Recently, my 1989 300 SE (W 126) had a speedometer glitch. We set off on the highway, and the speedo was working as usual. Suddenly, the needle dropped to the peg and it did not work for about 40 miles or so. The odometer continued to work properly. Then, without warning, the speedo worked again, and has been working ever since.

Looking at a spare cluster I have, I discovered that these are not cable-driven speedometers. I assume that they are electronic, with a signal being transmitted to the speedo by way of a wire. Is there a common fault area I should look at, should this problem reoccur?

Anyone with expertise, I would most appreciate your opinions.

Joe B.