Speed Signal Generator for standard Speed Cable


Status: complete


In this section I would like to show how to build an inexpensive Unit which delivers a speed signal, which could be used by the ums.

For generating a generic signal we can use standard hardware manufactured by Kienzle Automotive (aka VDO). Their Hall-Effect Puls Generator is commonly used in many cars from General Motors (GM) or Opel(Vauxhall).

As a first step we need to decide if we would like to keep the original mechanical Speedometer, or if we are going to replace it with an electronic Display. Reason for asking is: if we do not need the mechanical Speedo, we can simply mount the Sensor at the end of the Speedo Cable, using the connector normally plugged ito the Speedo.

If you would like to keep the mechanical Speedo, you need mounting the sensor somewhere into the speedo cable, where it runs in a straight line. This requires the Cable to be cut and involves more parts than only the sensor. For this you will need the complete kit from Kienzle Automotive, not only the sensor.

In any case w
e need the `AA` Version of the sensor, as there are different versions available. The AA type has a mechanical input and output, which enables the speedo cable to be ´sloped through´. Please see the picture below.







When mounting the Sensor at the end of the Cable, you only need a piece of shrink sleeving, two nylon straps and you are done.





The installation inside the cable as ´slope through´ solution is more complex, but an exaustive guide is included inside the kit from Kienzle Automotive, which you need anyway. So I will skip this section at this place.





Now as we have the sensor mounted, there are three cables. One is connected to 12V, one to chassis ground and the last one is an open collector drive. Togehter with a Pull-Up Resistor here yoiu will get pulses depending on the speed you are driving. For us it is more desirable having an analog voltage, as this is easily used by the ums. Therefor we need buiding a simple frequency-to-voltage converter, by using an inexpensive but high accuracy IC called LM2709. I already did that and tested it thoroughly. The output voltage needs to swing between 0V and 5V, where 0V is stand still and 5V is Vmax.
Anybody can adjust Vmax to his preferences by driving exactly half of Vmax on the highway (by use of a navigation device showing the driven speed)and adjust the output voltage of our converter to exactly 2,5V.
There you are!

Questions? ;-)








Click here to download bigger picture of schema