Auto Service World
Feature   June 1, 2008   by Prof. Tom Brown, Automotive Air Conditioning, Apprenticeship Training, Centennial College, Toronto

Clutchology 101

Sometimes knowing the basics can help one quickly track down a problem

Ever wonder how that electric clutch works on the A/C compressor? And what goes wrong with them? What causes that grinding noise, the smoking bearings and the shattered pulleys? Well, to understand what goes wrong, let’s look at how they work.

There is a coil mounted on the front of the compressor that gets current from the A/C clutch relay. It becomes an electromagnet when it is energized.

The coil fits snugly inside the pulley, which transfers the lines of magnetic force into the steel of the pulley. Since what was taught in trade school about the behaviour of magnetism is actually true, the lines of force are always trying to circle the coil. We know that magnetism travels well in steel (ferrous metal) and poorly in air. So the coil always has a steel bottom and sides, but an open top. For weather protection, the magnetically open top of the coil is covered with epoxy, ceramic or plastic.

After the lines of force jump into the pulley to traverse the top of the coil, they begin their journey towards the centre. But what’s in the way? A huge air gap, over, 0.100″ in size. That’s those slots you see in the pulley. Well, what’s closer? The hub if it is properly gapped at about 0.020″. So now the lines of force jump up in to the hub, pulling it down. As they travel along the hub towards the centre, they encounter another huge air gap. So now the pulley is closer, so down they go. This process is repeated twice on some and three times on others. Since a change in direction is called a pole change, the ones with three slots on the pulley are called six pole clutches and the two slot type are of course, four pole.

Well, the pulley is full of slots which means its not solid, sort of like a spoked wheel. Some applications cycle on-and- off so that the two surfaces continually wear, making the pulley surface thinner. Then it is subject to breakage when under extra load, as with a seized belt tensioner. A broken pulley is a sure sign its time to change the belt and tensioner too.

What goes wrong with them?

Broken pulleys

Seized bearings

The bearings usually seize because they were overheated and the seals and grease burned out. That’s from slippage between the pulley and the hub. Look for excessive high-side pressures and low voltage to the coil.

High side pressures can get out of control by faulty fan operation. If it’s a clutch type fan, look for leakage of the silicone. See if it is locking up when the high side begins to rise. Electric fans are triggered by a pressure sensor on the high side. Does it turn the fans on before the pressure hits 300 psi?

Poor air flow makes the high side go up too. Every two years, its time to power wash the condenser and rad, both front and back. That means separating them and flushing out all the dirt, leaves, plastic and cardboard that has accumulated in there.

If all these potential causes are covered, how about air in the system or an overcharge. This could only happen if the system was serviced in the past year and the technician was sloppy, not vacuuming properly, purging the air from the service lines or bleeding the air from the recovered refrigerant. Overcharging, well there is a tag under the hood to tell the tech the correct charge.

Low voltage can be the result of rotten wiring or connectors, burnt up relays or low system voltage from an ancient battery. Be sure to check hot and ground sides at the clutch connector.

Melted plastic

The top of the coil must be magnetically open to allow the flux lines to pass into the pulley. But, some have a plastic top. If the belt is slipping, the heat generated can melt the plastic and then it oozes through the pulley slots and hardens. That’s now a seized compressor. If you hear the belt squealing, there’s a comeback in the making.

Air gap too big

There has to be an air gap between the hub and the pulley so that the hub can pull away when disengaged and it is clear of the spinning pulley. In most cycling systems, this air gap increases as the clutch engages and disengages, so metal is worn away and the air gap increases. Then each time the clutch tries

to engage, the lines of magnetic flux must jump over a larger air gap to pull the hub into the pulley. The air makes the magnetic pull weaker so the hub spins against the pulley as it engages. That causes friction and heat, so eventually the bearing overheats and burns up.

This is something that can be checked easily by a tech. Simply measure the air gap and if its over 0.025″, take the hub off and change the spacer shims underneath to get it back to 0.020-0.025″.

Internal compressor problems

When a compressor seizes, often the clutch burns up too. If the compressor shaft is seized and won’t rotate, its time for a new compressor. But remember that simply replacing a seized compressor fixes nothing. The cause of the failure must be diagnosed and repaired or the replacement will burn up too.

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