Mechanical/Hydraulic Systems Diagnosis Flashcards

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?

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.

A technician finds automatic transmission fluid in the vacuum hose that runs to the vacuum modulator. What is the most likely cause?

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.

Transmission fluid on the dipstick looks milky and pink. What is the most probable cause?

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.

A transmission has been overfilled by about a quart. Which symptom is most likely?

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.

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?

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.

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?

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.

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?

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.

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?

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.

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?

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.

What is the maximum time a stall test should normally run at full throttle with the brakes applied?

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.