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Diseasel300 · #3 09-20-2017, 11:52 PM

The speedometer/odometer in the W126 is all electronic, there is no cable involved. The odometer portion is driven off a stepper motor.

I took the dash cluster out this evening to see if there is anything obvious going on. The main odometer and trip odometer are geared together and tight. There is no possible way that the trip odometer can slip while the main odometer doesn't. All of the number wheels are good and tight on the shafts and there is no slop in the mechanism. What I did find was a binding reset button. I can only assume that this was rattling in the cluster or holding the reset mechanism just so that the numbers would increment, but not reliably. I resolved the sticking reset button with a dab of silicone grease and now it operates smoothly.

In addition to the binding reset button, I noticed an unusually large amount of play in the odometer stepper-motor shaft. The amount of slop was such that the drive pinion could be ~85% disengaged from the driven gear. With the wiggle room allowed in the driven gear, it could become fully disengaged and indeed had been doing so as evidenced by a beveled wear pattern on the ends of the teeth. I remember reading somewhere on this forum (I can't find the thread to give the OP credit!) about inserting an O-ring behind the plastic bearing on the stepper motor to eliminate the runout. Apparently at one time there was a rubber bushing installed that has since gone missing. I used a 5.5mm x 1.5mm O-ring which fit perfectly and gave a firm fit without causing binding.

Took the car on a test-drive (the same 6.8 mile trip as earlier) and it read 6.8 miles. I then took it on a couple of other known-distances and the distances agreed with the known-values. I will let the mileage accumulate and see if I still have problems at >100 miles.

As an interesting sidenote, installing the O-ring completely eliminated the "Quartz-Clock" noise that the stepper motor made as it indexed the odometer. The odometer drive is now completely silent.

If anyone reading this thread remembers the thread I'm referencing above (or if you're the OP in that thread!) please chime in to give credit where due!