** For E30 speedometer / odometer troubleshooting and repair guide see THIS POST. **
The arrangement is the same with both VDO and Motometer type gauges.
Pins are:
R - 12v +
31 - Ground
31b - Speed sensor INPUT Pin 1
A - Speed sensor Pin 2 *NOT USED*
Bench testing can be achieved by powering the unit with 12v and simulating the pulse of the speed sensor over the two remaining pins, either way around. This is essentially done by creating a pulse across the circuit of pin A and A1. You can do this by just bridging the two pins together with a piece of wire and touching the wire against one of the pins on and off to make the ‘pulse’, as this is basically how the speed sensor works, but simulate any kind of actual road speed you will need to be doing the on/off pretty quickly. A far better way to bench test is to use some kind of ‘pulse-generator’, such as an oscilloscope or something set up using an Arduino, which is what I used to create a pulse with 5 volts. I will post a guide soon.
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