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Retro electric radiator fan indicator light

I have installed an auxiliary electric fan at the radiator on our 1973 Mercury Capri and it’s controlled by an adjustable thermostatic switch. Wanting to know when the fan comes on, I wired an LED indicator light to fan side of the thermostatic switch and mounted it inside the car. Mission failure. The light comes on just from leakage through the thermostat. Ok, I figured maybe this is an LED issue. But no, a test with an incandescent bulb showed the same issue.

If the pilot light is going to glow all the time (dim at idle and brighter with revs, by the way), then it’s no good for the purpose intended. I wondered if anyone has a fix for this they can offer. A resistor, maybe? If so, what rating would be best? Appreciate any useful input.

Rich O’Brien
Bowie Maryland USA

1973 Mercury Capri
1954 Citroen Traction Avant
1973 Beetle

How do you have the light wired? It seems to me that the light should be wired to the fan power wire. The light should only come on when the fan motor is running. Just use any ground to chassis and stay away from the thermostatic switch.

Don’t forget that you also need a resistor in series with the LED as ‘ballast’ or you will toast it when it’s operating. LEDs generate light more as a by product of current passing through than imposing a load like an incandescent lamp does. If you just wire an LED straight through, you are basically making a direct short through it and it will burn up. If you are using an LED with a built-in resistor, disregard my rant.

Here is a site that will help you choose the correct resistor value for your LED:

Leakage thru a thermostatic switch is hard to believe. Can you check the switch resistance with an ohmeter? Can you try a different switch?

What’s the voltage across the LED when it’s seeing only the leakage? Are you sure you have the lamp connected from switch’s “output” side to ground, and not wired across the switch?

If NOTA, then maybe there is a sneak circuit across the switch.

If there is enough current available to light an incandescent bulb, then a series resistor will probably not help.

Thanks for all the feedback.

First, yes, the light circuit is connected on the fan side of the thermostatic switch (TS).

Putting aside LEDs and resistors . . . a bit more fiddling around did not solve the problem but provided a bit more information.

First, a test showed +/- 3volts coming across the TS when the engine is revved but none with the engine off. Also, no continuity with the engine off, so the leakage is sneaky.

Now it gets to be more of a head scratcher. I did not have a relay on this fan since the fan kit didn’t come with one or recommend one. But I figured a relay would block the leakage from the TS.

I installed the relay using a standard fan relay circuit: pins #30 and #86 to positive power; pin #85 to the output side of the TS; pin #87 to fan pos input. And input to the TS is coming off ground.

I was very surprised to find the bulb behaving exactly as before. That is, when sharing the fan power output pin on the relay (#87), the bulb glows with the engine revved but without the relay activated. I’m no electrical engineer but this seems impossible to me.

I don’t understand the leakage through the relay but I do know the TS leaks, so I’m replacing it. Hopefully that solves the problem. I’ll report back.


You have the thermostat wired to trigger the relay by grounding the control circuit. When the TS is cool there would be battery voltage at 85 and when the TS is hot and closes the voltage would drop to near zero. Connecting the LED to pin 87 would appear to be the route to take.

Rod, the bulb is on pin 87 sharing it with the fan pos lead. Are you saying my circuit is right?

If your wiring operates the fan as desired and the LED is on pin 87 with the power to the fan the indicator should work. So yes, your circuit seems correct but where is the 3v feedback coming from?

Unplug bat+ from pin 30 and check voltage at the LED.

Reconnect pin 30 and unplug bat+ from pin 85 and check voltage to the LED.

Post the results.

3vdc is enough to trigger a 5vdc led. You need a 1k resistor in line with the LED. Tie the pos side of the diode to the fan power wire and the neg side to chassis ground. A 12vdc led will work.|pcrid|41566056141|plid|&CMP=KNC-GPLA

My guess would be that the fan is being spun by air passing through it and generating a voltage across the LED.

Whoha!!! BMW2007, I like your theory: Motor as generator, driven by the stock radiator fan! It makes perfect sense since I get no problem with the car engine off. I will test the circuit with the motor disconnected and report the results.

The LED indicators that I have bought at parts stores have been 12v and need no resistor when used in a 12v automotive circuit.

If Bat + is attached to pin 30 and 85, the fan attached to pin 87 and a grounded switch attached at pin 86 there should be 0v to the fan with the switch off and Bat+ at the fan when the switch is on. If a 12v indicator is attached to pin 87 and ground it should light when the fan is on and be off when the fan is off. I have installed fans just as stated here but never felt that an indicator was needed. The fan could be heard when stopped even when the engine was running.

Note for @Rich, If you use the AC compressor clutch POS connection at pin 85 the fan will only operate when the AC is on and the thermostat is heated above the threshold you set. But don’t connect pin 30 to that circuit as it will overload the AC compressor clutch fuse.

Thanks again to everyone for the ideas. In the end, BMW2007 nailed it. I checked the voltage coming off the freewheeling fan and that was the culprit – the fan was generating.

I rewired the TS and relay to activate from + output from the TS. Now the pilot light is connected to the TS output and the fan is connected to + coming off the other side of the relay. Problem solved. The light cannot get any juice coming from the freewheeling fan and everything is now working as it should.

Couple of notes: The fan TS is direct wired to the battery via fuse. I like the idea of the fan running on after the car is shut off as long as needed and this is how many factory installations are designed. As for the LED, although it doesn’t say that it has a resistor, I’m guessing that it does as it was sold for 12v application. If it dies early, I’ll know why.

Why the pilot light anyway? It is not always so easy to hear the fan come on at idle and for sure I don’t hear it go off when driving, so now I will know when it’s working as it should.