Many transmission problems are related to the operation of the torque converter. To test the operation of the torque converter, a stall test can be performed. The stall test checks the holding capacity of the converter's stator overrunning clutch assembly and the clutches and bands in the transmission.
Torque converter problems can often be identified by the symptoms; therefore, the need for conducting a stall test is minimized.
Stall testing is not recommended on many late model transmissions. The test places extreme stress on the transmission and should only be conducted if recommended by the manufacturer.
If the vehicle lacks power when it is pulling away from a stop or when passing, it has a plugged catalytic converter or the torque converter's one-way stator clutch is slipping. To determine which of these problems is causing the power loss, test for a restricted exhaust on the engine. If there is no evidence of a restricted exhaust, the torque converter's stator clutch is slipping and not allowing any torque multiplication to take place in the converter. To repair this problem, the torque converter should be replaced.
Restricted Exhaust Test
You can hear cars with restricted exhaust systems if you ever have occasion to be stopped on a hill or grade. A restriction in the exhaust causes a hissing sound from the tailpipe when under load. To test for a restricted exhaust:
- Use a fuel pump vacuum/pressure tester hooked either to the air (smog pump) lines into the intake manifold, or an adapter can be substituted in place of the egr valve.
According to TRW, removing the oxygen sensor to perform the test can give an inaccurate reading due to a venturi effect in the exhaust system.
- Raise the engine rpm quickly to 2000 to cause a vacuum reading that is momentarily low.
- Then, release the throttle quickly.
- Vacuum should return smoothly and quickly to higher-than-normal levels.
- A slow, hesitating return usually indicates a restriction.
Specifications vary among manufacturers. As a general rule, pressure should not exceed 1.75 psi at wide open throttle (WOT) under full load.
Lockup Converter Testing
All late-model automatic transmissions are equipped with lockup torque converters. The action of the lockup converter should be observed during a road test.
Poor lockup action can be the result of:
- Electrical problems,
- A malfunctioning lockup clutch assembly,
- An engine problem, or
- A problem in the torque converter itself.
To properly diagnose lockup converter problems, you must understand their normal operation and the controls involved with the system.
Although the actual controls of the lockup clutch assembly vary with the different manufacturers and models of transmissions, they all will have certain operating conditions that must be met before the lockup clutch can be engaged.
- The vehicle must be travelling at a certain speed before lockup will occur.The vehicle speed sensor sends this speed information to the computer.
- The converter clutch should not be able to engage when the engine is cold; therefore, a coolant temperature sensor provides the computer with information regarding temperature.
- During sudden deceleration or acceleration, the lockup clutch should be disengaged.
- One of the sensors used to tell the control computer when these driving modes are present is the throttle position sensor.
- Some transmissions use a third or fourth gear switch to signal to the computer when the transmission is in that gear and to allow for converter lockup.
- A brake switch is also used in some lockup circuits to disengage the clutch when the brakes are applied.
Proper adjustment of the brake light switch is essential for proper operation of the lockup torque converter. Courtesy of General Motors Corporation - Hydra-Matic Division.
The system's sensors should be visually checked as part of your diagnosis of converter problems.