I'm a novice on this stuff, so go easy on me!

I built a car several years back. I wired in an LED that lights up when the cooling fan is on - but, it sometimes comes on even when the fan is not powered. This generally occurs when I'm driving at highway speeds. It is a 12v DC, negative ground system like almost all modern cars.

I am assuming airflow through the fan is causing the blades to spin the motor enough for it to generate power.

So, I'm thinking I need a diode in the circuit to prevent that from reaching my LED (and presumably the rest of my electrical system too).


  1. Does it make sense that a diode is the right solution?

  2. If so, would I need a rectifier or zener diode?

  3. Is it safe to assume I just need to select one rated to handle the max automotive voltage (about 14.5v DC) and the max current the fan draws (I'm sure it's not spinning as fast as it does when it's powered on high)?

  4. Is the diode then wired inline with the power wire or across the power and ground?

Thanks in advance.

  • \$\begingroup\$ How to fix the LED being turned on depends on how you have wired it. Show a drawing how the LED is connected to the system. \$\endgroup\$
    – Justme
    Jun 28, 2021 at 18:45
  • \$\begingroup\$ How do you know the fan is not powered while driving at highway speeds? I could not tell when the fan was running in a 1991 Mustang. In that car a temperature sensor would send a 5 volt analog signal to the computer. The computer would use some algorithm to turn on an open collector transistor to activate the fan relay under the hood. I wanted to wire an LED to the open collector output and a parallel switch that could ground the relay instead of the transistor for manual fan control. But I never implemented the system due to hassle of running wires. I agree a wiring diagram would be helpful. \$\endgroup\$ Jun 28, 2021 at 20:48
  • \$\begingroup\$ I'll have to figure out what I can use to draw the diagram to post it but it's pretty straight forward. The fan has 3 wires. One is ground, one is power for low speed, and one is power for high speed. Ground is connected to chassis ground. Each of the power wires are connected to the load side of a relay (the #30 pins) (2 relays; one relay for low speed and one for high speed). The other side (the 87 pins)coils and the \$\endgroup\$
    – user40603
    Jun 29, 2021 at 20:54
  • \$\begingroup\$ I inadvertently hit enter and now it won't let me edit the comment. Just disregard and I'll figure out how to do a diagram. \$\endgroup\$
    – user40603
    Jun 29, 2021 at 21:03

2 Answers 2


I think you've pretty much got it.

I'm not sure how much an automotive cooling fan draws, but it might be hard to find a suitable diode. If it's drawing 10's of amps then the 0.7V drop of a diode can result in a lot of heat (10A * 0.7V = 7W).

The diode type doesn't much matter for this application, just focus on the current, voltage and power ratings. Besides meeting your voltage and current requirements lower voltage drop is better.

I would add the diode inline in the positive lead, since that's the insulated part of an automotive system.

One more thing, the fan could potentially generate HIGHER voltage than the alternator if it spins fast enough. Probably not much higher but I would give the diode some voltage margin.

There may be a relay somewhere that operates the fan, if you can tap into the signal that goes into the relay, then you can bypass the problem.


Car fans are usually quite beefy but your idea in principle is correct. A rectifier diode in series with the fan wire would fix it if indeed the fan is working as a generator.

Working voltage is not an issue since your fan would probably not make much more than 10V (nominal supply should be 8-16V). A Schottky instead of silicon will usually have a lower Vf and 20-40V is a sweet spot for these. Be sure to derate the current since under the hood it goes really hot. Plan for an ambient temperature of 120°C. Luckily you are near a fan so there should be plenty of cooling air available.

  • \$\begingroup\$ Thanks for the replies. It occurred to me that the fan should not be back feeding my electrical system. It's wired through a relay. Only the fan and the LED are on the load side of the relay so while the fan is, I think, acting as a generator to power the LED, the relay wouldn't be energized so nothing should be passing through it back to the rest of the electrical system. Thanks again for the info; very helpful. \$\endgroup\$
    – user40603
    Jun 29, 2021 at 15:12

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