1992 Pontiac Sunbird Fan is always on

DOES THE VEHICLE HAVE AC?

Im going on all data to check wiring diagrams, so we can get to the bottom of this. Ill be back as soon as I can. Hang in there.

Ok heres where it gets tricky. Im assuming your vehicle came from the factory with no ac. If your computer reads neg 39 there is a problem in cooling temp sensor circuit. You need to check voltage at the yellow wire at the cooling temp sensor. Key on engine off there should be 5 volts.

You need to put it on dc volts. ****I forget, have you replaced the sensor already? **Use your ohm meter and check the resistance of the coolant temp sensor. I have the schematic at work. I think a low ohm reading indicates high coolant temp. If you have an open in the coolant temp sensor, your ohm meter will not read anything. Than your problem would be a bad coolant temp sensor. ** The reason the low temperature would turn the fan on is because the computer knows neg 39 degrees is an error so it goes into failsafe and turns the fan on. **Also use your ohm meter from battery neg terminal to the black wire at the plug, you should have 0 - 5 ohms resistance. If ohms is higher than 5 ohms than you have a bad ground on the black wire.

Id install the new one onto the harness with the old one still in vehicle. This will tell the ecm a temp of 60 degrees. See if the fan stops running with the new sensor plugged in to harness.

You said the brown wire had 10 ohms it should be less than 5 ohms. Id hook a test light to 12 v and check brown wire, the test light should light. Technically if your ecm coolant temp is really neg 39 and your guy who scanned it did not make a mistake, and you said you had 3-5 volts on the yellow wire, which tells us the integrity of the wiring from the ecm to sensor is ok. Id double check the ground with the test light. If all that tests out ok. The green wire with white stripe on it from the ecm to the fan relay is shorted to ground somewhere. **You said the vehicle came from the factory without ac. Correct. But your vehicle is still wired for ac. So id check harness for a plug that might be chafed somewhere. Because the ac high pressure switch also will turn on the fan

Id cut the green and white wire at a spot that is easy to put back together. Then lets see if the fan turns off.

Ok heres what we do. You said you put in a new ecm. If you can access the green white wire at the ecm. Cut the wire there, then reconnect the other end that you just cut. Turn key on, if fan comes on the green white wire is shorted to ground somewhere, if the fan still is off than there is something triggering ecm to turn on fan. Were almost there!

Well heres what we have. We can go on tracing wires that input and output the ecm which could take a real long time for us to find on line. What you can do to save alot of time and aggravation. This repair will cost you about $20 and take you about an hour to do. Go to parts store and get a cooling fan switch for a 1994 camaro with a 5.0l engine. Usually in the help section of parts store you will find the direct plug in plug for this sensor. There are numerous spots to install this sensor in the head, manifold or the engine block where coolant is. Install the camaro coolant fan switch and plug, now run a wire from the green and white at the relay to the switch you just installed. This is your best bet to fix this car. Make sure you get the cooling fan switch not the coolant sensor for the ecm. The cooling fan switch will have only 1 terminal in it. Your cooling fan will now come on at 228 degrees and shut off at about 200. Exactly like the fan should.