Speedometer issue- 73 S3 V12

How can this happen? has anyone seen this before?

I replaced this drive gear after replacing the speedometer cable and finding the Speedometer still not working. After about 100 miles, the speedometer started bobbing and then finally stopped working again. Assuming this is the root cause of the speedometer not working, what can be done to resolve this? Does anyone have any experience with this?

I can’t say I’ve seen that on a speedo drive but I have on a distributor drive on a V8 Daimler engine. Unless you have cleaned the drive it looks like its been running very dry.

My ('72 S3 V12) has 16 teeth and OD is 20 mm.

The speedometer gear sits in the transmission tail section, and usually doesn’t see much wear. For this to happen, there had to have been a problem upstream: in the angle drive, cable, or speedometer head itself. I don’t know of any source for replacement gears. Worse, the drive gear on the tailshaft may be similarly worn.

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It’s not entirely clear from your post whether this is the second such shaft/gear that you have replaced, or there has only been one. If this is the second, was the first similarly damaged? If so, one would have to conclude that there is a problem with the mating gear in the gearbox tail shaft.

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This is the second gear replacement. is it possible to access the mating gear without dropping the gearbox?

Walter,

I am not that familiar with the details of the gearbox extension on the Series 3 cars. Hopefully someone else here can better advise. From what I’ve read in the S3 ROM it seems to me that it might be possible to remove the extension with the gearbox in the car, but what do I know…?

I’m not sure if it’s possible to replace in the car. But before you go there, why not use a borescope to inspect the drive gear? It should be pretty straightforward to see inside with the bullet removed.

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If your current replacement is similarly worn like the original shown in your picture, there is clearly something amiss with the gear on the Main Shaft that drives the Speedo Drive. However, if the new Speedo Drive gear of the Speedo Drive unit is OK, it may be as simple as the Companion Flange Nut not being sufficiently tight.

The Speedo Drive gear on the Main Shaft is secured and driven by nothing more than the clamping force from the Companion Flange nut. If this nut is not torqued up sufficiently, then the gear on the Main Shaft can slip. I’ve seen this many times, therefore, that would be the first thing to check. If this is the problem, the Gearbox Oil Pump won’t be working either, for the Drive Sleeve for the Oil Pump is also driven via the clamping force of the Companion Flange nut.

There is no torque specification for the Companion Flange nut, for the nut is castellated and relies on a split pin to stop it loosening and therein is the main reason why this nut may not be torque up well enough.

As mentioned, this nut must be torqued up sufficiently to drive both the Oil Pump Drive sleeve and the Speedo Drive gear on the Main Shaft. The inexperienced, or the Numb Nuts that sometime work on these gearboxes, when they find that a slot in the Castellated Nut and the split pin hole in the shaft don’t align, they back the nut off until it does, this is the worst thing they could do. I torque this nut to circa 110 lb/ft and if no slot in the Castellated Nut aligns with the hole in the shaft, a small amount of metal should be removed from the inside face of the nut, or the thick washer it interfaces with. I have a well-equipped machine shop; therefore, this is a simple task for me, but the same result can be achieved by lapping the face of the thick washer with, say, 180 grit wet and dry abrasive paper backed up by something flat such as a flat piece of glass. The aim is to get a slot in the castellated nut and the hole in the shaft to align when the nut is torqued up tight.

I doubt that the Gearbox extension of a long wheel base E Type (S1/S2 2+2 and all S3 cars) can be moved to the rear far enough to clear the Main Shaft with the gearbox in the car, but the replacement of the Speedo Drive gear on the Main Shaft can be done with the Gear Box and Extension in place, albeit not a pleasant job.

If the gear on your replacement Speedo Drive unit is chewed up like your original in such a short time, I would be concerned about the metal debris that would be present and would be removing the gearbox to rectify for sure.

Regards,

Bill