Originally Posted by Montegue
My plan is to keep the original switch and knob, but just mechanically limit it to two positions, ON & OFF.

I'll then use it to ground the coil in the forward relay on this inexpensive 12V 60A PWM motor speed controller.

[Linked Image]

https://www.ebay.co.uk/itm/166609762129?itmmeta=01HQMQ959DHV1B9PRKFCKH68QD&hash=item26cab73751:g:dqoAAOSwr2Zl1ksg&itmprp=enc%3AAQAIAAAA0NKPvzZVtPzZLsrOCjbNAIhC9juse%2FBuvF6yroWAKwSFLIVitNdTy9rC9CI59QucmRzhdnLyLj0ql4CuImguEqYIBbLaxv3jg8EFVRriegVlkgGCLRjDZMQgj8y9jjmMJllRxlq9CgAtQk3TWy6CO6zsFkVN%2FtV0Vht58GGveAT76zpNh8UbbXlOwZPuRrjhTqjWZG0IKyoDFeKfn42%2BYdlngQjgqR8Up9pszALVzXCKTu%2Fzo5Mc%2Bbu3cNHr%2BWLScLnf8u8P8WUzeYiaU7%2FaX%2FM%3D%7Ctkp%3ABlBMUJLVpJe9Yw

All the higher amp rated PWM controllers seem to have a forward and a reverse relay on the board, but obviously I only need to drive the fan motor in one direction.

[Linked Image]

The controller with its surface mounted relays allows me to bypass the problematic PCB, and with the potentiometer mounted in a convenient position under the dash I will enjoy infinitely variable speed control over the heater fan, from super slow to full blast.
A novel solution. Just make sure that those heat sinks are well. away ( and stay away) from any wiring insulation when it’s fitted into the confines behind the dash. 🔥

Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900