Your problem sounds very like the one on my '86 300E. Here's what I did:
1. Remove cluster from dash
2. Remove all the screws holding the gauges in the cluster. These include the screws holding the light dimmer rheostat and the outside temperature display
3. With everything now loose, you can lift out the speedometer. Always be very careful of the speedo pointer. What I do is remove the gauges on each side of the speedo and then set the speedo back into the cluster thus protecting the pointer. 'Careful of the other pointers too!
4.Take off the 4 screws holding the white plastic back cover on the speedo.
5. Now you may be able to see the problem. I had two problems.
a. There is a pot metal lever which has a U-shaped fork on the outer end. This lever is pushed in when you press the trip odometer reset button on the front of the cluster. Mine was broken in half. I had to buy a speedometer on Ebay and salvage this part from it. Swapping this lever is a bit tricky but not rocket science.
b. The second problem was this. Look for a loose, flat piece of metal made of spring steel. This piece of metal has, I think, 4 fingers on it. I don't know what to call it so let's refer to it as the 'piece'. This 'piece' was attached to a black plastic cylinder which is rotated when the reset lever is pressed. If you move the reset lever and its linkage, you'll see this cylinder move. The 'piece' was fastened to this plastic cylinder so that the fingers on the piece pressed against the trip odometer rollers which have the numbers on them.
When you press the trip reset. the lever and linkage rotates the cylinder, the fingers press against the rollers, and the rollers are rotated to reset the numbers to zero. If the 'piece' is loose or off the roller, the numbers will not reset. I don't know if this is common, but both my original trip odometer and the one I bought on Ebay had the same problem. I'd bet your's does too.
The 'piece' was originally held to the plastic cylinder by a sort of heat weld where small plastic nubs projected out of the cylinder. These nubs fit into two holes on the 'piece'. The plastic nubs were melted so they spread out over the edges of the holes like plastic rivets. It's these nub/rivets which break off.
The 'piece' needs to be refastened to the cylinder. The remains of the nubs serve to locate the position of the 'piece'. I used JB-Weld, an epoxy repair which has a very good reputation. I lightly scratched the surface of the cylinder and cleaned off the 'piece'. Then I put a thin layer of the epoxy on the cylinder and placed the 'piece' on the cylinder using the nubs to be sure it was properly located. I also spread a little epoxy over the holes where the nubs were and on the edges of the 'piece'. I held the 'piece' on tightly while the epoxy hardened overnight. I don't recall if I used a small clamp or whatever to do this.
I did make a mistake at this point. I left too much epoxy on the top of the 'piece'. There isn't much space above the cylinder when the reset lever rotates the cylinder and moves the 'piece' fingers against the rollers. The epoxy I left got in the way thus not allowing the reset to fully take place. I had to scrape the epoxy down a bit to allow a full reset. To see if you have too much epoxy, move the reset lever to rotate the cylinder, look at the epoxy on the top of the 'piece' to see if any of it touches the frame above the cylinder. If it does, scrape off the epoxy until it no longer touches. Reassemble. You may want to put in fresh bulbs (2) which light the guages.
I tested the trip odometer by spinning the speedometer cable connection with an electric drill. I put a Q-tip in the drill and pressed this into the place where the cable fits. I think I may have had to reverse the drill. Let the drill run intil the trip odometer shows a mile or two, the try the reset.
This was a fun repair since this is one of those parts you just can't buy. Of course you could buy a used speedometer but be sure the seller guarantees the trip odometer reset works.
