If you look at the signal with a scope you will see a square wave at battery voltage with a constant frequency and a varying duty cycle.
What this means is that the valve is being pulsed against the spring that opens it. Depending how your volt meter sees this one usually sees the average voltage with a 50% duty cycle and 13v idle voltage most meters would see 6.5volts. The controller has the capability to go to 70% duty cycle on or off (I think). This would place the control range between 9.1 and 3.9v with an averaging voltmeter. Instantaneously the voltage is either battery voltage or zero.
Bosch Master, ASE Master, L1
33 years MB technician