Failure Analysis

As the manager of warranty and product support at a major HVAC systems and component supplier, I see a lot of things I just shouldn't see: compressor clutches that look like an exploded stopwatch (with copper windings bursting out all over the place), melted clutch bearing seals, burned-up epoxy potting-compound around the coil seal.

I'm not saying these things are a shock to my senses. I mean, I literally should not be seeing seized bearings, melted shaft seals, and other heat-related failures because they aren't typically covered under warranty. Warrantable failures involve the root cause of the problem.

What then causes A/C clutches and compressors to fail prematurely?

One of the leading reasons trucks repeatedly experience short A/C clutch or compressor life is low voltage at the clutch coil lead wire. On a 12-volt system, the compressor's clutch needs a minimum of 11.5 volts to create the electromagnetic field necessary to engage the clutch pulley so refrigerant can flow.

Inadequate voltage will cause the clutch hub to slip against the face of the pulley, generating friction and heat in excess of 1,000 degrees F. It can quickly melt the clutch bearing seal, which causes the loss of bearing lubrication, as well as the epoxy potting compound that seals the coil.

Poor ground wire connections are an obvious suspect in voltage-related problems. But more and more, truck owners and shade-tree electricians are tapping into the electrical system to power auxiliary items like radios, marker lamps, heaters, and appliances. And when they do, they risk stealing the voltage the clutch needs to do its job.

Standard alternators are around 85 amps. When they're required to supply more than their capacity, the additional power comes from the batteries. As the batteries drain, power to components like the compressor clutch drops from 13.5 volts to less than the required minimum of 11.5 volts.

This causes the clutch to not fully engage. You get slippage, friction, and heat. Eventually, the clutch fails. So does its replacement. When the clutch seizes and the compressor locks up, the chances are good that you'll throw a belt. If you're lucky, you'll lose only your air conditioning. In some cases, you may snap a double-V or multi-groove belt that drives other important components, like a fan, water pump, or alternator.

Troubleshooting electrical problems is never easy, but there's one essential step you can't miss: you need to recreate the peak demand for voltage when you make your diagnosis. With the engine and air conditioner running, turn on lights, wipers, radios—anything that draws from the truck's electrical system. Then take a reading. Ideally, we want 13.5 volts but not less than 11.5 volts through the clutch coil.

With regular preventive maintenance and the proper amounts of oil, refrigerant, and voltage, the compressor and clutch assembly should last the life of the vehicle. Instead, compressors are the Number One warranty-cost item in an air-conditioning system, because people wait for the system to fail before they fix it.

That can be an expensive job.

First, clutches and compressors are such a tightly integrated package now that when one component fails you're faced with replacing the entire assembly. Second, you're having to replace a part before the end of its expected usable life. The part didn't fail; the system failed the part, because of a poor ground wire connection or insufficient voltage. Finally, replacing the compressor involves pulling down the system, which adds refrigerant recovery and recharge to your repair cost.

Several companies offer devices that protect against compressor failure. For example, my company makes a device that monitors refrigerant pressure and shuts down the system in case of refrigerant loss, excessive overcharge of refrigerant, or low voltage. It also has built-in diagnostics and communicates with the SAE J1708 serial bus.

Integrated system protection may be required if you want an extended warranty for HVAC system components. As a technician or maintenance manager, these compressor protection devices can help you troubleshoot problems. And they can remind you that when you add electronics on the vehicle (or when one of your drivers does it on his own), you shouldn't rob voltage from Peter to pay Paul—in this case, the clutch on the A/C compressor.