A few basic reasons why an air conditioning clutch does not engage, even if you have a power supply down to the compressor.

Let’s first of all understand how the compressor starts to ‘pump’ when we switch the air conditioning on inside the car.

At the front of the compressor, within the pulley, is a magnetic coil. When this is energised it pulls in the front plate of the compressor with a distinctive ‘click’. The refrigerant is then pumped around the system.

But what happens if we do not get this distinctive ‘click’ and there is power to the coil:-

1 BAD EARTH: the coil relies on a good earth to complete the circuit, check this by proving the earth circuit.

2 AIR GAP: if the distance between the pulley and the clutch plate is too far, the magnetism of the coil will not be able to ‘stretch’ across to pull the clutch plate in . A normal type of air gap is between 0.5mm and 1.5mm. If the air gap exceeds this level, you will have to replace the clutch plate and probably skim the pulley to remove any ridges.

3 COIL FAULTY: This is where most people go wrong, they measure the coil for continuity – bad mistake. YOU NEED TO MEASURE THE COIL FOR RESISTANCE. If the resistance of the coil is incorrect you will still probably get a continuity reading.

Lets see how we can prove this:- 

It goes back to Ohm’s law: V=1xR

V=voltage of car (12 volts)

1=current (amps) to coil

R=resistance of coil (ohms)

The current (1) to nearly all 12-volt cars is 4 amps. So the resistance should be 3 ohms. Measure this on; your multimeter to ensure it is correct.

Let’s see what happens if the resistance is low (say 1 ohm)

V=1xR 12=1×1 the current would be 12 amps- blown fuse!