Transmission sensor - How It Works

General points

Transmission sensors record the gear speed. This is required by the control unit to regulate the transmission pressure during gear shifting and to decide when to switch to which gear.

Function

There are two types of transmission sensor designs: Hall-type sensors and inductive sensors. The rotary movement of the gear rim affects a change in the magnetic field which changes the voltage. The transmission sensor sends these voltage signals to the control unit.

Effects of failure

A faulty transmission sensor can become noticeable as follows: 

■ Failure of the transmission control, control unit switches to limp-home programme 

■ Engine warning light comes on 

Causes of failure can be: 

■ Internal short-circuits 

■ Interrupted cables 

■ Cable short-circuits 

■ Mechanical damage to the sensor wheel 

■ Soiling through metal abrasion

The following test steps should be taken into account during troubleshooting: 

1. Check the sensor for soiling 

2. Check the sensor wheel for damage 

3. Read out the fault code 

4. Measure the resistance of the inductive sensor using the ohmmeter, reference value at 80 °C approx. 1000 ohm. 

5. Test the supply voltage of the Hall-type sensor using the voltmeter (circuit diagram for pin assignment necessary). Note: Do not carry out resistance measurement on the Hall-type sensor since this could destroy the sensor. 

6. Check the sensor connection cables between the control unit and sensor connector for transmission (circuit diagram for pin assignment necessary). Ref. value: 0 ohm. 

7. Check the sensor connection cables for ground connection, use the ohmmeter to measure against ground at the sensor connector with the control unit connector removed. Ref. value: >30 MOhm.