There is a speed sensor in the rear cover of the gearbox.
It sends pulses to the instrument cluster (the frequency of the signal changes with the speed).
In the instrument cluster there is an integrated circuit (UAF2115) with some surrounding electronics, that translates the pulses to other signals.
The chip generates separate signals for the speedometer, the odometer (mileage counter) and a signal that can be used by several modules in the car.
That last signal for example goes to the engine management module, the idle control unit, the fuel pump relay, the cruise control (or ASR if you have ASR).
The three functions of the UAF2115 chip are mostly independent of each other, it is possible that one function does not work any more, while the other two still do work as intended.
So, if it is just the speedometer that does not work, maybe the UAF2115 chip is partially defective, but it can also be caused by a broken circuit board track, or a defective speedometer instrument.
If the car has more problems, maybe the speed sensor in the gearbox is broken, or the cable that connects the sensor to the instrument cluster, or maybe the signal does arrive at the instrument cluster, but the circuit board track that leads the signal to the UAF2115 chip is broken, or one of the surrounding electronic components is defective.
The easiest way to check where the problem lies, is swapping the instrument cluster with one that you know is functioning correctly.
If that does not fix the problem(s), probably the sensor or its cable is defective.
If it does fix the problem(s), the fault is in the instrument cluster and you have to replace or repair it.
Greetings from the Netherlands
1-mal bearbeitet. Zuletzt am 16.04.21 22:42.