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I’m installing two 12V 80W cooling fans in an old BMW which originally uses a PWM electric fan.

This original fan has a three pin connector (constant 12v, ground and PWM signal from the ECU). I gather it has a controller built into the fan which actually controls the voltage that the motor sees.

How can I use the PWM signal from the ECU to control the speed of my two new fans (identical and rated at 6.6A)?

Not sure if a low pass filter would do the trick or something else is required.

Thanks in advance!

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If the new fans have PWM control then they can be wired in parallel, PWM too. Can you source this type? It will make your life easier.

If not, the problem becomes that the new fans may not work well depending on how finicky they are about their voltage.

There’s two approaches.

  • Direct PWM chop
  • PWM to voltage conversion

The first approach might work if the fan motors can tolerate the PWM chop action. This gives a nice linear control with minimal losses. If they're brushed motors, no problem. If not.. you’d need to ask the vendor or try it experimentally to see if the fans will work with that chop. As near as I can tell, BMW uses about 300Hz chop from the ECU.

The second approach is problematic as the range of control will be limited by the fan stall voltage. And, it may not be practical on a fan that size due to losses in the conversion, unless you use a fairly sophisticated DCDC power supply that switches at a higher rate. It is possible to coax such a supply to work under PWM control.


More: did a bit of digging. Siemens / VDO part number 410-65008-01 is a purpose-built module for this. AC Delco makes one too, part number 15-80803.

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