Truck air systems

Jan. 1, 2008
Cold weather has an adverse impact on trucks and can create a multitude of problems, including weak or dead batteries, frozen locks, problems with fuel

Cold weather has an adverse impact on trucks and can create a multitude of problems, including weak or dead batteries, frozen locks, problems with fuel flow, and air system freeze-ups. To help fleets maintain optimum air system operation, officials at Bendix Commercial Vehicle Systems offer some basic tips.

The first is to avoid idling the engine for extended periods of time. “Winter idling is a major factor in compressor discharge line freeze-ups, and account for a significant number of compressor failures annually,” they say.

Discharge lines

Discharge lines should slope downward from the compressor discharge port, without forming water traps, kinks, or restrictions. If the line crosses over from one side of the frame rail to the other, it should occur as close to the compressor as possible.

The maximum length for a discharge line is 16 feet, note the officials.

For pick-up and delivery and line-haul trucks, the last three feet of the discharge line, including the fitting at the end of the discharge line, must be insulated with ½-inch thick closed cell polyethylene pipe insulation.

System leakage

The Bendix officials recommend checking the air brake system for excessive leakage. Excessive system leakage causes the compressor to pump more air and moisture into the brake system.

Reservoir draining

For an air system without an air dryer, routine reservoir draining is the most basic step - although not completely effective - in reducing the possibility of freeze-up, say the officials. All reservoirs in a brake system can accumulate water and other contaminants, which must be drained. “The best practice is to drain all reservoirs daily.”

The suggested procedure: After turning off the truck's engine, drain all of the air from each reservoir. An alternate method: Open the drain cocks on all reservoirs and leave them open over night to assure all contamination is drained.

If automatic drain valves are installed, their operation should be checked before the weather turns cold. “While the need for daily reservoir draining is eliminated through the use of an automatic drain valve,” they say, “periodic manual draining is still required.”

Air dryers

Proper air dryer operation is “critical” to helping the air system run clean and dry. The officials say to confirm the proper operation and function of each vehicle's air dryer, and to change the desiccant cartridge and purge valve as necessary.

Air lines

Some dos and don'ts for preventing air lines from freezing, and for thawing them in the event they do:


  • Thaw out frozen air lines and valves by placing the vehicle in a warmed building. “This is the only method for thawing that will not cause damage to the air system or its components,” say the officials.

  • Use dummy hose couplings on the tractor and trailer.

  • Check for drooping air lines, which could form water traps.


  • Do not apply an open flame to air lines or valves. “This practice is unsafe and can result in a vehicle fire,” they warn. It can also damage the valves and melt the air lines.

  • Do not pour fluids into air lines or glad hands. Certain fluids can cause immediate and severe damage to rubber components.

    “Fluids poured into the system wash lubricants out of valves, collect in brake chambers and valves, and can cause malfunction,” say the officials. “Loss of lubricant can affect valve operating characteristics, accelerate wear, and cause premature replacement.”

  • Do not immediately park a vehicle outside after thawing its air system indoors. Condensation will form in the system and re-freeze. Place the vehicle in operation when it is removed to the outdoors.

Air, electrical lines

Ensure tie wraps are replaced and support brackets are re-attached, if removed. These items prevent the weight of ice and snow accumulations from breaking or disconnecting air lines and wires.

Assuring reliable transportation in cold weather, say the Bendix officials, boils down to performing good preventive maintenance on all vehicles.

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