have a look under the bonnet and see if the clutch is spinning when its on low! if its not tap it with a spanner and see if it then grabs? i had a problem with lack of voltage at the clutch and it wasnt enough to pull it over, i gave it a tap and it would fly across. with the engine off you can see the movement when you press it and this is done by electromagnet when power put to it.
you can check the voltage in the wires that disapeer under the pulley on the compressor to see what you have, i pushed the multi meter probes through the insulation of the wires to do this.....