Speedometer & Odometer Failures

From my readings, failure of speedometer in 940 is normal. The symptoms include either speedometer not functioning as well as the odometer.

How Speedometer & Odometer Works?

First, a speed sensor is attached on the rear drive axle. This sensor will read the speed of the axle; which translates to the speed of the car.

This sensor is wired and connected to the in-dash instrument cluster inside the car. This includes usage of sockets and wires (that also prone for ruptures/damages).

Upon receiving signal through the wires, speedometer will display the speed accordingly.

Speedometer is integrated to odometer by a small electric motor with gear; which translates how many kilometers the car had been driven based on the speed recorded. This achieved with calculated time of motor rotation and gear teeth (the engineer knows their job).

Logically, when speedometer did not work, odometer will not give any reading as it consider the car stops. Vice versa, if the speedometer works and odometer stops, something might be wrong with the motor or the popular plastic gear. (In most cases, the gear teeth broke after years).

How To Troubleshoot Speedo/Odometer Failures ?

First, perform visual inspection on the sensor attached to rear axle and it's socket. If any obvious ruptures of socket/wires could be seen, perhaps that the culprit.

Second, detach the instrument cluster. This guide is very useful; contain how to remove the cluster, and wiring diagram. To cut it short, visit this guide, turn to page 5 and you can see an L-shape connector (with arrow in the diagram). This is the one coming from speedometer sensor. Below illustration is on a 240, perhaps 940 have the same type of wires/socket. Note that wire color codes have been changed in later models, however it still serve the same purpose.


Third, using an analog multimeter (can use any type of multimeter, even an oscilloscope), connect the -/COM lead to the BLACK wire terminal, and the + lead to the BLUE wire. Note that the RED wire supply voltage for the board, because we are not connecting the board, leave it. BLUE wire is transmitting signals from the speedo sensor.

Set multimeter to read AC voltage as follows;

Then, start the car, and get moving, slowly. You should be able to see the multimeter readings increasing with the speed of your car. This indicates that the signals from the speedo sensor at the axle was fine.

So What?

If your multimeter shows reading while the car is moving, this means you sensor, wires, and sockets are OK. The board itself must be checked (or sometimes to replace with a used unit is cheaper).

If your multimeter did not show any reading, perhaps you got a broken sensor/wires. An auto electrician can help to perform continuity test of the socket/wires from axle to in-dash socket to confirm they are OK. If wires are OK, get the sensor replaced!


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