83 300sd speedometer
 

I have just started having some problems w/ my speedometer. It started not working and then bouncing around some. Today it seems to be working but definately not at the correct speed( reads 55-60 @ 70mph). I also noticed the gas gauge was not registering imeadiately and then coming up after several seconds. The Tach has also been flaky as well( I have already done the quick fix -shimming the amp). I think there may be grounding issue. I removed the cluster yesterday and could not find any obvious problems. All the connectors looked Ok. How or where can I check the ground to the cluster? Should I check anything else?Where does the speedo input come from? I can't find a ground strap to the motor( similar to W123 onthe bell housing). I added an addition strap from the chasis ground to the block( didn't seem to help).

Paul

Anybody??

Look for the brown wires.
 

The ground wires are brown in color. According the the wiring diagram, there is a ground behind the instrument cluster. It should have a bunch of brown wires attached to it. This ground does a lot. It is used for the cluster, hazard switch, radio, all the cluster switches and lights, rear defogger, etc.

You probably have a bad connection from the cluster to this ground contact.

Let me know if you want me to send you the ground distribution pages from the circuit diagram.

I can see the bunch of grounds behind the cluster. I need to find where tehy come into the cluster. It must be on one of the plugs. I assume the wire should be brown as well. I guess I can measure resistance between the plug ground and the chasis.

The ground wires in the cluster are also brown.