Gear sensor on manual gearbox

On vehicles with a manual gearbox, a gear sensor is installed on the top of the transmission housing and has the task of detecting the individual gears. The gear sensor is a PLCD sensor (PLCD stands for Permanentmagnetic Linear Contactless Displacement). The gear shift causes the gearshift rod and therefore the magnet in the transmission to move. The Digital Engine Electronics (DME) identifies the position of the gearshift lever from the PLCD sensor.

The gear sensor detects the following gears:

No gear engaged: important input signal for the automatic engine start-stop function.
Reverse gear engaged: important signal for switching on the reversing lights. The reversing light switch can be omitted here in the case of a manual gearbox
Individual forward gears engaged

Courtesy of BMW OF NORTH AMERICA, INC.

Item	Explanation
1	Manual gearbox
2	Gear sensor (PLCD sensor)
3	4 pin plug connection

Teach-in is necessary to compensate for tolerances in the mechanical and electrical systems. The Digital Engine Electronics (DME) then evaluates the maximum ranges in which the engine may start with clutch control as a ratio of the learnt zero point of the gearshift lever. When the clutch pedal is fully operated the engine can start within a gearshift lever articulation of 1.89 mm (10 percent pulse width modulation).

The PLCD sensor is supplied with 5 volts. The output voltage is a pulse-width modulated signal. The duty cycle changes depending on the position of the gearshift lever (10 to 90 percent).

NOTE: The gear sensor must be taught in after exchanging the engine control unit. There is a service function to teach in the gear sensor.

The gear sensor transmits the information via a SENT signal.

The following information is transmitted:

Longitudinal direction and transverse direction of the gear sensor
Gearbox temperature
Sensor fault

NOTE: The SENT signal was specified by American automobile manufacturers. It is characterized by its simplicity and works on an unshielded three-wire connection which also transmits the supply voltage to the sensors. The transmission via a SENT signal is digital and relatively uncritical against fault signals. That is because the information content is located in the falling signal edge only, whereas the time between two falling pulse edges contains the information of the nibbles. The value transmitted by a nibble is proportional to the duration between two falling pulse edges.