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Radiator fans turn on immediately on cold start, turn off on operating temp

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Hello. My brother seems to have a bit of a weird problem. He recently swapped his belt driven fans for an electrical ones. He did all the wirings and changed the temp Sensor. Although the AC fan works fine, It seems that the radiator fan turns on instantly when the engine is cold, but when the engine reaches the operating temperature it turns off and stays off.


What could cause this? Did he flip a wire somewhere?

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He converted the fans from a belt driven one to electrical.


He made a custom wiring harness and installed a new Mercedes temp sensor because he was told it worked better in a hot climate.


The problem he has now is that the radiator fan works in reverse. When I put the key on the "On" position and start the engine cold the fan is still on. And when the engine reaches the operating temp, which is about half way of the gauge, the fan turns off.


Basically the radiator fan works in the reverse order of what it should. That's the problem.

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The temp sender/sensor . . .

A sensor acts like a thermometer and sends out a varying signal that is interpreted by a guage. It typically is a variable resistor either highly resistant when cold or highly resistant when hot. It has to be matched with the gauge to interpret correctly

A sender acts more like a switch that opens it closes at specific temperatures.

Earlier cars would have both. Later cars might have just an sensor and the ecu uses programming to interpret when to turn on the fan.

Your brother probably installed a sensor that is highly resistive when hot.

As for Mercedes vs other - it doesn't really matter, all mtg are designed to be installed into metal manifold surrounding very hot water. I've had GM cars (truck, Carl) where I have never changed them - 15-25 years running.

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Update... Changing the temp sensor to the original GM didn't do anything. Here are pictures of the custom harness placed in the fuse box that's made for the fans. Does anything appear out of the ordinary?


If two of the wires' places are switched, would they cause this problem?



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  • 3 months later...

A relay is basically an electrical switch that is operated by electrical signal also. Its capalbe of normally closed (always on) or normally open (always off) and switching the opposite. Is ENTIRELY possible the relay is able to do both. Which my gut tells me that you have the fans wired to N.O. (always on) and when the sender closes to activate, it OPENS the relay thus disconnecting from power.

If you have zero experience with relays I'd recommend going to Google university course in how they work before continuing. A custom automotive or a car audio shop could fix your issue if you prefer to do that instead. Sometimes a local mom/pop garage would be able to do this for you also.

I can offer you a starting point if you like:

In this diagram, replace their toggle switch with your temp sensor. Its nothing more than a temperature activated switch.
Also noteworthy is that they are showing two ways to interrupt a circuit. One is positive and one is negative interruption. 
Simplest way is to use the positive interruption. If your temperature switch breaks negative, it gets slightly more complicated but its still very do-able.

Cheers. :-D

Edited by crnext
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