I can't get part cooler fan to work.MKS GEN L V1.1 / Marlin

Hello my first post here so be gentle!

I have built a FLSun I3 plus and, with some modifications, it works OK. I added a part cooling fan and plugged it in to the "Fan" / "D9" socket (= arduino D9 pin) and try as I might I can't get the fan to move.

I have tried:

Another fan.

changing this line in marlin/Configuration_adv.h
#define FANMUX0_PIN 9

Any ideas?

I am assuming using the touchscreen controller to set the fan to a non zero speed should work anytime. I have printed a PLA object to test too.

FYI:
The supplied hardware loaded with recent Marlin and configuration seems OK apart from this issue.
I added a simple "discount" controller as well as the touchscreen.
Control arduino mega RAMPS board is a MKS GEN L V1.1

Edited 1 time(s). Last edit at 05/17/2018 03:44PM by Tryit.

Question is, how did you configure your board? In Marlin you configure Ramps as EFB* or EEB or EFF, depending how you want to use the three MosFets.
Is there an option like this for the MKS board?

*E = extruder, F = fan, B = bed

Trying to see if I can figure this out - I am looking to find out how to send debug messages from marlin.

It works!

After hours dipping into marlin code debugging etc. it turned out the fan manufacturer had put the connector on the wrong way round!

Thanks for your help