Cam/Crankshaft Sensor TTC2.0

What it does

It measures the crankshaft/camshaft rotational speed/position. It signals the ECU with a frequency signal that increases with engine speed. There are two basic types of sensor, the Hall type or the Inductive type. Information on engine speed/position is probably the most important piece of information that the ECU receives.

How does it work?

Of the two types of sensor in common automotive use the most popular is the Inductive type.
The sensor contains a soft iron core around which a fine wire is wound. A permanent magnet is connected to the soft iron core resulting in a magnetic flux which extends beyond the sensor.
Each time this magnetic field is “broken” a small electric current is generated in the solenoid wiring. The component that is responsible for breaking this magnetic flux is the rotating component of the engine such as the teeth on the flywheel or a stator wheel mounted on the front pulley of the crank/cam shaft. The output frequency of the sensor depends upon the number of teeth on the stator (fixed at manufacture and dependant upon application) and the speed of the rotating component (varies with engine speed). There is often a tooth missing off the stator at a strategic position in order to notify the ECU of the engine’s position (relevant to TDC). One of the main downfalls with the Inductive type sensor is its inability to register accurately from low RPM. Often an engine speed of at least 100 RPM is necessary for it to function. The output of an Inductive sensor is a sine wave type signal.

The other type of speed sensor in current use is the Hall Effect type sensor. This type of sensor incorporates an integrated circuit (IC) and a permanent magnet. Once again a similar principle of a magnetic field being “broken” by a rotating stator is used but this time the IC is responsible for sensing, modifying and to some extent amplifying the signal. The resulting signal is a square wave pulse signal alternating between a high and low voltage (the actual voltage outputs vary with application. Due to its higher inbuilt accuracy (especially from low RPM) its key uses include camshaft and wheel speed sensors. They are also more able to cope with larger air gaps than the Inductive type, this is particularly useful where a degree of bearing movement is to be expected (wheel speed sensors).

Reasons for Failure:

As with most engine management sensors, the sensor component itself is usually close to indestructible. It’s when the component is adapted to its use in a motor vehicle environment that things start to go wrong. Failings seem to revolve around its connections to the rest of the system through the lead and connector.

Internal lead failures due to the angles which it is routed through plus electrical interference caused by the lead being routed close to high tension/high current leads are way up in the failure charts of this type of sensor. Connection failure due to the harsh environment in which the sensor operates is also common.
Failures can occur that have been caused by physical damage during a major mechanical repair, such as when the end of the sensor is damaged whilst the clutch is being changed. Contamination of the tip by rust or debris is another area where failures have been noted.

Testing the sensor:

Before testing the sensor you must determine the type of sensor fitted. The two types of sensor can be very similar in appearance and often the only way to determine the difference is to measure the output with accurate test equipment such as an oscilloscope. A clue can often be given by the number of wires to the sensor; the Hall type sensor needs an input voltage and thus will always have more than two wires (usually three). If the sensor has only two wires it must be an Inductive sensor.
To test an Inductive type sensor the first area that should be checked is the resistance of the internal wiring. Measure this from as far away, electrically, from the sensor as possible, e.g. at the ECU, with a conventional Ohmmeter (disconnect the ECU to carry out the test). The resistance reading should correspond to the manufacturer’s data books or check the FPUK catalogue.
Should a third wire be fitted to an Inductive type sensor this will be an earth/shield wire. Check this wire for earth continuity. Check for shorts to this wire and the other two signal wires; also check for a short to positive which is quite common where the sensor wire runs inside a main loom.
There are extra test/checks involved in the diagnosis of a hall type sensor. Check the supply voltage, test this with the sensor connected to “load” the feed to highlight connection faults. All hall type sensors will have an earth/shield test this as with the inductive type.
The preferred method of testing either Inductive or hall type sensors is dynamically with a piece of equipment such as an oscilloscope. Connect your ‘scope up, turn on the ignition, crank the engine and monitor the resulting outputs. Look out for interference problems, intermittent signals and specific outputs. The difference between the two types of sensor is obvious when connected to a ‘scope, the Hall Effect being a square wave pattern and the Inductive type a typical sine wave.

Note:

When checking for a cam/crankshaft sensor fault via a code reader the results can be misleading. This is particularly relevant with the Vauxhall ECOTEC engine where an incorrectly tensioned cam belt can display a cam sensor failure fault.

For further technical information concerning crank/cam sensors please phone our technical help line 01527 839307