Vintage Cars: 83 500 SEL Speedometer, electronic speedometer, mechanical wire

Question
QUESTION: The speedometer in my 500 SEL hovers @ 40 mph until I have driven about 30 minutes. I am told it is a grounding problem. How do I go about fixing this?

ANSWER: If I remember corectly the S series W126 had an electronic speedometer....so its likely an issue with the pickup/sender on the transmission, or an issue with the gauge itself....as there is no mechanical wire connecting.  I am not aware of a fix that deals with grounding, unless your dash pod is not grounding in the frame or one of the traces on the board has broken

---------- FOLLOW-UP ----------

QUESTION: You are correct. It is electronic. How would I find where the dash pod is grounded or where the traces are?

More info - If I turn the key on without starting, it hovers @ 20 mph. Once the car starts it goes to 40 mph. If I put the windows all the way up, the speedo drops a little until I release the window switch,then it goes back to 40.

Answer
sounds like a ground issue.  The dashpod grounds to the frame via metal tabs.  If you can try to get under and behind it and connect a ground wire to the pod  and then connect the other end to a known good ground.  See if that helps.  I had to do this on an W124 and it worked . You may or may not have to pull the pod, depends on how much room you have behind it to work
good luck
Ron