Engine Speed & Position Sensor (Crankshaft Sensor): Description

This sensor is the most important sensor on the vehicle, without it the engine cannot run. There is no backup strategy or limp home facility should it fail. The sensor produces the signal which enables the ECM to determine the angle of the crankshaft, and the engine RPM. From this, the point of ignition, fuel injection, etc. is calculated. If the signal wires are reversed, a 3° advance in timing will occur, as the electronics within the ECM uses the falling edge of the signal waveform as its reference/timing point for each tooth.

The reluctor is machined and has a tooth pattern based on 60 teeth at 6 degrees intervals and 3 degrees wide: two of the teeth are removed to provide a hardware reference mark which is 60 degrees before top dead center No. 1 cylinder. See Fig 1 .

Fig 1: Illustrating Missing Crankshaft Sensor Reluctor Teeth

Courtesy of LAND ROVER NORTH AMERICA, INC.

The sensor operates by generating an output voltage caused by the change in magnetic field, which occurs as the teeth pass in front of the sensor. The output voltage varies with the speed of the teeth passing the sensor; the higher the engine speed, the higher the output voltage. Note that the output is also dependent on the air gap between the sensor and the teeth (the larger the gap, the weaker the signal, the lower the output voltage).

There are two diagnostic checks on the output signal of this sensor:

1. The hardware reference mark created by the missing teeth is outside the search window and the engine speed is greater than 500 RPM.
2. The hardware reference mark is outside the search window by more than one tooth and the engine speed is greater than 500 RPM.

The ECM transmits the engine speed to the automatic TCM using CAN, while all other control modules are hardwired. See Fig 2 .

Fig 2: Engine Speed & Position Sensor DTC Testing

Courtesy of LAND ROVER NORTH AMERICA, INC.