Many air compressors are fed turbocharged air right out of the intake manifold. This air has been slightly compressed by the turbo and has gone through the charge air cooler, so this makes the compressor more powerful and efficient. Because of air compressor turbocharging, the purge valve has another function. It incorporates a turbo cutoff piston that closes off the supply port through which air normally flows from the compressor into the dryer. This keeps this air from escaping through the dryer and out the exhaust port while the compressor is off. It also eliminates the chugging sound that would come out of the exhaust port even with normally aspirated compressors when the compressor is off.
Listening for a regular purge cycle can alert you to air dryer problems, says Canale. If the purge cycle occurs too frequently, “that noise may be a symptom of another problem. Do deeper troubleshooting,” says Kern.
Air system leaks, including leaks within the dryer itself, can cause the compressor to cycle more frequently. Leakage causes the unit to come on after less off time and stay on longer. If the leak is big enough, it will stay on all the time. In this case, says Kern, “Figure out where the leak is. It could be in the dryer or anywhere in the system.”
To find the leak, run the engine to charge the system to the point where the governor cuts out. Then listen to the purge of air from the dryer. There will be “a significant initial blast,” indicating that the dryer is purging freely, Canale says. Listen, but don’t feel for this because you could get injured by debris. But once the blast slows down, put your hand under the exhaust port. You should feel a slight flow for 15-30 seconds, after which it will shut off. If flow continues, the most likely cause is a failed turbo cutoff valve, assuming the air compressor is turbocharged. You can quickly determine you have this type system by tracing the inlet line to the air compressor. If it comes from the engine intake manifold, you have a turbocharged compressor. If it comes from upstream of the turbocharger (for example between the air cleaner and the turbocharger), and the turbo valve has failed, you’ll hear a chugging noise. In a few cases, leakage during the off cycle can result from a failed check valve on the dryer discharge line leading to the brake system.
Next have someone dump system pressure by fanning the brakes. Soon you’ll hear the compressor cut in. After the system charges back up to nearly the cutoff pressure, shut the engine off. Now feel for air escaping from the purge valve’s exhaust port. If there is leakage from the port, the problem is a failed purge valve. Leakage elsewhere in the brake system, for example in tank drain valve O-rings, will tax the compressor and dryer and should be fixed. Leaks can be located by coating every connection with soapy water, which will bubble from the leak.
Another common cause of too-frequent purge cycles is a failing air compressor governor or “unloader” as it’s sometimes called. Kern explains that it’s supposed to cycle the compressor off and send a signal to the dryer to exhaust at 120 psi. After that, the brake system uses the air in the tanks till the pressure has dropped to 90 psi. Then it cuts back in. The governor “is a likely item to fail, but low cost. It can begin to send the signal too often as it wears, causing frequent cycling.” Even though the air gauge on the dash is normally connected to the secondary air storage tank, and the governor to the primary one, you will likely notice the indication moving up and down less if this is the trouble.