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Electrical Sensors that can affect automatic transmission operation.

Sensors deliver information to the powertrain control module and transmission control module

The Powertrain Control Module controls engine and transmission function by way of a programmatically adaptive microprocessor. It is responsible for all shift timing and shift feel characteristics. If the PCM fails were malfunctions, the transmission may stop shifting, have firm or soft shifts, or cause complete transmission failure. Some vehicles use a TCM (Transmission Control Module) that is separate from the PCM. Most vehicles have several electronic control modules. Other modules such as the BCM (Body Control Module) can play a role in transmission function.

Vehicle Speed Sensor

The Vehicle Speed Sensor measures vehicle speed. If it fails or malfunctions the automatic transmission may not operate properly. It may also cause the automatic transmission to go into failsafe mode making it appear that the problem is more severe than it actually is. Recently, a customer came in believing he would need a complete transmission overhaul. After diagnosis, it was determined that it was only a speed sensor issue costing over $3000 less. That's obviously quite the savings.

Transmission Input Speed Sensor

The input speed sensor measures the RPM of the input shaft of the transmission. With the engine running and the vehicle in gear (forward or reverse), if the car or truck is not moving, the input shaft of the transmission is also not moving. This is because the torque converter, which is a fluid drive, allows this effect, which is perceived as 'slipping'. When the input shaft moves, the input speed sensor detects this motion, and wires deliver this signal to the control module. When the input shaft is moving, the car or truck is also moving, unless there is an internal transmission slipping problem or other damage. While the vehicle is moving, the transmission input shaft is most often rotating near or at the same RPM as the engine. While the speed of the input shaft is directly proportional to the speed of the vehicle, the vehicle's speed depends on both the input speed and the current gear ratio.

Transmission Output Speed Sensor

An output speed sensor of a car, truck, or SUV measures the rotational speed of the output shaft in RPMs. This is not the same as the measurement of the vehicle speed, which is measured by the vehicle speed sensor, or by the wheel speed sensors. The wheel speed sensors are also used for anti-lock braking systems and traction control systems.

Turbine Speed Shaft Sensor

The Turbine Shaft Sensor measures the input shaft speed of the automatic transmission. The electronic transmission control module uses the information provided by the TSS to determine the amount of torque converter clutch slippage. If it fails it usually produces an automatic transmission fault code.

Intake Air Temperature Sensor

The Intake Air Temperature sensor measures the temperature of the air as it enters the engine's intake assembly. It controls the air/fuel mixture within the engine, so as to provide the proper ratio required for the most efficient operation. It is also a component of the pressure control system for the automatic transmission. If it malfunctions or fails, the transmission may produce hard shifting or soft shifting, or produce a transmission fault code.

Manifold Airflow Sensor

The Mass Airflow Sensor measures airflow into the engine intake assembly and is used to determine engine load. If it fails it can cause automatic transmission problems such as late harsh shifts, early soft shifts, or can result in no shifting at all. The mass airflow sensor is directly related to engine performance, so an issue may affect the drivability of the vehicle and a poor running engine.

Manifold Absolute Pressure Sensor

The Manifold Absolute Pressure sensor measures the pressure within the intake manifold and is used to measure engine load. If it fails it can cause hard shifts or slipping shifts. It also measures altitude above sea level and controls engine performance. The manifold absolute pressure sensor is used by the electronic engine management system to determine barometric pressure, which is necessary to calculate the proper air-fuel ratio for the engine

Coolant Temperature Sensor

The Coolant Temperature sensor measures the engine coolant temperature. It is used to inhibit overdrive and the converter clutch when the engine is too cold. If it fails it may or may not produce an automatic transmission fault code. The coolant temperature sensor is also used by the power train control module to adjust the engine's air-fuel mixture ratio, making it richer when the engine is cold.

Throttle Position Sensor

The Throttle Position sensor measures the throttle position, which is controlled by the gas pedal. It is used to determine engine load and if it fails it can cause automatic transmission shifting problems. It is used by the vehicle's computer to control engine performance by increasing the amount of fuel delivered as the throttle opens. In theory, the throttle position sensor is not necessary for proper engine operation because the mass airflow sensor and the manifold absolute pressure sensors can be used to determine the amount of air traveling into and engine at any point in time. However, the throttle position sensor is beneficial for proper automatic transmission control because it immediately establishes the driver's intent, and transfers this information to the transmission control module.

Differential Speed Sensor

The Differential Speed Sensor measures vehicle speed. If it malfunctions or fails, the transmission can stop shifting or shift late and harsh. It may also cause the automatic transmission to go into fail-safe.

Overdrive Switch

The driver of the vehicle uses the Overdrive Switch to disable overdrive. If it stops working, either the transmission will not shift into overdrive, or the driver will not be able to disable overdrive.

Brake Switch

The Brake Switch measures brake pedal position. Its primary function as it relates to the transmission is to release the converter clutch while braking. If it fails the torque converter clutch may not apply, or it may chug the engine while coming to a stop.

Transmission Range Sensor or manual lever position switch

The Transmission Range sensor tells the PCM the position of the transmission shifter. The PCM uses this information to control which gears of the transmission to enable or disable. When the transmission range sensor fails it can cause wrong gear starts, no upshifts, or what feels like a falling-out-of gear condition.

Transmission Fluid Temperature Sensor

The transmission fluid temperature sensor measures the temperature of the transmission’s automatic transmission fluid. It is often used to delay the engagement of overdrive and the converter clutch when the vehicle's engine is not warmed up. If it fails it is sometimes unnoticeable, but may produce an automatic transmission fault code.


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