Speedometer Pinion Gear

Shear age weakens and destroys the plastic drive pinion in the W126 speedometer (Mine is out of an Australian-compliance 1989 560SEC). A replacement 12 tooth pinion is difficult to obtain in Australia, so I resorted to printing a spare one on my Mendel Prusa II.

Design was based on the Openscad design file “parametric_involute_gear_v5.0” by Greg Frost (2010). Gregs original design was fiddle, fudged and stripped until the pinion looked “right” and printed correctly.
The modified Scad file has been included so anyone wanting to try for a different configuration can have a go. I printed it direct from the STL file provided.

So far the odometer is still working with the new pinion. The original printed pinion is a trifle “small”, so the current STL file has got a slightly larger gear profile so it should mesh better. (Update 1/8/15 - new slightly larger gear printed and fitted. No problem with bore size, and much better mesh than original design - STL file in "downloads" was always correct version)

Thanks to Greg Frost for his original work.

It was printed on a Mendel Prusa II in silver ABS using a 0.3mm nozzle. Extruder/Bed settings were set to 250/110 degrees. First layer was set to 0.15mm with the rest at 0.1mm.
On my printer, the printed pinion ended up with the correct bore size, but the hub bore was slightly smaller due the material flow. The smaller bore was carefully reamed out with a small round file so that it became a firm press-fit onto the ribbed brass hub that is left after the old pinion has fallen off.
It is much, much easier to fit the new pinion with the speedometer circuit board separated from the speedometer proper. This requires desoldering and resoldering of the 5 pins (2 for the speed meter, and three for the odometer motor.
The clear cover and the first large gear must be removed from the “gearbox” end of the speedometer assembly (2 screws). Also remove the odometer drive motor from the other end of the speedometer to expose the drive rotor (4 screws).
It is VITAL to support the shaft on the hub of the rotor as the new pinion is pressed on to prevent damage to the speedometer assembly. Be careful not to press the pinion so far on that all the “end-float” of the shaft is gone – a few thou clearance is all that is needed to ensure free running of the shaft.
Replace the drive motor and the gearbox before re-attaching the circuit board.
If you are having problems with speedometer readings, and replacing those two capacitors didn’t fix it, I’m working on an method of adding a speed adjuster to this type of speedometer (does not affect the odometer ratios). As this project progresses it is being posted to Peachparts.com.