Looks a bit broken to me

Slightly Broken

Fairly sure the above picture describes it all. I must’ve ridden about 20, 30 miles before I realised this wasn’t going to fix itself and resigned myself to taking it apart to fix.

Bezel Removal

Bezel shown in red, main body in black

No proper pictures for this, sorry, but the bezels on speedometers of al ages and species are generally retained by the peening over of the soft bezel material.

Undoing this process is long and tedious, but is quite possible if you bend the tip of a knife over, and VERY slowly work around the body, doing only a little bit at a time. Doing a little bit should stop the bezel warping enough to be reused. The same goes for refitting it, tap it down a little bit at a time, keeping the bezel roughly even around the entire diameter, otherwise you’ll end up with a buckled bezel that doesn’t seal against the o-ring that stops all that moisture getting in and out.

Speedometer Dial

Input shaft So the speedometer dial itself is magnetically driven and in this case worked fine. As far as I could tell, the first gear in the system from the cable input was a helical gear driving either the bucket or magnet side of the speedometer needle, as well as the series of worm driven gears that scale the input ration down for driving the odometer.

Odometer

Scaling down the input shaft speed to the odometer mechanism

Lots of reduction required to get the correct ratio for the odometer. The main odometer and trip meter appear to have separate drives, presumably as odometer requires 10x more reduction than trip meter because it counts miles not 1/10th miles. The trip shaft is linked to the odometer shaft by a couple of gears in the middle of the whole assembly.

The shaft for the trip reset is shown vertically on the left of the above image.

Wheeeels MORE Wheeeels

The shaft going through the odometer is linked to the least significant digit wheel. This digit wheel engages with the white paddled wheel on a parallel shaft, which thereby engages the next significant digit, which links on it’s other side to the next white paddled wheel whic…… etc etc.

The white paddled wheels have a center rib which keeps them aligned with the odometer digit wheels, and give the next digit along a shove by one position (due to a nice little cam shaped thingy I couldn’t get a good look at sadly) for each revolution of the previous digit. Voila, denary.

Fixing The Issue

After lots of rattling around, shaking, brief moments of it working, more moments of it not, I found the issue was due to a badly constrained shaft onto which the odometer wheels are placed.

This shaft has a brass straight cut gear on one end that engages with the end of the worm drive for the odometer. I found that if I pushed it out to one side, the gear could be disengaged from the worm drive and all the odometer digits shuffled around in such a way as to manipulate any of them to any position (hence the ridiculous reading it had obtained).

The odometer shaft pushed clear of engaging with the worm drive

The other end of the shaft was only retained by the end of the shaft being tapered and inserted partially through a hole. I can only assume that the most likely cause of this whole issue was this tapered end wearing away and enlarging the hole to a point where it stopping the gear engaging at all.

Tapered end of the odometer shaft locates in a hole in the other side

Overall the solution was to support the tapered end of the shaft, and give the brass gear a little tap onto the shaft so that it was properly engaging with the worm, and so the digit wheels were no longer slopping about. Obviously I had to correct the mileage to the last number I could remember before doing so. This involved a fair amount of fiddling, but there was enough room to push the assembly all to one side, then pull the outermost digit clear of engaging with any white counter wheels, then turn it to correct number, pull counter wheel over, pull next digit over a little bit so it doesn’t engage with either of its counter wheels, adjust, rinse and repeat.

Tip: Use gravity to hold the digits engaged with the counter wheels for a much easier time adjusting. The digit wheels only engage in a single position with the counter wheels (if I remember correctly) so make sure they’re all engaged properly before tapping that brass gear down into position.

The fixed speedometer all operating correctly]