[Walt Fricke]

The speedometer ground is a bit tricky. There is a ground from the speedometer case to the chassis ground. This serves as the ground for any lights in this instrument.

However, the speedometer positive and speedometer signal contacts are grouped with another ground, and these three are isolated from the case ground. Note that these three contacts stick through a non-conductive plastic of some sort. If you fail to jumper this ground to the case, or separately to the chassis, the speedometer won't work.

I think that that this was Bosch allowing for the possibility of positive ground automobiles, but that is just my speculation.

In any event, if the case is grounded, but the instrument part is not, you will burn out a diode inside the speedometer. I hypothesize that the lack of a proper ground here means that the ground through the speedometer sensor (turning the rear wheels alternately opens and closes a switch to ground) routes excessive current through this diode.

In this event, it won't work, even if you fix the ground issue. You can repair this by opening the case, locating the diode (thankfully, it is close to the surface of things), replace it (any common diode of its type will do), and put things back together.

I know this because my '77 Euro, when converted to meet US requirements with a miles per hour speedometer, had one which didn't work. And a brand new replacement which also didn't work. All due to the fact that someone didn't realize this.

I'll send a picture. Maybe by 1980 they had changed this setup. I'll take a photo of the backside of a speedometer.

Here is how to test your speedometer: hook up the +12v and the speedo ground I have described - the one which is part of this cluster of three contacts. Attach a wire to the sensor contace which has a bare other end. Rapidly touch this bare end to a ground and remove it: tap tap tap tap. This mimics what the sensor does. If the speedometer needle moves, the speedometer is probably fine. If nothing happens, something is wrong with the speedometer itself.

Walt