'87 560SL Speedometer Pegged with Key On
 

I'm still working through a bunch of little issues on this vehicle and this forum has helped immensly with everything so far.

I'm now trying to determine what would cause the speedometer to stay pegged at the max end when the key is turned on. When the key is off, it returns to zero, but as soon as it gets power, it just stays pegged. I see lots of posts about speedometer problems, but haven't found anything talking about this specific symptom.

Does anyone have any thoughts on the cause?

Thanks as always!

The speedometer is electronic. It's entirely possible you have a failed or shorted component on the circuit card that the speedo unit is attached to. A forum member by the name JamesDean services a lot of electronic parts for these cars, not sure if he does speedo repairs, but it may be worth a PM to him to find out.

This kind of weirdness is usually a product of poor electrical grounding.

When you pull the complete cluster out, you will find a large grounding block behind it. Using a clip lead, ground your cluster from one of the speedo mounting screws to that ground. If that cures the problem, then wire a positive ground from your cluster to the ground block.

Is there any way to get at the speedo mount screw without removing the cluster? I'd rather not go through the process of removing it if there's a way around it.

Thanks!

No but the cluster is a light press fit and not difficult to remove. Two methods:
1. Remove driver side underdash panel, reach up and push out from back. You will first have to remove the knee pad (2 screws).
or
2. Fashion two hooks from hanger wire, push thru left and right side of panel then pull
out.
There is enough cable slack to get at back of the cluster and to the ground block.