Frozen Air System?
Check the separator's purge valve.
TOM BERG
EQUIPMENT EDITOR
Cold weather brings many miseries, including an occasional frozen air system on a truck or tractor. This is most likely to happen after the rig has been parked overnight in bitter-cold temperatures. The engine starts, but the brakes won't work.
It doesn't happen near as often as it once did, because most air-braked vehicles come with air dryers that strip compressed air of moisture and other contaminants. These can freeze or become sluggish enough to interfere with proper operation of air valves and other parts.
If it happens, a guy might first assume that the air dryer's not working. Well, not quite. The technical brains at Bendix Commercial Vehicle Systems say the "help" calls they get are most often traced to a frozen purge valve on the dryer. This valve can stick partially or completely open, allowing air to escape when it should be going into the system.
Most purge valves have a heater that keeps the valve from freezing, but it takes a little while to heat up on a cold morning. If the engine's been started and the compressor's running, air could be vented from the system by a balky purge valve. So shut off the engine and turn the ignition back on for two to three minutes to give the heating element time to heat. The colder the temp, the longer it'll take, but it shouldn't be more than five or so minutes.
Drivers can avoid this hassle the night before with a trick suggested by Bendix people. After shutting the engine off, fan the brake pedal until system pressure drops below the compressor cut-in point. Usually that's 100 psi, so fan the brakes until pressure drops to 90 psi to be sure it's well below cut-in. This will close the purge valve and it'll still be closed in the morning, when the compressor begins running.
If you've still got problems, look at the air dryer. Does anything seem amiss? Does the air dryer purge as it is supposed to or does it cycle continually between the purge and charge modes? This can indicate a problem with the air dryer, or it may be caused by system leakage.
Does the air dryer make an odd sputtering sound after the initial purge has occurred? This can indicate a problem with the compressor unloader mechanism. A desiccant cartridge change may be in order if, during the periodic reservoir draining inspection, there are indications water and/or oil is accumulating in the reservoirs.
Warming parts of the system may be necessary to get it working. The best way is to move the vehicle into a warm building. If that's not possible, get a "torpedo" heater -- never an open flame -- and direct its hot air to valves, tanks and low-hanging hoses, where moisture can collect. Take note of those dips in air lines and at the first opportunity remove them so that contaminates, water included, will fall to the lowest point, the reservoir. Be sure to drain air tanks every day, to expel most of the moisture in them.
The first reaction of old-timers to a frozen system might be to grab a can of alcohol and somehow pour a bit of the stuff into the lines, like through a trailer's gladhands. We won't cover how to do that because it is not recommended by people who make air valves and other sensitive components in the air system. Alcohol, in liquid form, can really cause problems. Remember, this is an air system and the components in it are designed to function with air, not liquid.
However, some older trucks were equipped with methanol-alcohol injection systems. If your trucks have one, be sure they're operating properly and that the injector reservoir has enough approved fluid in it.
Trailer air systems can be contaminated if their gladhands are left open as trailers sit outside in wet weather. Dust, dirt and other crud can also get in that way. Yes, the tractor's air dryer should eventually dry the air, but it cannot remove the solid stuff. So use dummy gladhands to keep stuff from getting in, or consider gladhands with shut-offs.
Bendix makes an air dryer for trailers that tend to be pulled by dryerless tractors, but the trailer's air system still needs periodic "cleaning" by a tractor with a functioning air dryer.
Contaminants can get into a truck or tractor's air system in a variety of ways. Air comes through the vehicle's air cleaner, so a dirty element can cause contamination, as can a worn or defective compressor, which can pump in excessive oil. The air dryer should catch this stuff, too, but only if it's working properly.
Overall, prevention is the best cure. A properly maintained air dryer will keep the system dry and clean. Therefore, get instructions for the type of unit you have and follow them. Do that and use the other tips, and your trucks should always be good to go.
