Hotpoint Quatro 1300 won't spin?

Motor works OK in wash cycle, but does not turn when it should be spinning. My mains tester lights up on all four power wires to the motor during the spin cycle (the middle two wires are for the tach sensor, which is giving a pulsed resistance as the shaft turns). I am also getting variable resistance at the spin speed switch when I turn the setting.

Although not an electronics person, I suspect that the motor is intact, and that the fault is somewhere on a switching device after the motor, i.e. on the negative wire. This would (I think) explain power to all 4 wires.

Can anybody help me with this? With no wiring diagram, a description of the switching (logic) sequence of spin control would be a great help.

Thanks in advance,

B