Transmission sensors record the transmission speed. This information is needed by the ECU to adjust the shifting pressure and for the decision which gear must be chosen.
There are two different types of transmission sensors, a hall sensor and an inductive pickup. Their function is send voltage signals produced by changing magnetic fields developed by the rotating ring gear.
Effects of failure
- A faulty transmission sensor can cause the following:
- transmission control system not operating, ECU switches to limp mode
- engine warning light illumination
Causes of failure:
- mechanical damage of the ring gear
- wire short circuit/open circuit
- internal short circuit
- soiling through metal abrasion
For fault recognition consider the following system tests:
- check sensor for soiling
- check ring gear for damage
- read out trouble codes/fault codes
- measurement of the sensor coil resistor(if on-vehicle) with a ohmmeter, measured value at 80°C about 1000 ohm
- check operation voltage of the hall sensor(if on vehicle)with a multimeter (circuit diagram needed) Attention: Do not check the resistor of the hall sensor it can be damaged.
Wiring harness to the ECU plug check for continuity and short circuit to earth. Measurement with a ohmmeter between the sensor plug and removed ECU plug, measured value: <1 ohm. Respective pin from the sensor plug checked for short circuit to earth, ECU plug removed, measured value: >30 Mohm