>>> Sorry for sounding daft but what does pwm stand for? Is it the speed of earth switching from ecu? <<<

Not the speed, that's called the frequency, measured in Hz.

PWM is the DUTY CYCLE within the blocks of time.

If we take one second in time and split it into 100 separate "blocks" of time each block would be 10ms long and the frequency of the signal would be 100Hz.

Now if we look at one 10ms "block" of time and provide an earth path to the consumer for 3ms of the 10ms period and remove it for the remaining 7ms we would have a duty cycle of 30%.

When we increase the "ON" time (time it is supplied an earth) to 5ms we will have a duty cycle of 50% and 7ms would give us a 70% duty cycle.

The result of doing this is that we (the ECU) can control the CURRENT flowing within the circuit and therefore the speed of a motor, the brightness of a bulb, the strength of an electro magnet etc etc.

Most Frequencies are fixed but watch out because others are not.

Regards

Mark

PS. I'm with Kevin and the training front, especially and time saved part. there is no way I could fix the vehicles I do without the training that I have gone on.