This is interesting. If I have time tomorrow I will look at the diagram. What year is it? Typically on the ECC the voltage is constant and the ground is controlled through resistor (the gizmo mounted to the left of the motor with the cooling fins). Volvo is notorious for bad solder joints in the electronics.
Sometimes you may even read 12 volts until you put it under load then the resistance is so high at the loose pin that no current can be conducted through it.
It could be a bad relay with loads of resistance in the contacts also. Just don't know exactly. If I find out more will pass it on.