[Greg Elmassian]

I'd make it run full speed whenever the loco is "on" just because power on dcc is not usually a problem. I don't think paralleling with the motor is the best way, you can load the motor at lower voltages, and the fan might be going too slow. A function output that is on whenever the loco is moving might be ok, but would shut the fan off even if you just finished a very hard pull.

Regards, Greg

 

[Greg Elmassian]

I don't know what the current draw of the fan is. If it's low enough to run from a function output, then you can pick a function and map it to something that is "on" in forward and reverse. I suspect if that was hard to find, 2 outputs, one on in forward and one on in reverse, combined with a coupler of diodes would do the trick. (sort of like if either the headlight was on or the backup light was on the motor was going)... in this way, you might find a way to do what you want.

If the fan draws too much, then a small relay would do the trick.

This method does not use a function button, only a function output, which many decoders have plenty of.

Regards, Greg

 

[Greg Elmassian]

Wire the decoder common to the fan positive
wire the fan negative to 2 diodes, the connection point would be 3 wires, the fan, and the cathodes of 2 diodes.

Now take one of the diodes (anode) and connect to the F0 output, and the other to the output for the backup light.

Now the fan runs whenever the loco is in forward or reverse.

Regards, Greg