Camshaft Position (CMP) Sensor

The CKP sensor is variable reluctance sensor and provides an analogue voltage output to the ECM relative to the speed and position of the target on the flywheel. A permanent magnet inside the sensor applies a magnetic flux to a sensing coil winding. This creates an output voltage which is read by the ECM.

As the gaps between the poles of the target pass the sensor tip, the magnetic flux is interrupted and this causes a change to the output voltage (e.m.f.).

It is important to note that the ECM is unable to determine the exact position of the engine with its four stroke cycle from the CKP sensor alone: the CMP sensor must also be referenced to provide sufficient data for ignition control and sequential injection.

The ’spaces’ on the target are spaced at a rate of one hole per 10°. There are only 32 holes, this leaves four ’spaces’ where a single hole is missing. When the crankshaft is positioned at TDC (cylinder number one firing position) the CKP sensor is positioned at 55° BTDC. The ’missing’ holes are positioned at 80°, 110° , 260° and 300° before the CKP sensor position.

Camshaft Position (CMP) Sensor

The CMP sensor provides a signal which enables the ECM to determine the position of the camshaft relative to the crankshaft. This allows the ECM to synchronise fuel injection and, on VVC engines, monitor valve timing.

CMP Sensor - MPi Engines

The CMP sensor on MPi engines is located on the camshaft cover (under the plastic cover) at the opposite end to the camshaft drive and reads off a reluctor on the exhaust camshaft.

The sensor is a hall effect sensor which detects the reluctor mounted on the exhaust camshaft. The sensor receives a battery supply from the main relay. The sensor operates on the principle of a voltage generated when the sensor is exposed to a magnetic flux. This causes a potential difference in voltage as the reluctor passes the sensor which is detected as a digital signal by the ECM.