Speedometer / odometer acting strange

[Cees Klumper]

A few weeks ago, my speedometer started acting up. First, the daycounter stopped counting, for no apparent reason. Then, a couple of hundred miles after, the odometer stopped as well. The speedometer still works, although it is slightly jittery between, say, 50 and 60 MPH which is never was before.

I checked both connections today, at the transmission AND at the back of the speedo. Both seem ok, and the cable itself looks relatively new. The speedo was replaced by the previous owner for a brand-new (KM) unit only 6 years ago.

Does anyone recognize these symptoms? It seems a bit odd that the speedo still works, but the odometer / day trip counter have stopped, and not at the same time.

Any tips, advice more than welcome!

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Ricardo]

Cees
When I opened up my speedo a couple of years ago, I found that there is a plastic worm gear driven by a brass crown gear and the plastic was chewed up from wear and this caused the gears to slip out of contact. I was able to slice a little off the end of the plastic gear where it was damaged and placed a washer at the other end to shim it into contact with the brass gear. This is from memory so I may not be describing it accurately, but it's still working fine three years later. The speedo is actually a spinning aluminum cup/magnet affair and there is no direct connection to the cable drive, so it can still work when the odo has stopped.
Ricardo

[Cees Klumper]

Encouraged by Ricardo's post, I decided to take out my speedo again tonight, to see if I could find the problem myself. This time, it took only 15 minutes for me to take out the tachometer, central instrument cluster and speedometer. I opened up the back of the speedo (it still looks new, being only about 6 years old) and here's what I found. One of the worm gears is kept in place by a brass bushing. This bushing had come loose, allowing the worm gear to disengage. That was all. I put the bushing back in, making sure it was tightly seated this time and checked whether the mechanism now worked by manually turning the connection to the speedo cable - all seemed fine. I then did the 'installation is the reverse of removal' routine and went for a test drive. It works perfectly again!
Thanks Ricardo for the tip. I must say I am just a little bit disappointed at how a relatively new (and expensive) instrument can malfunction like this. But, in all fairness, this is the very first time something has broken down during my 5 year ownership, so I guess I can't complain. At least I mananaged to finally clean the central instrument cluster glass.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[Ricardo]

Cees
It's nice when you can fix something that seems too difficult at first, or when the expense is not in the budget.
I found that I could drive the speedo, to check it, with a square drive bit (Robertson bit) in a battery powered drill, you can even run it backwards to get your mileage down too. Of course this is illegal, so I don't want anyone following my suggestion, unless it's just to show the wife that you really were at home working all weekend
Ricardo