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Your Impulse Sensor (Camshaft Sensor & Crankshaft Sensor) worn out? » Read more about common malfunctions • Repair • Replacement manual
All angles in check with the shaft sensors
The control unit needs data for the car's effective operation. The most important data are provided by the shaft sensors. With the information on the precise rotation angle of the shafts the control unit is able to determine the moment of ignition and fuel injection. These sensors are also known as angle sensors, as they provide information about the rotation angle.
Indispensable for modern engines
Until the early nineties, engines were relatively simple constructions. The downward moving piston drew the air through the air filter, creating volume, which generated the air/fuel mix in the mechanically operated down-draft carburettor. The spark plug was engaged via a mechanically operated contact-breaker point – and that was it. All systems' interaction was purely mechanical. A central control was not necessary. No matter how simple, this technology was all but efficient. Even smaller vehicles, like e.g. the Volkswagen 1303 "beetle car" provided a mere 34 HP, using 10 – 12 litres fuel per 100 km. Nowadays, these performance values are inconceivable. Increasing performance of smaller engines required plenty of change in the technological and constructional sense, including central engine control. This functions by way of a control unit, depending on data from several sensors in the engine, such as the camshaft and crankshaft position sensor, the latter a.k.a. the crank sensor.
Camshaft position sensor and crankshaft position sensor: construction and location
Both are so-called proximity sensors, registering the proximity of a magnetic field. They contain a small electric coil, generating a weak magnetic field. An approaching magnetic field interacts with the generated field. This in its turn causes a change in the sensor's resistance, resulting in a reading. The approaching magnetic fields are created by pins on the crank- or camshaft. Separate magnets are not necessary. These solutions would bear the risk of magnets loosening and causing damage in the engine. A crown gear wheel or a pin adequately reads measuring values. The camshaft position sensor sits at the top of the engine. It is a cylindrical component with cables leading to the control unit. The crank sensor is located at the bottom of the engine. It is not as easily accessible, though of similar construction. The crankshaft position sensor is alternatively called the TDC sensor. TDC is short for top dead centre, signifying the position of the piston in the cylinder when it is at the top end of its up-and-down movement.
Camshaft position sensor/TDC sensor defects
When sensors fail, the control unit no longer knows when to activate ignition and injection. For this contingency, cars have an emergency running program, enabling the car to slowly drive on at maximally 80 kph. Check engine lights are on. The defect can only be definitely identified by reading the error memory. Before replacing the sensors, cables and connection should be checked first. A worn or bitten-through cable causes a similar effect and therefore a thorough preventive check can save a lot of work and costs.
Angle sensors: repair
The camshaft position sensor is the easier one to replace. It can be accessed by opening the bonnet. The sensor is fixed to the engine with two bolts and connected to the onboard electronics with a plug. Bolts are unscrewed, the plug is pulled out and now the cylindrical camshaft position sensor can be pulled out. Upon installing a new sensor, it might have to be programmed into the control unit. The replacement of the crank sensor is similar although it requires working underneath the car. Depending on location, removal of exhaust and other components might be necessary. For all repairs underneath the car applies: if no pit or lifting platform is available, safety is priority number one. For these repairs, ramps are the most suitable. They offer ample space and allow the car to stand securely. Ideally, the car is additionally secured against rolling away by kegs.
Buying shaft sensors
With 12 – 20 EUR (± £11 - £18), these sensors are amazingly cheap. Replacement is not a very complicated repair. Upon replacement the control unit's error memory has to be deleted. Any garage with modern equipment will take care of this for a few bob.