alright heres my problem, the car was overheating so i replaced the thermostat and everything seemed to be fine, but then it started overheating again and i realized as soon as i turned on the a/c the engine would cool down, so i came to the conclusion it was the fan. so i started the car and let it run for a while and the engine fan never kicked on. it comes on when the a/c fan is running, but not when the a/c is off. i checked all the wiring and everything seems to be good, no burnt fuses or anything. what could it be, and how can i fix it?
-Todd

The ECU controls the fan. Might want to check that out.

Yeah dude. The fans don't turn on, so the ECU is bad? No. That's totally ridiculous.

Jumper the center peg on the condensor fan relay to the center peg on the radiator fan's low speed relay and turn your AC on. if the fan comes on, at least you know the fan itself isn't bad.

Now, the fan's function is controlled by coolant temp. Can you verify the coolant temp sensor is plugged in, and that its resistance is within spec? I believe it's 11K ohms at 70* and 8-900 Ohms at operating temp. Check it with a multimeter.

I have noticed that the fan will come on every once in a while, but other times it wont come on and it will overheat. where is the coolant temp sensor located at?
-Todd

The fan relay is under the hood fusebox, so you may want to check that also..The relay could be bad, or the relay pins are corroded..Just as info, the driver side fan should kick on at around 206 degrees, from many datalogged runs i've done. The pass side fan should come on with the AC and when coolant temps get approx 212 or so..

i found the coolant temp sensor, what does the relay look like?
-Todd

Originally posted by MitsuEclipseGS
i found the coolant temp sensor, what does the relay look like? -Todd

The location of the relay should be printed right on top of the fuse box lid. The relay is a black, square/rectangular shaped looking plastic box, so to speak.. If you still cant get it to work, then you will need to test if the ECU is actually sending a signal to the relay to turn it on, and also, if the relay is sending out power to the fan. You will need a multimeter to do this..The ECU sends a ground to the relay to turn it on, and then you should have power at your fan..Also, you may be able to feel the relay "click" on, then you can test to see if its sending power to the fan. There should be at least 2 of the same relays there in the fusebox, so try switching the fan relays(there is one for each fan), and see what happens.

Originally posted by DarkOne
Yeah dude. The fans don't turn on, so the ECU is bad? No. That's totally ridiculous. Jumper the center peg on the condensor fan relay to the center peg on the radiator fan's low speed relay and turn your AC on. if the fan comes on, at least you know the fan itself isn't bad. Now, the fan's function is controlled by coolant temp. Can you verify the coolant temp sensor is plugged in, and that its resistance is within spec? I believe it's 11K ohms at 70* and 8-900 Ohms at operating temp. Check it with a multimeter.

It might be ridiculous on the Eclipse but on an Evo this can be the case. I have an EMS in there now but this controls the fans especially at startup and idle. When the EMS is in downlowd mode(logs) the fans stay on because the ecu doesnt fully function when transfering data. Some people have also had bad basemaps which didnt allow the fans to turn on and casuing overheating. So his idea is not that far off.

Originally posted by DarkOne
Yeah dude. The fans don't turn on, so the ECU is bad? No. That's totally ridiculous. Jumper the center peg on the condensor fan relay to the center peg on the radiator fan's low speed relay and turn your AC on. if the fan comes on, at least you know the fan itself isn't bad. Now, the fan's function is controlled by coolant temp. Can you verify the coolant temp sensor is plugged in, and that its resistance is within spec? I believe it's 11K ohms at 70* and 8-900 Ohms at operating temp. Check it with a multimeter.

The ECU might not be the problem on his car but its definitely possible. Its not ridiculous. It was the problem on my car when the fan wouldn't turn on.

Originally posted by pr0
The ECU might not be the problem on his car but its definitely possible. Its not ridiculous. It was the problem on my car when the fan wouldn't turn on.

I just don't believe that. The fan functions are so simplistic i just can't see an ECU failing in that way and not showing other symptoms first. This is like saying you're having a motherboard failure because the HDD LED on the case doesn't light up anymore.

Originally posted by gofaster87
It might be ridiculous on the Eclipse but on an Evo this can be the case.

Well, it is an Eclipse we're talking about, after all..

its your coolant temp sensor, cuz when you turn on the AC it kicks both fans on, i had this problem, check/replace the sensor and you will not only not overheat, your gas mileage will go up cuz it wont constantly be in openloop cold-start mode!