Nissan Repair: Engine overheating, 1990 Nissan Maxima, 2 cooling fans, coolant temp

Question
QUESTION: Hi Calvin,
My 1990 Nissan Maxima has been overheating for the last couple of weeks. I had a coolant flush and replaced the thermostat but the problem still persists! The coolant keeps evaporating, I guess because of the heat. I did notice that the cooling fan never seems to come on except when I turn on the Air Conditioning, recently when the car overheated I turned on the AC and the engine temperature indicator immediately started falling! Any suggestions on what could be the problem?

Thanks a ton,
Husein

ANSWER: Hi Husein -

You have hit the problem - First of all - do you have 2 cooling fans, or one?

Let me know, and I will walk you through a diagnosis.

If you have 2, one goes on with the A/C, and one goes on when the engine reaches a certain temp.

Let me know -


Aloha
calvin

---------- FOLLOW-UP ----------

QUESTION: Hi Calvin,
Thanks a lot for the reply. Yes, I do have a two fan system. Strangely enough though both of them come on when I switch on the AC! Also, after reading your first reply I took the car to a mechanic. He pressure checked the system for leaks and didn't find any, then he turned his attention to the cooling fan. Supposedly, the fan motor, the relays and the wiring were okay so he started looking for the coolant sensor but couldn't find it! So his diagnosis was that my ECU (engine computer) had gone out and a new one would be $800.00!!! I find it hard to believe that my old car needs a $800.00 system to control my coolant fan! Please help!

Thanks again,
Husein

Answer
Hi Husein -

OK, since both fans go on when the A/C is activated (this is normal), I can assume the motors for the fans are OK.  Only one is supposed to activate when the A/C is off and the engine reaches a certain temp. This fan is controlled by a coolant temp sensor, a switch, and a relay, located in the engine compartment by the battery. Should be with a bunch of other relays and large circuit fuses.

In order of probability, this is what fails first:

Fuse
Fan motor
Switch
Sensor
Relay

Check the fuse below the drivers side dash. It should be labeled "cool fan" or something close to that. If the metal strip is burnt, replace the fuse and run the car, checking the fuse once again when the engine is warmed up. If the fuse blows again, then there is a short somewhere.

The fan motor is obviously OK. There is no need to check that.

The fan switch is located on the radiator most of the time. Look for a hex shaped device that has a couple wires connected to it, screwed directly into the radiator. Sometimes it is on the engine, near the water outlet, but always has 2 wires. With the ignition on, and temp hot, check voltage at both wires. One should have voltage. Jump the 2 wires. If the fan goes on, then the switch needs replacing. If not, then probably check the relay.

I do NOT think it is the computer. That is a shot in the dark assumption.

Hope this helps -
calvin