W126 Odometer Repair

My ‘new’ 300SE suffered from a common problem with Mercedes of that era, a non functional odometer.   The mechanical odometer is driven by a small electric motor instead of a traditional speedometer cable and associated gears.   This is because the speedometer is electric yet the odometer is mechanical.   This combination was common in the late 80s and early 90s before cars moved to fully electric speedometers and odometers.    The Series II W126 uses a VDO odometer unit shared with many Mercedes and BMW models of the era.     There are a number of articles on the internet that already cover W126 Odometer repair, however it took me a while to sift through them all.   This was one of the best.

The most common point of failure is a set of small gears that connect the electric motor to the odometer.   They are made from a very soft plastic to ensure noise is not transmitted to the cabin.   Over time, they loose teeth which stops them from working.     Replacement of the gears is a fairly simple procedure.   There are a few different companies who can supply the replacement gears – I got mine from Garagistic.

Removal of the instrument cluster

The W126 instrument cluster is simple to remove.   The cluster is only friction fit in the dash, and unlike some models the steering wheel does not need to be removed.   The official procedure calls for two special hooks to drag out the cluster.  Some people have made their own copies of these hooks. I found it easier to remove the drivers side speaker and simply push it out from behind.    There are many different connections to the cluster, and the trickiest part is removing the ones with the least slack first.  I found this was the vacuum line for the economy gauge.  That line is friction fit into its rubber tube.

Dismantling the instrument cluster

The speedometer is removed from the rear.    The outside temperature gauge is removed first.    There are after market replacement units available although I do not know their quality.   They are available in Celsius and Fahrenheit.

Next the plastic housing is removed.   Care needs to be taken with the speedometer face and needle!   This allows access to the rear of the speedometer.    It appears that the circuit board can be removed from the speedometer, but it doesn’t easily.   Its not really necessary, as once the back cover is off, the odometer gears can bs seen on one side and their cover removed.     From here the Garagistic instructions are useful, even if they are for a BMW.

Replacing the gears

The gears are easy to remove, and my small gear was pretty much disintegrated.   The only slightly fiddly job is that there is a metal bushing that also needs to be removed as the new gears do not need it.  The first photo shows two of the three gears removed and the second shows the new gears installed.

Some people recommend some grease to ensure the gears are silent.   I didn’t have any on hand, so I’ll need to see if they are annoying and I need to remove the cluster and add some.   There was also a 4th tiny gear in the pack I didn’t seem to need.   I am not sure if this is for another model, or I missed it.   Re-assembly is simple and only takes a few minutes.

UPDATE September 2018.   The repair above worked.  However, one of the gears was the wrong ratio, causing the odometer to run low.  I also chose to lubricate the gears to try and reduce noise.   For more details click here.  I also fixed the Odometer in my 560SEC using the same procedure.