The wheel rotation signal is used as a measuring value for determining mileage, as well as in the operation of the service interval indicator, fuel consumption indicator, in adjusting the volume of the radio receiver and in the operation of the on-board computer. Therefore, the idea arose to send this signal to the speedometer and to abandon the speedometer shaft, which provides a drive in old mechanical speedometers.
If the speedometer does not work, first check the appropriate fuse in the fuse box (chapter Electrical equipment).
Odometer
Indication of the total and daily run in Audi A4 is carried out by the liquid crystal display. To prevent the mileage data from being manipulated, the mileage data is continuously stored in a voltage-independent memory located in the combined instrument panel.
Speedometer impulse sensor
Wheel rotation pulses are fed into the speedometer by a reed contact sensor at the rear of the gearbox. A reed contact is a block of contacts that are placed in an airtight gas envelope. The contact closes when an electromagnetic field acts on it.
In order for a reed contact to be able to output a pulse sequence, it must open and close in time with the wheel speed. Therefore, the notched wheel that rotates in front of the sensor influences the magnetic field acting on the contact block.
Checking the speedometer pulse sensor
1. Raise the left front wheel.
2. Press the retaining clip on the connector and remove the connector.
3. Connect an ohmmeter to both contacts of the sensor.
4. Now slowly turn the left front wheel by hand: the ohmmeter indicator should fluctuate between 0 ohms and ∞ ohms.
5. If not, replace the sensor.
Dismantling the speedometer pulse sensor
1. Press the retaining clip on the connector and remove the connector.
2. Lightly press down on the sensor retaining clip and rotate the sensor slightly to the right.
3. Remove the sensor from the gearbox.
4. During installation, make sure that the O-rings are installed correctly.
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