation is affected by the TCS (temp control sensor) and can also be upset by a faulty thermostat.
The ECM is unlikely to be the problem.

 

Thanks rspi and difflock54! To clarify, with the AC on both fans start off running and continue to run while the car is sitting still in a garage but then after around 5 minutes or less of driving both fans cut out and the AC starts blowing warm air. I could live with that issue (and have for a few summers now) but my mechanic pointed out that the radiator fan cutting off will cause it to overheat again.

I agree that it's likely not an ECM issue since putting a different ECM in didn't solve the problem so I will look into the TCS (temp control sensor) and thermostat and let you know what I find. It does seem to be something about driving the car that causes both fans to shut off even though the AC is still running though.

Thanks!

 

Are you sure the AC is still running? Which fan cuts off, the motor fan or the fan inside the cabin?

 

When you start the engine (especially from cold) the main radiator cooling fan should not operate at all.
That one should only cut in if the engine begins to run too hot so as to assist in cooling the motor back down to the normal running range temperatures. (cut in is around 212 degrees F plus.)
Normal running temp should be around 180-190 degrees F.
Thats where the possible thermostat and temp' sensor problems may be allowing the fan to run outside its proper parameters.

 

UNLESS --> the AC is turned on or it's in defrost. If you have the AC ON or defrost then as soon as it starts the engines cooling fan should start up too. Normally the coolant temperature sensor controls it and yes the engine needs to heat up and turn the sensor on to get the fan going but in AC or defrost the AC compressor is turned on and the AC system needs air flow through the condenser in the front of the car to cool the interior. With AC or defrost on a signal is sent to turn on the cooling fan no matter how cold the engine is.

I would find the relay and check that. Could be that it heats up and cuts out.

You could also jump the connector for the coolant temp sensor and see if the fan comes on and stays on or if it cuts out. For now I'd turn the AC off and just focus on the basic cooling fan circuit first and once that's working correctly then see IF there is a problem with the AC operation.

 

As for the radiator fan, mine turns on at 216 F and shuts back off at 204 F. My car runs between 183 F to 216 F, rarely gets up to 218 F (never seen it higher than that). At 178 F my needle is at 3 O'clock and it stays there through 218 F. Not sure what temp makes it raise above 3 O'clock.

On my 3249 mile road trip that I just finished, almost the entire time on the highway my temp was between 188 F and 190 F. Again fan does not come on unless the temp is 216 F. It also comes on when I turn on the AC.

 

So when I start the car up and turn the AC on without driving anywhere, the radiator fan comes on. Then I drive the car and recently checked after 10 minutes of highway driving with the AC on, kept the AC on as I was parked and the radiator fan is off.

I'm going to have my mechanic recheck the cooling fan circuit! We did try replacing the thermostat already.