Repeated Air Dryer Purge on a Loaded Tractor-Trailer
You are driving a 2021 Kenworth T680 tractor pulling a 53-foot dry van, gross combination weight 76,000 lb, eastbound on I-76 near Fort Morgan. At a truck stop, you build air pressure from 80 psi. The dash gauge climbs to 125 psi, then within 20 seconds drops to 90 psi while you hear a sharp, sustained hiss from the passenger-side air dryer. The hiss stops after about 15 seconds, and the cycle repeats every 2 to 3 minutes. The low-pressure warning has not sounded. The ambient temperature is 28°F, and the air dryer was last serviced 11 months ago. You have a delivery appointment in Denver in 2 hours and 110 miles to drive. The tractor's governor cut-out is specified at 125 psi and cut-in at 100 psi. There is no visible ice or water on the dryer.
1. Explain what is happening in the compressor, governor, and air dryer sequence, and identify the most likely failed component.
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The compressor charges the system until the governor reaches cut-out at about 125 psi. At cut-out, the governor unloads the compressor and sends a signal to the air dryer to purge moisture. A normal purge is brief, usually 1 to 2 seconds, and causes only a small pressure dip. Here the gauge falls 35 psi over 20 seconds and the hiss lasts about 15 seconds, which is far too long and too large for a normal purge. The most likely fault is a purge valve stuck open or leaking, or a purge valve held open by ice or contamination. The governor is probably still signaling cut-out because pressure reaches 125 psi, but the dryer is dumping stored air. Because the compressor can rebuild pressure afterward, the cycle repeats. The 28°F temperature raises the possibility of ice in the dryer, even without visible ice on the outside.
2. May you drive the truck to Denver as-is? Support your answer with pressure and brake-performance reasoning.
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No. A 35 psi loss in 20 seconds is a serious air leak, not normal dryer operation. The compressor is cycling every 2 to 3 minutes just to replace lost air, which means the system has little reserve. On a downgrade, in traffic, or during repeated brake applications, pressure could fall to the low-pressure warning threshold and then to the spring-brake application range. Even though the warning has not sounded yet, the vehicle is not safe to dispatch. Driving 110 miles with a known air loss also risks a brake emergency at highway speed. The correct judgement is to keep the vehicle out of service until the dryer or purge valve is repaired and the system passes a leak test.
3. State your immediate actions and what you should tell maintenance and dispatch.
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Secure the vehicle: park on level ground if possible, set the parking brake, chock the wheels, and do not continue the trip. Notify dispatch that the load will be delayed and tell maintenance exactly what you observed: pressure reaching 125 psi, a 35 psi drop in about 20 seconds, a 15-second hiss at the passenger-side dryer, and a 2-to-3-minute repeat cycle at 28°F. Ask them to inspect the air dryer purge valve, heater, desiccant, and governor signal line. If ice is suspected, the vehicle should be thawed in a heated shop before testing; do not pour hot water on the dryer or tamper with the governor. After repair, perform the standard leak check before moving.