Question - when the engine is running, the belt turns at full tilt from the crank pulley to the a/c compressor, but the compressor itself is only turning at half speed, and intermitently. Should the compressor be turning at full speed too? Do I need to retension it? Turning on the a/c doesn't seem to change the engagement of the belt either.

 

There is a clutch that cuts it in and out as needed.

 

With the clutch engaged,the inside of the clutch drive should be turning at the same speed as the outer part, the sheave that the belt rides on. With the clutch disengaged, the inside should not turn at all.

 

OK, that makes sense. Should the outer sleeve turn at the same rate as the crank pulley?

 

Not sure what you mean by speed. Short answer is yes. If the pulleys are the same diameter, their speeds will be the same (one to one ratio), and as Dr. Bob said, the outer turns continuously whenever the crank turns. With the A/C on, when the sensors call for cooling, the inner should be locked to the outer pulley engaging the compressor. If the inner isn't turning the same speed, it is slipping and will burn out, YMMV. I believe it could occur if the electrical impulse is somehow compromised (not strong enough to hold the mechanism to the outer pulley) or the compressor (bearings?) is/are locking up. Do you hear any squeal or whine when the inner is turning?

 

Ed--

The electromagnetic clutch on the front of the compresor is a strictly engage or disengage item. There's not supoosed to be any slippage. The belt, unless it's slipping on either the crank or clutch pulleys, will turn at crank pulley speed and will turn the clutch at whatever speed the belt is moving at. It looks like the compressor spins at a greater speed than the crankshaft, since the pulley on the crankshaft is larger than the one on the compressor.

The clutch is triggered by twelve volts, and is supposed to be just on or off. There's a relay in the AC control head in the dash that's a notorious weak spot in the system. The relay is undersized for the duty, and over time will pass less and less current to the clutch until at some point it will fail to engage. If it's 'sort of' working, the clutch may slip, so if it is you can do some soldering-iron surgery on the control head or get another one. Lots of documentation on the fix at the various tips pages.

 

It seems I have an issue then. The pulley on the compressor is turning erratically by the belt - it looks like it is very loose, and is slipping quite a bit. It isn't making any noise, and the belt feels nice and tight, but the pulley is turning in fits and starts, and at a much slower speed than indicated by the belt velocity. Maybe I'll tesion further and see what happens. Maybe this is why my a/c doesn't feel strong.

 

Make sure you have the proper refridgerant charge in the system. The clutch could be engaging and dis-engaging very quickly if the pressure switch is sensing low refridgerant. This could give the impression of "fits and starts"

 

Ed--

Before you add tension to the belt (that won't help unless the belt itself is slipping...), grab your voltmeter and get into the area at the base of the windshield under the hood, under that plastic rain shield. In the middle and forward (towards the front of the car) against the firewall bulkhead, there's the anti-freeze switch with two wire connections and a capilary tube coming out. The maybe 1.5" cube silver switch hangs from a sheet metal bracket. Current passes through that switch between the control head relay and the compressor clutch. Get a meter connection to ground and the other to either of those wires on the anti-freeze switch, and test for DC volts. Turn the key on to 'engine run' position, AC button depressed, and you should see full battery voltage on both of the switch terminals, and hear the compressor clutch clank as it engages. You don't need to actually run the engine while you test, but the wires need to stay connected to the switch while you test. As that control head relay deteriorates, less current will pass through it, and that shows up as lower voltage in the clutch coil circuit. If you see less than battery voltage when testing this way, it's a pretty safe bet that the control head relay is failing.

There's some more troubleshooting if you find full voltage there but no clutch actuation, since the clutch coil current passes through the anti-freeze switch there, the pressure switch on the high-side plumbing next to the receiver-dryer (in front of the radiator and condenser), then back through the 14-pin connector on the fender wall, through that and into the FOE (Front of Engine) harness, and finally drops out by the oil dipstick to a spade connector where the actual clutch coil wire connects to wire in the harness.

 

I've seen instances where the infamous AC relay provided 12V through the freeze switch and low pressure switch to the clutch but the amperage was very low and the clutch would not actuate. It should get 3 amps or so.

 

thanks Dr Bob - will try that test this weekend.

 

Ed...............given the recent coolant leak that was right above the a/c belt that is perhaps the slipping issue and hopefully an easy fix

 

The one's I've looked at, with resistance high in the relay contacts, all present low voltage at the freeze switch terminals. Load in ohms at the coil is the same (~~4-6 IIRC), so the drop in current flow due to added contact resistance means lower voltage.

My friends 'Fig' Newton and Judge Ohm make the laws around these parts, pahdnuh. Or, "Resistance is Futile".