Esc won't power on with sensor lead connected?

[Monkeysmate]

Right. I've a strange problem, which I'd be grateful of any advice on... I'm just shuffling motors and esc between cars, and the toro 120a esc will not power up when the sensor lead is connected to the motor. If I disconnect the sensor lead from the motor, the esc powers up fine. I have tried it with another motor, and it powers up fine when connected...
So it would appear to be the original motor that's the issue, but what would be the fault?
Can anyone help please?
Thank you.