@zapta Thanks for your interest, much appreciated,

Interesting pump. Looks like the 6 litre-per-minute brushless version with pwm and tachometer. At $9.50 each, I may have to grab a few to play with.

So I measured the voltage again. I plugged a connector with 2 cables at the fan1 pins and connected this to a multimeter.

If I measure between VFAN and FAN1- I see 7.6V when the fan is at 0% on the web control and 12V when the fan is at 100%.

@dc42 Thank you, that pointed me into the right direction.
I tried to use a different circuit for the Heatbed as the Main Vin of the board has a 6A Breaker.
The Heatbed is now switched externally by a separate Mosfet, this works perfectly, also directs some of the High Current Stuff away from the Duet.