Downshift Test

CAUTION: Refer to Road Test Caution in Service Precautions.

1. On a smooth,level road, accelerate to the speed at which the concern vibration occurs. Observe the engine RPM.
2. Decelerate and safely downshift to the next lower gear.
3. Operate the vehicle at the previous engine RPM.
If the vibration returns at the same engine RPM, the following conditions are the most probable causes of the vibration:
- The engine
- The clutch disc (manual transmission)
- The propeller shaft
- The transmission flywheel (automatic transmission)
- The torque converter
Repeat this test in lower gears, and in NEUTRAL gear, in order to confirm the results.
In some cases, a vibration may also be sensitive to torque or engine load, as well as being related to a specific engine speed or vehicle speed. These vibrations can be most difficult to diagnose, and require additional testing. A systematic approach usually leads to isolating the problem.