156 selespeed pump

Hi guys,
I have a question about the 156 selespeed pump, where does the pump get its signal from to start, i thought that it ran continuous whilst driving but it cuts in a out when required. I would like to know what tells it to start.
Any one got any ideas

thanks
jim

the M1.3 ECU is the operator

I'm guessing that it is triggered by a drop in pressure so most be pressure sensor somewhere?

I think the pressure sensor must be part of the actuator.

But yes the Selespeed ECU triggers a relay (one of the 50A in front of the battery) which runs the pump.

Thanks for the info guys