Five free practice questions on engine brakes: content area H of the ASE T2 Diesel Engines test, 3 of its 55 scored questions (5.5%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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What does a compression-release engine brake do to slow a truck?
- Opens the exhaust valves near top dead center of the compression stroke
- Closes a valve in the exhaust pipe so the pistons push against back pressure
- Closes an intake throttle so the pistons pull against manifold vacuum
- Cuts fuel so the engine's own compression strokes hold the truck back
Show the answer
A. Correct. The piston uses crankshaft energy to compress the air, and the brake opens the exhaust valves just before the piston would be pushed back down, so the compressed air escapes instead of returning that energy. The work of each compression stroke is lost, and that loss is what holds the truck back.
Why not the others
B. That describes an exhaust brake, which restricts the exhaust so the pistons work against back pressure on the exhaust stroke. A compression-release brake works on the compression stroke by opening the exhaust valves near top dead center.
C. That is closed-throttle braking on a gasoline engine. A compression-release brake gets its effect by releasing the compressed air near top dead center of the compression stroke, not by creating intake vacuum.
D. Cutting fuel alone retards very little, because the air squeezed on each compression stroke pushes the piston back down on the next stroke and returns most of that work to the crankshaft. The brake opens the exhaust valves near top dead center of the compression stroke so the compressed air escapes and the work is lost.
ID T2-H-001 · Task T2-H-1 (ASE task list)
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A tractor's compression brake, designed to turn off whenever the clutch pedal is pressed, applies with the switch on and the foot off the accelerator and turns off when the accelerator is pressed or the switch is turned off, but stays on when the clutch pedal is pressed. This could be caused by:
- a dash switch with contacts welded in the on position.
- a clutch switch that never signals a pressed pedal.
- a solenoid valve sticking in its energized position.
- a pedal sensor reading zero with the pedal pressed.
Show the answer
B. Correct. The clutch switch tells the control system that the pedal is down so the brake releases during shifts and cannot pull the engine down toward a stall. Every other release path works, so only the clutch input is failing; check its adjustment and its signal with the pedal pressed.
Why not the others
A. A dash switch welded on could not turn the brake off, yet the stem says turning the switch off releases the brake. The fault is limited to the clutch input, so the clutch switch is the cause.
C. A solenoid stuck energized would keep oil flowing to the brake no matter which input asked it to release, so the accelerator and the dash switch could not turn it off. Only the clutch input fails here, which points to the clutch switch.
D. A pedal sensor stuck at zero would keep the brake on while the driver is on the accelerator, but the stem says pressing the accelerator releases it. The input that fails is the clutch switch.
ID T2-H-002 · Task T2-H-2 (ASE task list)
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On a tractor with air brakes, the exhaust brake's butterfly valve is moved by an air cylinder. What closes the valve when the driver turns the exhaust brake on?
- Engine oil pressure, admitted by a solenoid-operated control valve
- Turbocharger boost, admitted by a pressure-operated check valve
- Air from the truck's air system, admitted by a solenoid valve
- The cylinder's return spring, freed when a solenoid vents the air
Show the answer
C. Correct. Turning the brake on energizes a solenoid valve that passes air from the truck's air tank to the cylinder, and the cylinder swings the valve closed across the exhaust. Because the cylinder runs on the truck's air, a leak in that feed or a solenoid that does not pass air leaves the brake weak or dead, so check for air at the cylinder with the brake on.
Why not the others
A. Engine oil working through a solenoid and control valves is how a compression-release brake operates. An air-cylinder exhaust brake is closed by air from the truck's air system, admitted by a solenoid valve.
B. Boost falls almost to nothing when the driver lifts off the accelerator, which is exactly when the exhaust brake is used, so it could not hold the valve closed. The cylinder is fed from the truck's air system through a solenoid valve.
D. That is the spring-brake chamber habit carried over to the wrong part. The exhaust brake's solenoid sends air to the cylinder when the brake is turned on, and that air closes the valve; the cylinder is not held off by air the way a spring brake is.
ID T2-H-003 · Task T2-H-2 (ASE task list)
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After an overhead adjustment, cylinder 5 misfires at idle with the engine brake off. Leakage on cylinder 5 is 38 percent with air heard at the exhaust; its exhaust valve lash is within specification. With cylinder 5's slave piston adjusting screw backed out one turn, leakage drops to 6 percent. This was caused by:
- exhaust valve lash set too tight on cylinder 5.
- an exhaust valve crosshead adjusted unevenly on cylinder 5.
- a slave piston held down in its bore by varnish.
- a slave piston adjusted tight enough to unseat the valves.
Show the answer
D. Correct. With the brake off, the slave piston rests against its adjusting screw; screwed down too far, it pushes on the exhaust valve crosshead and holds the valves off their seats. Backing out the screw let them seat, so reset the slave lash to the manufacturer's figure with the exhaust valves closed.
Why not the others
A. Too little valve lash can hold an exhaust valve open, but the stem says the lash is within specification, and backing out the slave screw does not change valve lash. The slave piston, adjusted with no clearance, was holding the valves open.
B. A misadjusted crosshead can hold one exhaust valve open, but backing out the slave piston screw would not change it, and the leakage would have stayed high. The leak stopped when the slave piston was backed off, so the slave adjustment held the valves open.
C. A slave piston stuck down in its bore would also hold the exhaust valves open, but backing out the adjusting screw could not lift it, because the screw is only the stop its return spring pulls it against. Leakage would have stayed high; the drop to 6 percent shows a free piston that was adjusted too tight.
ID T2-H-004 · Task T2-H-3 (ASE task list)
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In a housing-type compression brake, what does the solenoid valve do when it is energized?
- Sends engine oil to the control valves in that housing
- Pushes the slave pistons down onto the exhaust valves
- Vents the oil trapped between master and slave pistons
- Sends compressed air to the slave piston bores
Show the answer
A. Correct. The energized solenoid lets pressurized engine oil flow through the control valve to the master and slave pistons, charging the circuit the master piston then pressurizes. When the solenoid is de-energized, that oil feed is cut off and the brake releases.
Why not the others
B. The solenoid does no mechanical work on the valves. It only admits engine oil; the slave piston is driven down by oil that the master piston pressurizes.
C. Letting the trapped oil go is part of releasing the brake, which happens when the solenoid is de-energized, not energized. Energized, the solenoid sends engine oil through the control valve to charge the circuit.
D. A compression-release brake is hydraulic and runs on engine lubricating oil. Air is the working medium of an air-cylinder exhaust brake, not of the slave pistons.
ID T2-H-005 · Task T2-H-2 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | B |
| Q3 | C |
| Q4 | D |
| Q5 | A |
03What area H covers
This content area covers the engine brakes that help slow a truck: compression-release brakes, which open the exhaust valves near the top of the compression stroke, and exhaust brakes, which close a valve across the exhaust. It covers testing and adjusting them, their switches, wiring, solenoids and enable settings, and the housings, valves, pistons, springs and seals inside them.
The ASE test specification calls this content area “Engine Brakes”. It carries 3 of the 55 scored questions, 5.5% of the test.
- Test and adjust compression-release and exhaust brakes
- Diagnose control switches, wiring and solenoids
- Check enable conditions and control parameter settings
- Service brake housings, valves, springs and seals
- Set slave piston lash to specification
04What the questions turn on
Every enable condition counts
The ECM commands the brake only when every enable condition is met. A pedal reading 6 percent with the foot off looks like a request for fuel, so the pedal sensor is diagnosed before the brake hardware.
Slave lash, tight or loose
Slave piston lash set too tight can hold the exhaust valves off their seats with the brake off, so the cylinder misfires and shows high leakage. Set too loose, it opens the valves later and less, so the lash is closed up to specification.
One housing or all three
An open solenoid coil stops its housing, and the two cylinders it serves, from braking at all, so it cannot explain weak braking from every housing. Voltage at the pass-through but none at the solenoid places an open under the valve cover.
Cold oil, slow brake
A compression brake works on engine oil, so after a cold start the thick oil flows slowly through the solenoid and control valve passages and the brake acts weakly until it warms. Leakage past a worn part is worst with hot, thin oil.
05The other content areas
The T2 test has eight content areas. Each has its own page of free questions:
The ASE T2 Diesel Engines practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
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