An air dryer purge occurs at about the same time as
- a full application of the service brakes
- compressor cut-out ✓
- compressor cut-in
- application of the parking brakes
Answer: B. compressor cut-out
An air dryer purge happens at about the same time as compressor cut-out.
Why this is the answer
The functional inspection checklist says the air dryer purge also signals compressor cut-out. The air dryer section explains that moisture collected in the dryer is expelled with high-pressure air when the compressor reaches cut-out pressure. The governor testing section adds that when a vehicle has an air dryer, its exhaust cycle indicates the compressor has reached the cut-out setting. Together these passages tie the purge to the moment the compressor stops pumping air at cut-out, not to a brake application or to cut-in.
Why the other answers are wrong
- A. a full application of the service brakes — A full application of the service brakes is not what the passages link to the dryer purge; the purge is tied to the compressor reaching cut-out pressure.
- C. compressor cut-in — Compressor cut-in is when the compressor starts pumping again, while the passages say the dryer purge signals cut-out.
- D. application of the parking brakes — Applying the parking brakes is not connected to the air dryer purge in the passages; the purge happens when the compressor reaches cut-out.
Remember it
Listen for the whoosh: the dryer purges when the compressor has finished pumping, not when it starts.