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MG MGB Technical - Speedometer/odometer 1974 B

Having trouble with the speedometer/odometer on my early 1974 B. Neither function works.
Replaced cable, swivel and nylon gear that goes into the transmission. No go.
Connected cable to electric drill and speedo and odometer work!
Have been told that there is a gear on a shaft in the transmission that may have slipped out of place, thus no contact with the gear on the end of the cable.
Could this be the problem?
Can you or someone direct me to a schematic of the transmission showing how this is set up? (Most parts displays don’t have this part shown in sequence.)
If this is the problem, can it be fixed without pulling the engine and transmission to service it?
Thanks for the help and/or direction.
Tim Beaton
TJ Beaton

Sudden failure or never worked in your ownership?

If it's never worked then the worm drive may not even be present. Logic dictates that with the drive pinion out you should be able to see it, and turning with one wheel off the ramp to turn the prop-shaft.

With the pinion out of the gearbox but connected to the right-angle drive (?) and cable if turning the pinion (be patient) turns the odo then yes it has to be the worm drive on the shaft.

From what I have seen the right-angle drive at the gearbox end is only needed for 3-synch boxes.

I don't think the worm drive can slip out of position as such, and whilst it could strip I would think it unlikely for the whole circumference to strip i.e. if parts of it had stripped then the speedo would be jerky and read low.

If you don't have overdrive then it is supposed to be possible to remove the gearbox without removing the engine (http://www.mossmotoring.com/pull-mgb-engine-read/) but for me that's far fiddlier then removing the two together or engine first then gearbox.

With overdrive it _might_ just be possible to change the worm drive in-situ by removing the output flange, oil seal and bearing as behind the bearing there is a spacer and then the worm drive (14 in the attached), it depends how tight the bearing is on the shaft and in the casing. It's clamped there by the output flange, so maybe a sliding fit.

But on non-OD boxes the speedo drive is much further forwards and the rear extension has to be taken off the gearbox proper.


paulh4

The speedo/odo worked from our purchase in 1979 (55,000 miles) through about 2000 (about 80,000 miles)
The cable seems to be okay. As noted, if I turn it, the speedo/odo works. However, once reinserted back into the transmission, no go.
No overdrive.
TJ Beaton

You say 'if I turn it, it works OK'. Are you just turning the cable? If so you need to remove the angle drive and pinion and turn those while connected to the cable.

You can also do the other thing and rotate one back wheel (the other one locked in some way) so as to turn the propshaft and see if the output of the angle drive turns. If so then put some load on the output and see if it still turns.

Also with the wheels locked try turning the output of the right-angle drive.

If it turns when it should, and doesn't turn when it shouldn't. I can't help feeling you are missing something.

I assume the cable is fully seated in the speedo head. It's possible to get the knurled nut on a couple of turns but the inner isn't correctly engaged.

If you look at where the speedo take-off is on a non-OD gearbox you will see why the extension almost certainly needs to be removed to access the worm gear, hence gearbox (at least) out.
paulh4

If the worm drive gear is indeed loose on the shaft then it would be worth just checking that the drive flange nut is done up to the correct torque since it is that that clamps the rear shaft assembly together.

No need to remove gearbox, just the propshaft. The worm gear is steel and the pinion plastic so I very much doubt the worm is damaged.
Chris at Octarine Services

Incidentally there is a 'key' shown for the speedo gear on the non-OD output shaft, but not for the OD output shaft. So on that basis unless the key has failed or is missing it would seem less likely that a loose drive flange would cause the worm gear to slip on the non-OD arrangement than the OD arrangement.
paulh4

Thank you to all of you who have responded to my post. VERY much appreciated. No true British auto mechanics around here and I am now too old and fat to slide under my car so I may have take your suggestions to a young mechanic I know and "coach" him through your suggestions. Thankfully the B is not my daily driver so I can afford to wait for his help.
Tim
TJ Beaton

Ran across a You Tube from John Twist of Moss. It is at John Twist 234 Flickering Speedometer.
Chris, he shows how to do what you suggest. It looks like the worm gear in the demo is nylon. In the you tube, John says that older cars had a key way but not the newer cars. Wonder if that also applies to the steel vs. nylon worm gear?
TJ Beaton

I have seen both steel & nylon gears, keyed and unkeyed.

You only have to look in the hole having removed the pinion gear to see which you have and a quick finger test will show if it is loose or not.
Chris at Octarine Services

Tim,
John Twist is not of Moss, he used to be of his own company University Motors. I don't know what Moss is like on your side of the pond but it wouldn't necessarily be a compliment if relating to side's Moss.

He put up literally hundreds of useful videos.

https://www.youtube.com/user/Universitymotorsltd/videos

https://universitymotorsltd.com/

Along with Paul's mgb-stuff website and Chris's website there's a vast wealth of knowledge and information on Bs.

http://www.mgb-stuff.org.uk/hammertext.htm

https://octarine-services.uk/forum/
Nigel Atkins

This thread was discussed between 01/08/2019 and 07/08/2019

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