– The Powertrain Control Module controls the engine and transmission functions. It is responsible for all shift timing and shift feel characteristics. When the PCM fails, the transmission can stop shifting, shift harsh or soft, or cause complete transmission failure. Some vehicles use TCM (transmission control module) rather than PCM. Most vehicles have several modules: some have over ten. Often, other modules like BCM play a role in transmission function.
– The Throttle Position sensor creates a signal based on how far you're pressing the gas pedal; an indication of vehicle load. A faulty TP sensor can cause late, harsh shifts, early / soft shifts, or even prevent the transmission from shifting at all.
– The Vehicle Speed Sensor creates a signal based on road speed. A VSS failure can prevent the transmission from shifting, or cause late, harsh shifts. It may also inhibit the converter clutch or overdrive.
– The Overdrive Switch allows the driver to disable overdrive. Depending on how it fails, the transmission may not shift into overdrive, or the driver won’t be able to disable overdrive.
– The Differential Speed Sensor creates a signal the computer can use to measure vehicle speed. A DSS failure can prevent the transmission from shifting, or cause late, harsh shifts. It may also inhibit the converter clutch or overdrive.
– The Brake On-OFF switch indicates whether the brake is applied or released. The computer usually uses the signal to control the converter clutch; a failed BOO switch can prevent lockup, or may cause the car to chuggle
when coming to a stop.
– The Transmission Range sensor indicates the position of the gearshift lever; the computer uses this to enable or disable. A failed TR sensor can cause wrong gear starts, prevent upshifts, or cause the transmission to seem like its falling out of gear.
– The Transmission Fluid Temperature sensor creates a signal based on transmission operating temperature. The computer uses this signal to alter or inhibit certain gear ranges under cold or overheated operating conditions. A failure can cause wrong gear starts, late shifts, or prevent overdrive or converter clutch apply.
– The Turbine Shaft Speed sensor creates a signal based on the rotating speed of the transmission input shaft. From this, the computer can verify gear ranges, or determine whether the transmission is slipping. A failure can cause any number of shift timing or shift quality problems.
– The Coolant Temperature sensor measures engine coolant temperature. It’s used to inhibit overdrive and the converter clutch when the engine is too cold. When it fails, it can result in no overdrive or converter clutch. It also affects engine performance.
– The Air Conditioning sensor indicates whether the A/C clutch is on or off. The computer uses this signal to adjust engine idle speeds. A failure can cause a number of problems that appear to be transmission related.
– The Intake Air Temperature sensor provides a signal based on the temperature of the air entering the engine. The computer uses this signal to modify engine air / fuel mixtures. A failure can cause the transmission to shift harder than normal.
– The Mass Airflow
Sensor measures airflow into the engine and is used to determine engine load. When it fails, it can cause late, harsh shifts, early / soft shifts, or it can result in no shifts at all. It also affects engine performance.
– The Manifold Absolute Pressure sensor creates a signal based on vacuum levels in the intake manifold, which indicates engine load. A faulty MAP sensor can cause late, harsh shifts, early / soft shifts, or even prevent the transmission from shifting at all.