cooling fan always on

when I start car cooling fan comes on, tested temp switch, relay, all okay it seems to me that message is coming from computer that tells fan to go on! when i romove relay fan does go out. My question is is the comuter bad? other than the temperature switch is there something else that could tell computer to turn fan on.Sal

Is the a/c or defrost on?

What year is it? What engine?

If it has a high side pressure sensor (most of the ones left on the road do), the PCM only commands the cooling fan on if the A/C high side pressure is roughly 180 PSI, so it shouldn't be on immediately after starting the car. If it has a pressure switch that info is correct. On some years/engine, the PCM will command the fan on if certain codes are set, check for codes and fix those if there are any. If the sensor is stuck reading high pressure, that could cause the fan to stay on. I couldn't tell you what the check there, I use a scan tool to look at the datastream. If it's an older one with a 2 wire switch opposed to a 3 wire sensor, that's easy. But if it's got 3 wires I don't know how to test it the old fashioned way without looking at a wiring diagram and a resistance table. If that doesn't solve it, you could connect a test light to the motor that you can see from inside the car and drive it at 55 MPH. The PCM is programmed to turn it off from speeds ranging from 30 to 45 from what I've seen, so I'd bet any car (with the programming) would turn them off at 55. If it doesn't turn off then, check for a short to ignition switch controlled power on the controlling side of the relay. If you're good there, then it's probably the PCM.

Sorry, 96 cavalier 2.4 I hear its a pain in behind engine to work on. Ok I pluged a tester to read codes and it gave this (operatins error ckeck conections try again) thannks for your time.Sal

Check whatever documentation you got with that code reader. Usually when I get that message (similar, anyway) it turns out to be a blown cigarette lighter fuse, that fuse also powers the DLC on most GMs, including the Cavalier.

when I start car cooling fan comes on, tested temp switch, relay, all okay it seems to me that message is coming from computer that tells fan to go on! when i romove relay fan does go out. My question is is the comuter bad? other than the temperature switch is there something else that could tell computer to turn fan on.Sal

Sometimes, on the cavaliers, if there is a check engine light on, the fan will stay on.Try to clear any codes if any with scan tool and see if the fan goes off

On some years/engine, the PCM will command the fan on if certain codes are set, check for codes and fix those if there are any

Sometimes, on the cavaliers, if there is a check engine light on, the fan will stay on.Try to clear any codes if any with scan tool and see if the fan goes off

:runaround:

when I start car cooling fan comes on, tested temp switch, relay, all okay it seems to me that message is coming from computer that tells fan to go on! when i romove relay fan does go out. My question is is the comuter bad? other than the temperature switch is there something else that could tell computer to turn fan on.Sal

You say you tested the Temp Switch (on/off)? Do you mean the Engine Coolant Temperature sensor (thermistor, no on/off)?

If the Cooling Fan turns off when the car is turned off proves the relay is switching normally. The coil in the Cooling Fan Relay has one end tied directly to a fused battery feed-with the other end of the coil (switching feed) tied directly to the PCM.

With the fan on, this switching feed line must be at 0 volts or ground potential. You must determine if this line wire is just shorted to ground or if it is actually being switched to ground through the PCM.

Case 1: wire just shorted (repair)-PCM ideally has built in short circuit
protection.
Case 2: PCM working normally, either the ECT sensor or the A/C refridgerant pressure
sensor (as J-Ri mentioned) at fault or their subsequent systems at fault.
Case 3: As "youstillmydog" mentioned (post 6)
Case 4: PCM faulty

Would the Engine Coolant Temperature Sensor cause the cooling fan not to come on automatically, but allow the fans to come on when the AC is turned on?

if so, where would the ECT sensor be located on an 85 Cavalier with the 2.0 4cyl