Mercedes-Benz automatic transmission speed sensor
The transmission speed sensor (TCM) is a device that measures the current rotation speed of the input shaft in an automatic transmission (AT). This signal is used by the transmission control module (TCM) to determine the current gear and shift the gears correctly.
Sensor location
The automatic transmission speed sensor is usually located on the gearbox housing, next to the input shaft.
Operating principle
The speed sensor uses two basic principles to measure rotation speed:
* Hall Effect: The sensor uses a magnetic Hall effect sensor, which generates an electrical signal when exposed to a magnetic field. A magnetic disk on the input shaft rotates continuously, creating a magnetic field that interacts with the Hall sensor.
* Infrared: Some sensors use infrared LEDs and photodetectors to measure rotation speed. Infrared LEDs emit light that is reflected from the input shaft, and photodetectors detect the reflected light. The faster the shaft rotates, the shorter the intervals between reflected signals.
Diagnostics and replacement
The automatic transmission speed sensor is an important component necessary for the proper operation of the transmission. If the sensor fails, it can result in improper shifting, loss of power, or even damage to the transmission.
Typically, the following signs indicate a malfunction of the automatic transmission speed sensor:
* Jumps and jerks when changing gears
* Check Engine light on
*Power loss
* Gear changes too fast or slow
To diagnose a malfunction of the speed sensor, a fault code scanner is used. If a fault code is detected associated with the sensor, it must be replaced.
Replacing an automatic transmission speed sensor is a relatively simple procedure that usually involves the following steps:
1. Disconnecting the battery
2. Removing the sensor from the gearbox housing side
3. Installation of a new sensor
4. Connecting the battery
5. Reset transmission adaptations (if required)
It is important to use the manufacturer’s recommended speed sensor and have it replaced by a qualified mechanic.