Crankshaft Position Sensor: Description and Operation
CRANKSHAFT POSITION SENSOR (CKP)
The crankshaft position sensor is mounted at the rear underside of the engine near the transmission bell housing. Connection between the sensor and the harness is via a link harness and a two-way connector. Both wires go directly to the ECM (engine control module). The sensor produces the signal which enables the ECM (engine control module) 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 (engine control module) uses the falling edge of the signal waveform as its reference/timing point for each tooth.
The reluctor is pressed into the flywheel and has a "tooth" pattern based on 36 teeth at 10° intervals and approximately 5° wide: one of the teeth is removed to provide a hardware reference mark which is 60 degrees BTDC (before top dead center) No.1 cylinder. Because of the crankshaft sensor's orientation, the target wheel uses windows machined into the face, rather than actual teeth.
The sensor operates by generating an output voltage caused by the change in magnetic field that occurs as the windows pass in front of the sensor. The output voltage varies with the speed of the windows 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). The ECM (engine control module) transmits the engine speed to other vehicle control modules on CAN (controller area network).
CKP (crankshaft position) Sensor
The characteristics of the signal supplied by the CKP (crankshaft position) sensor are learned by the ECM (engine control module). This enables the ECM (engine control module) to set an adaption and support the engine misfire detection function. Due to the small variation between different flywheels and different CKP (crankshaft position) sensors, the adaption must be reset if either component is renewed, or removed and refitted. It is also necessary to reset the flywheel adaption if the ECM (engine control module) is renewed or replaced. The ECM (engine control module) supports four flywheel adaptions for the CKP (crankshaft position) sensor. Each adaption relates to a specific engine speed range. The engine speed ranges are detailed in the table below:
Adaptions Engine Speed, rev/min
1 1800 - 3000
2 3001 - 3800
3 3801 - 4600
4 4601 - 5400
Misfire Detection
Legislation requires that the ECM (engine control module) must be able to detect the presence of an engine misfire. It must be able to detect misfires at two separate levels. The first level is a misfire that could lead to the vehicle emissions exceeding 1.5 times the Federal Test Procedure (FTP) requirements for the engine. The second level is a misfire that may cause catalyst damage.
The ECM (engine control module) monitors the number of misfire occurrences within two engine speed ranges. If the ECM (engine control module) detects more than a predetermined number of misfire occurrences within either of these two ranges, over two consecutive journeys, the ECM (engine control module) will record a fault code and details of the engine speed, engine load and engine coolant temperature. In addition, the ECM (engine control module) monitors the number of misfire occurrences that happen in a 'window' of 200 engine revolutions. The misfire occurrences are assigned a weighting according to their likely impact on the catalysts. If the number of misfires exceeds a certain value, the ECM (engine control module) stores catalyst-damaging fault codes, along with the engine speed, engine load and engine coolant temperature.
The signal from the crankshaft position sensor indicates how fast the poles on the flywheel are passing the sensor tip. A sine wave is generated each time a pole passes the sensor tip. The ECM (engine control module) can detect variations in flywheel speed by monitoring the sine wave signal supplied by the crankshaft position sensor.
By assessing this signal, the ECM (engine control module) can detect the presence of an engine misfire. At this time, the ECM (engine control module) will assess the amount of variation in the signal received from the crankshaft position sensor and assigns a roughness value to it. This roughness value can be viewed within the real time monitoring feature, using T4. The ECM (engine control module) will evaluate the signal against a number of factors and will decide whether to count the occurrence or ignore it. The ECM (engine control module) can assign a roughness and misfire signal for each cylinder, (i.e. identify which cylinder is misfiring).
Diagnostics
The diagnostic socket is located in the fascia, in the driver's stowage tray. The socket is secured in the fascia panel and is protected by a hinged cover.
The ECM (engine control module) stores faults as DTC (diagnostic trouble code), referred to as 'P' codes. The 'P' codes are defined by OBD (on-board diagnostic) legislation and, together with their associated environmental and freeze frame data, can be read using a third party scan tool or T4. T4 can also read real time data from each sensor, the adaptive values currently being employed and the current fueling, ignition and idle settings.
