ASE A2 Mechanical/Hydraulic Systems Diagnosis — Questions and Answers
Question 1: A vehicle with an automatic transmission accelerates normally from a stop, but the engine lacks power at highway speeds and the transmission fluid overheats. Which torque converter fault is most likely?
- Stator one-way clutch slipping in both directions
- Stator one-way clutch seized in the locked position (Correct answer)
- Torque converter clutch apply solenoid stuck off
- Turbine splines worn at the input shaft
Correct answer: Stator one-way clutch seized in the locked position
A stator that stays locked blocks fluid returning to the pump at coupling speed. This restricts flow, which causes poor top-end performance and overheating.
The stator sits between the pump and turbine and redirects returning fluid to multiply torque at low speed. Its one-way clutch lets it lock during multiplication and freewheel once the converter reaches the coupling phase. If the clutch seizes, the stator keeps obstructing fluid flow at higher speeds. The result is good low-speed launch but weak high-speed performance, overheated fluid and sometimes poor fuel economy. A stator clutch that slips both ways causes the opposite symptom, which is poor launch with no torque multiplication.
Question 2: A technician finds automatic transmission fluid in the vacuum hose that runs to the vacuum modulator. What is the most likely cause?
- Overfilled transmission
- A restricted transmission vent
- A worn converter pump bushing
- A ruptured modulator diaphragm (Correct answer)
Correct answer: A ruptured modulator diaphragm
A ruptured diaphragm lets fluid pass into the vacuum line and the engine intake. The modulator can then no longer regulate line pressure correctly.
A vacuum modulator uses a diaphragm to separate engine vacuum from transmission fluid pressure. If the diaphragm tears, fluid is drawn through the vacuum hose into the intake manifold. This can cause white or blue exhaust smoke and falling fluid level. Because the modulator no longer senses engine load correctly, line pressure is usually too high, which causes harsh shifts. Replace the modulator, then check the fluid level and condition.
Question 3: Transmission fluid on the dipstick looks milky and pink. What is the most probable cause?
- A leaking transmission cooler inside the radiator (Correct answer)
- A worn front pump seal
- A clogged internal filter
- A stuck converter clutch solenoid
Correct answer: A leaking transmission cooler inside the radiator
Coolant mixing with the fluid gives it a milky pink appearance. The usual entry point is a failed cooler in the radiator tank.
Many vehicles route transmission fluid through a cooler inside the radiator tank. If the cooler fails, engine coolant can enter the transmission fluid, and in the other direction fluid can enter the coolant. Coolant ruins friction material and seals, so the transmission usually needs a full overhaul along with replacement of the radiator or cooler. Pressure-test the cooling system to confirm the leak path.
Question 4: A transmission has been overfilled by about a quart. Which symptom is most likely?
- Fluid pressure that is too low because the pump runs dry
- Delayed engagement caused by a high fluid level
- Foamy, aerated fluid with erratic shifts and fluid expelled from the vent (Correct answer)
- Improved cooling and quieter operation
Correct answer: Foamy, aerated fluid with erratic shifts and fluid expelled from the vent
Rotating parts churn an overfilled pan and whip air into the fluid. Aerated fluid gives erratic pressure and can foam out of the vent.
If the fluid level is too high, the planetary gearset and rotating drums contact the fluid and aerate it. The pump then draws in foam instead of solid fluid, so hydraulic pressure becomes erratic. The result can be slipping or harsh shifts, overheating and fluid leaking from the vent or dipstick tube. Correct the level by draining to the proper mark. Check it at normal operating temperature using the procedure the manufacturer specifies.
Question 5: During a line pressure test, pressure is within specification in Park, Neutral, Drive and the manual ranges, but low only in Reverse. Which is the most likely cause?
- A clogged transmission filter
- A leak in the reverse clutch or servo circuit (Correct answer)
- A defective pump
- A stuck main pressure regulator valve
Correct answer: A leak in the reverse clutch or servo circuit
Pressure is good in every other range, so the pump, filter and regulator are working. The leak is in a circuit used only in Reverse, such as the reverse clutch seals.
Pressure tests narrow the fault by range. Low pressure in all ranges points to common components such as the pump, filter, main regulator or a low fluid level. Pressure that is low in only one range points to the circuit unique to that range. In Reverse, this includes the reverse clutch piston seals, the servo seals or leaking sealing rings on the shaft feeding that circuit.
Question 6: A vehicle with an automatic transmission stalls or bucks every time it is brought to a stop in gear, and runs normally in Neutral. Which fault is most likely?
- A torque converter clutch that fails to release (Correct answer)
- A failed stator one-way clutch
- A low fluid level
- A worn turbine shaft seal
Correct answer: A torque converter clutch that fails to release
If the lock-up clutch stays applied, the engine is mechanically tied to the drivetrain at a stop. The engine stalls like a manual-transmission car with the clutch released.
The torque converter clutch is meant to release below a set road speed so the engine can idle while the vehicle is stopped. A stuck lock-up solenoid, a stuck TCC control valve or a mechanically failed clutch keeps the clutch applied. The engine then stalls or shudders as the vehicle slows. Check for TCC solenoid command and for stored codes, then test the solenoid and valve body.
Question 7: A vehicle has a vibration, like driving over rumble strips, at a light throttle cruise of 40-50 mph. It goes away when the brake is lightly touched, which releases the torque converter clutch. What should be done first?
- Replace the engine mounts
- Replace the torque converter immediately
- Balance the driveshaft
- Check the fluid condition, and exchange it with the correct fluid before further repair (Correct answer)
Correct answer: Check the fluid condition, and exchange it with the correct fluid before further repair
Touching the brake releases the TCC and the shudder stops, so it is TCC shudder. Degraded or wrong fluid is a common cause and the simplest to correct first.
TCC shudder occurs while the lock-up clutch is applying or slipping, so it disappears when the clutch is released. Fluid that has lost its friction properties or is the wrong type is a very common cause. Check the fluid condition and the service history, then exchange the fluid with the specified type and recheck. If the shudder remains, further tests of the solenoid, valve body and converter are needed.
Question 8: A whining noise is heard with the transmission in Park and Neutral. It rises in pitch as engine speed increases, and it is not related to vehicle speed. Which is the most likely cause?
- Worn output shaft bearing
- Planetary gear damage
- Transmission pump wear or low fluid level (Correct answer)
- Worn drive axle
Correct answer: Transmission pump wear or low fluid level
The pump is driven by the converter hub at engine speed. A whine that follows engine rpm in Park or Neutral points to the pump or to aeration from low fluid.
Noise that changes with engine speed and is present in Park and Neutral originates in components driven by the engine, such as the pump. Noise that is present only in a specific gear or that follows vehicle speed points to the gearset or the output components. Always check fluid level and condition first. A low or aerated fluid causes pump whine, and the pump may also be worn or have a damaged pickup or filter.
Question 9: During a road test, engine speed rises sharply during a 1-2 upshift without a matching increase in vehicle speed, then the transmission catches. What does this indicate?
- A slipping clutch or band applying for second gear (Correct answer)
- A faulty torque converter clutch
- A worn drive axle joint
- A restricted air filter
Correct answer: A slipping clutch or band applying for second gear
Engine rpm flaring during a shift means the element that should apply is slipping. It often points to low apply pressure or worn friction material.
A shift flare is a sign that the element for the incoming gear is not holding fully. Causes include worn friction material, leaking seals or low apply pressure. A pressure test shows whether the fault is hydraulic or a mechanical wear problem. Check the fluid level and condition before testing further, because low or burnt fluid can cause the same symptom.
Question 10: What is the maximum time a stall test should normally run at full throttle with the brakes applied?
- 30 seconds
- 15 seconds
- 60 seconds
- 5 seconds (Correct answer)
Correct answer: 5 seconds
A stall test generates a lot of heat in the fluid, so each test is limited to about 5 seconds. Then the engine is idled in Neutral for cooling.
During a stall test, the torque converter turbine is held while the engine runs at full throttle, so all engine power turns to heat in the fluid. Test time must be kept very short, normally no more than 5 seconds. Afterward, shift to Neutral and idle the engine to let the fluid cool before any other test. Compare the stall speed to specification. Wheels must be chocked and the parking brake and service brakes firmly applied for safety.
ASE A2 Mechanical/Hydraulic Systems Diagnosis
A vehicle with an automatic transmission accelerates normally from a stop, but the engine lacks power at highway speeds and the transmission fluid overheats.
Which torque converter fault is most likely?
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