Pressure Build-Up and Air Dryer Problem at Minot
At a Minot terminal in February, a 2015 tractor with a 2012 trailer has an air-pressure build problem. With the engine at 1,200 rpm, the dash gauge rises from 85 psi to 100 psi in about 3 minutes, then stops at 100 psi. The governor cut-out is supposed to be around 125 psi, but you never hear it cut out. The air dryer purges every 20 to 30 seconds, much more often than normal. When you drain the wet tank, you get a cup of water. The compressor belt is tight and not slipping. The truck has a rebuilt compressor installed last month, and the shop is backed up. The dispatcher wants you to take a short 40-mile bobtail run to see if it clears up.
1. Which components control air-pressure build-up and cut-out, and what are two plausible causes of these symptoms?
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Components involved: compressor, governor/unloader, air dryer, reservoirs, check valves, and the safety valve. The compressor makes air; the governor tells it when to stop at cut-out, about 125 psi, and restart at cut-in; the dryer removes moisture and purges; check valves protect circuits. Plausible causes: (1) a faulty governor or leaking governor signal line, causing short-cycling, a low effective cut-out, and frequent dryer purges; (2) a sticking air-dryer purge valve or saturated desiccant, causing constant air loss, frequent purging, and water carryover. A discharge-line leak or weak compressor could also limit build-up. The rebuilt compressor is not proof of a good system.
2. Why is the dispatcher's short bobtail run unsafe, and what should be done instead?
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The run is unsafe. The system is not reaching cut-out, is purging abnormally often, and has water in the wet tank in February. Water can freeze in lines, valves, or the dryer and cause dragging brakes or a complete loss of braking on one or both circuits. A 40-mile run will not fix a mechanical purge, governor, or compressor fault; it only puts the truck on the road with a known defect. At 100 psi the spring brakes may release, but pressure can fall below 60 and then into spring-brake range. Drain all reservoirs, inspect and repair the dryer, governor, and compressor or leak path, then perform a proper build-up and leak test before dispatch.