Riding down the M40 yesterday (yawn) and the Speedo suddenly stopped working. What are the most likely causes?
inner cable is top of the list Gavin, most likely at the wheel end, just at the end of the square 'drive' section (cable run could have got dry/lightly corroded resulting in light stiction)
you didn't mean that to do cable though did you? Just undo knurled nut at end of cable/wheel end and all will be revealed - if square section is still sticking out of (now detached) cable assy then try and spin it with your fingers to see if speedo flicks if it does then sadly you are right and problem is almost certainly in the drive unit and it would be wheel off but no point until you have a new/secondhand unit ready to salvage. If it is the speedo drive unit then nylon gear is most popular fail.
The knurled nut just seems to turn and turn without actually undoing. It looks like there's another knurled part just sticking proud of the housing. Should the outside nut turn independently of this?
they have a habit of doing that unfortunately - the brass body with the male thread (which the knurled nut threads on to) is only fine-splined into the plastic body so if knurled nut is tight either by overtightening or light corrosion again then it tends to turn the whole body in the plastic. Bit crude (but works 9/10 times) but you need to search for a pair of pliers etc with a nice cupping action that will grip the plastic outer body diameter (failing this, any pliers flat-on-flat will probably work). It will leave slight tooth marks but hardly noticeable and it doesn't usually take much effort to restrain it before nut undoes.
Ok, gripped the plastic housing with one set of pliers and the knurled nut with another. Plastic housing broke. Managed to get everything out in the end and the inner cable has snapped. I'm looking at eBay now and all the Speedo drives from that era look the same, no matter what the model. Is this the case?
Yep**, almost just check the ends are the same* and of course length if anything longer is better than shorter as that's the very reason they usually break. The commonest replacement type has an adjustable collet-type clamp for one end only, obviously not as good as an original but it works okay I've used 3 of them so far and much cheaper than that place in Dorking *speedo cable-wise **don't forget there are 2 axle sizes - didn't realise you were asking about speedo drive unit at same time.
If you get stuck for a complete drive unit, I can lend you one until you get the right one......1996 model should be the larger axle. Check your wheel to be certain........the brake disc locating ring cast into the hub should be approx 4mm thick.......the earlier wheel for the smaller axle will have a locating ring approx 6mm thick. You can actually tell by looking at the drive plates as well....see below. I assume you didn't bugger the grease seal.
The replacement turned up from Durhamducs ebay shop very quickly. The good news is that the drive unit is correct for my axle size. The bad news is that the Knurled nut at the speedo end on mine needs to be 16mm but the replacement that I bought has a 10mm thread. I've identified what I think is the correct cable from the parts list. It has the part no. 40310041A. I've ordered a new one of these from Moto Rapido. Fingers crossed, it has the 16mm fixing.
My parts list says that is the Km speedo cable (quite how the cable knows the difference, I don't know)........... The speedo drive may be either 49810051A or 49810071A.......... I found that changing my front tyre profile from 60 (OEM 750) to 70 (OEM 900) the speedo became far less optimistic.........in fact the damn thing seems spot-on at 30mph so I have no tolerance in speed limits.........
