I want to wire up my old truck to turn an electric fan ON to keep the radiator cool.

The ECM has an engine coolant temperature sensor mounted in the engine that alters the resistance to determine coolant temperature.

From the service manual, I have this data:

engine coolant temperature sensor output

If you can not see the pic above:

__Condition________________|_Engine coolant temperature decided
Just as ignition switch is | 20°C (68°F)
turned ON or Start_________|___________________________________
More than 6 minutes after  | 80°C (176°F)
ignition ON or Start_______|___________________________________
                           | 20 - 80°C (68 - 176°F)
Except as shown above______| (Depends on the time)_____________

So, what would be the magic temperature to look for to turn on my fan?

If I set it to 80°C (176°F), the electric fan would always be ON after 6 minutes.

If the truck has a 180°F Thermostat, should I be looking for 185°F before turning on the electric fan?

schematic 2

  • A 1994 Nissan D21 w/ VG30e. Did not come with electric fan, but I am installing one from a 1993 Nissan Altima. So far, I have it installed CLICK and this thermoswitch relay ready to go on. Just ran out of time today.
    – jp2code
    May 2, 2013 at 1:36

3 Answers 3


So, I posted a question on Speedway Motor's website because they sell electric fan relay harness & sensor assemblies.

I asked:

My truck has a 170° Thermostat.

Does that mean I should get the kit that turns the fan (ON @ 185°)/(OFF @ 165°) or the kit that turns the fan (ON @ 195°)/(OFF @ 175°)?

The answer by Jason was:

Part number 91064028 has options for fan turn on and off and with the 170 degree thermostat the hot coolant will enter the radiator at 170 and in many cases may cool the engine to this temperature without the fan running. If you get the on at 185 and off at 165 when the coolant got to 185 the fan would come on to assist and this is what would be recommended.

So I purchased THERMOSWITCH Item #91064028.

An electric fan from a 1993 Nissan Altima has been grafted into my 1994 Nissan D21 "Hardbody" in the pic below, with their THERMOSWITCH mounting on the circled spots:

enter image description here


I can report that on my car, with 185 degree 'stat that the fans are supposed to come on at 200 (per the manual). When they actually come on varies from 200-210... Not sure why that is, the fan relay is ECU controlled and the gauge and ECU directly samples the temperature... :-)

  • Not sure how old your car is, Brian, but I know the chemical characteristics of the goo those sensors use can change characteristics as the parts get older. You might be able to change out the sensor and have your fan turn on when it is expected. Also, if your car is like my truck, it could have a separate sensor for the gauge in the dash. In that case, the gauge sensor could be off.
    – jp2code
    May 2, 2013 at 14:54
  • It's a '95. There are 2 sensors. The primary goes to the ECU, which controls the fans. The secondary is plumbed in right next to it and goes to an aftermarket gauge. The OBD-II (it's a Mitsubishi, they used a 99% complete OBD-II that year even though it didn't become law until '96) coolant temp matches up perfectly with the aftermarket gauge (which is pretty surprising itself). I suspect that there may be some other value other than pure temperature that the ECU must be factoring in. However, I've not identified it yet. Consistent the 12 years I've owned the car. :-) May 2, 2013 at 17:07

You probably don't want your electric fan to always be on, that would seem wasteful albiet harmless ( every time the fan dropped the coolant temperature below 180 the thermometer would shut off the flow to the radiator ). If your ideal operating temperature is 176-180 I'd agree with setting your fan to turn on at 185, that way it kicks on whenever your coolant temperature starts to climb above that temp your fan can keep things in check.

  • Are you just guessing at 185°? That's how I came up with that value. I'm looking for a definite answer. When a car manufacturer builds a vehicle with an electric fan, that fan is set to turn on at a specific temperature. Given a 180° thermostat (optional 170°F thermostat), I'd like to know what temperature an electric fan should be set to turn on.
    – jp2code
    Apr 7, 2013 at 21:21
  • 1
    Ideally you want to calibrate your electric fan to turn on soon enough to avoid big temperature fluctuations, but not so soon that you risk it always trying to turn on before your thermostat is even open, as I mentioned before. If you want perfection it could very well take some playing around. When you're at cruising speeds the wind might be enough to keep your coolant temperature from going above 185. Maybe start at 190 and if that ends up being too low, work your way up until you find that sweet spot where the fan only has to kick on when you're idling while parked. Apr 8, 2013 at 20:13

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