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MG MGB Technical - Speedometer driven gear failures

My speedometer has failed many times but the most recent failures have been due to the nylon teeth on the driven gear rounding off. The gear fails in the wintertime, in the first 4 or 5 miles of early morning driving. It failed twice on the same hill in Chihuahua in exactly the same place as the car was warming after an overnight stay in the city.

The speedometer has been checked, and checked, rebuilt and checked again. The angle drive at the overdrive unit has been replaced four times (it failed once). The cable was replaced following each failure with a Moss replacement.

It occurs to me that the gear strips when the cable binds, probably due to its serpentine route into the RH foot well and up under the dash into the second angle drive mounted on the speedometer. I have considered drilling a hole in front of the speedometer through the bulkhead so that the cable avoids the tighter bends and the second angle drive. I tried to order one of the new electronic Smiths speedometers from Moss but they are on back order. My hands get cut up reaching under the pillow dash to remove the unit and the transmission must be drained each time the gear is replaced. Minding the speedometer is one of the worst repairs on the car and on mine, has been the most frequent.

Any suggestions would be appreciated. i am getting sick of this.
Glenn Mallory

Having just replaced my pinion and seal (because of an oil leak that travelled up the cable and contaminated the speedo) I feel your pain.

As it fails in winter it sounds like the lubrication is too heavy as much as anything else, but the route behind the dash which needs the angle-drive on the speedo is less than ideal. Later LHD cars brought the cable across in the engine compartment then through the bulkhead behind the speedo and didn't use an angle drive so that is definitely something to look at. Clausager shows one of these, but the attached is probably clearer as the engine and gearbox had been removed. This has the EGR service counter (arrowed) but the direct cable takes the same path.

There are two holes the cable could be entering at the moment, the second image shows where they enter on an RHD, LHD can be there or the other one higher up that is arrowed, and in one example I've seen it enters in the RHD position then goes out again via the other hole!






paulh4

Thank you Paul. The images are very helpful. I believe that a non-overdrive cable run directly from the overdrive box through the engine compartment will work. My car is 1970, hence no counter box. The cable for the overdrive box will be too long for the straighter run. The new routing will bring the cable into hotter territory near the exhaust manifold but if the cable is supported from the heater box, it should be fine. The cable in the midget takes a similar route near the manifold and if lubricated prior to installation, it holds up OK.
Glenn Mallory

I'd always wondered what that stud on top of the heater casing was for till I saw this :o)
paulh4

This thread was discussed between 13/12/2020 and 15/12/2020

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