300SD speedo question
 

Fixing the odometer has been creeping toward the top of my to-do list. That is until yesterday. Neither tach nor odometer have worked since I bought the car last year. Took dash out and saw that plastic gears on odometer were shot. But the speedo worked. While driving yesterday, the speedo start jumping erratically. Then quit totally. I'm thinking that I should replace the entire unit, rather than try to fix both the speedo and the plastic gears. Any advice?

The write up by diesel giant is good regarding the gears. If the problem with the speedo is in the speedo itself and not elsewhere then yeah, a whole new instrument cluster would do well. I bought a new to me one on Ebay for around 50$ and the seller had actually rebuilt the Odo. (I assumed this because the gears that break were a different color and brand new.)

I am not sure what would cause a bouncing speedo.

Get some Carrot colored paint and a hobby paintbrush and repaint the needles while you are in there. I LOVE my bright needles.

http://boostnbenz.1baddsm.com/

Diesel Giant is fixing a W123 spedo which is all mechanical except for the 81TD which is driven electricaly.

The W126 us driven electricaly from the tail of the automatic trans. Part of the spedo is electric, and part is mechanical in the spedo head.

I assume the W126 is like the W123, the spedo is matched to the differential gear ratio. So if you replace the whole cluster, make sure it is matched to the Diff. E-Bay sellers usually have no idea where is came from, where they got it let alone what car it came out of, so it is a crap shoot of it being for your yr model.

I just completely dissassembled my 240 spedo down the all the number wheels off the shaft etc... cleaned it all up, fixed the slipping pot metal gear, and reassembled. I went from a 85mph to a 100mph, and matched the new odo to match the milage on the car. easy fix.
the W126 I have looks quite a bit more complicated to work on. I need to rip it apart to see what makes it tick.

I haven`t seen a W126 spedo DIY pictorial yet.

If you do decide to get another cluster, use your old spedo for the sacrifical Lamb.

Charlie

Fixed similar problem
 

I had this same problem with my 84 300SD.
Speedometer was jumping up and down intermittently and then quit working altogether.

The cause turned out to be the magnetic cylinder (about 2" x 3/4" IIRC)
that plugs into the underside of the transmission. Mine was just hanging
by its wire. The screw that holds it in place was missing. I simply slipped
it back in place, screwed it in and that fixed the problem.

Hope yours is as easy to fix.

Jeff

odometer gears & clusters
 

My odometer recently went out (unrelated to the speedometer problem I mention above) and I got a good set of gears from the junkyard and it fixed the problem. It's an easy fix.

If you go that route, look closely at the gears you need to make sure they match.

Not all W126 clusters are the same and there are differences within the 81-85 W126 clusters. I believe the 85 & 84 clusters are different.

Jeff

another thing that kills the speedo is the wire harness under the kick panel. but it also kills the ability to start the car with the key...

Many thanks for the good advice. Gonna look at the pickup (from trans) tomorrow. Had planned to replace gears, but if pickup looks good, will just search for pre - 86 cluster. Said it before - but - what a GREAT forum.