Table of contents: Odometer ↓ Speedometer pulse sensor ↓ Checking the speedometer pulse sensor ↓ Removal the speedometer pulse sensor ↓
The speedometer in the Audi A4 is, as it should be, an electronic measuring device just like the rev counter. It also processes the rev pulses it receives (not like a tachometer) receives from a sensor on the gearbox. It measures how fast the wheels rotate.
The wheel rotation signal is used as a measurement value to determine the 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 the drive in old mechanical speedometers.
If the speedometer does not work, first check the corresponding fuse in the fuse box (chapter Electrical equipment).
Odometer
The total and daily mileage in the Audi A4 is displayed via a liquid crystal display. To prevent manipulation of the mileage data, the mileage data is continuously stored in a memory device independent of the voltage supply and located in the combined instrument panel.
Speedometer pulse sensor
The wheel rotation pulses are fed to the speedometer by a sensor with a reed switch contact on the rear of the gearbox. The reed switch contact is a block of contacts that are placed in an airtight gas shell. The contact closes when it is exposed to an electromagnetic field.
In order for the reed switch to produce a pulse sequence, it must open and close in rhythm with the wheel speed. Therefore, the notched wheel that rotates in front of the sensor affects the magnetic field that acts on the contact block.
Checking the speedometer pulse sensor
1. Raise the left front wheel.
2. Press the retaining clip onto the connector and remove the connector.
3. Connect an ohmmeter to both sensor contacts.
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.
Removal the speedometer pulse sensor
1. Press the retaining clip onto the connector and remove the connector.
2. Lightly press down on the sensor mounting clamp lock and turn the sensor slightly to the right.
3. Remove the sensor from the gearbox.
4. During installation, ensure that the sealing rings are installed correctly.