Speedometer Needle vibrates
 

My speedometer needle vibrates a little below 40 MPH. Above that speed, it is stable and operates normaly.

Have a 94 E320 58K miles.

That is usually a sign of a slight "kink" somewhere in the cable. Most likely in the cabin.

The cable resides just behind the accelerator pedal. Make sure it has some slack to it and there are no unusual bends. A damaged cable could also do what you describe, but typically, the needle would vibrate at ALL speeds...

I cured this as follows;

Remove instrument pack (it just pulls out until you can reach behind to unscrew the speedo cable, then there should be enough slack on the wiring so you don't have to disconnect anything else)

Run a few drops of oil (I used EP90 like for the diff) down the inside of the cable.

Put it all back together.

I followed G-Benz's suggestion and looked under the accelerator pedal. I saw a black cable going into a body opening and a white cable going up, which one is the speedo cable?

I moved both of them around. The black one was almost flat at the turn into the opening. Anyway, now the needle vibrates less. I did not want to further without being sure which cable was the speedo.

G-Benz, Mick J which one is it?

The speedometer cable is the one that connects to the speedometer. Follow GBenz' directions and you will find it connected to the speedometer. Lubricating the cable is definitely a good thing.

Good luck,

Hey guys, FYI. Before pulling out the instrument cluster, dis-connect the cable at the transmission. 124 cables don't have much slack and the cable can be damaged by simply pulling the cluster out far enough to get your hand behind to un-screw it. I've seen this happen many times before. The result is exactly what zafarhayatkhan describes.

Yup, always disconnect the mechanical speedo cable before pulling a cluster. They really should ban the use of those pulling hooks, since you can just push the cluster out from the rear once you have the cable undone.