• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB 72 mgb speedo drive seized.....easy fix?

Share this page

B

borjis

Senior Member
Offline
1972 mgb WITH overdrive.

driving last summer I noticed my speedometer was starting to slow down. 35mph would read as 20mph. It gradually got worse until it would barely move then it stopped moving at all.

today I pulled the the angle drive from the overdrive
and was really hoping it was broken. It was not.
the angle drive spins and drives the cable just fine.

I put one end of another speedo cable I have into the input on the overdrive. Its seized. I see there is a bolt and a bracket holding it up in there. Before I open it up and potentially make it worse I wonder if anyone here knows if this can be easily repaired WITHOUT pulling the transmission.

I'm willing to live without the speedo working if it required pulling the engine and tranny to fix it.

thanks.
 
You're telling us, I think, that the gears in the transmission itself are frozen?

If so, they're easily removed by taking bolt out...new ones are available from Moss...depends on whether you have 20 or 21 pinion.
 
hi tony thanks for the fast reply.

The "sending unit" for the speedo cable coming out of my overdrive unit is seized.

If I turn the the drive shaft with the car out of gear
it won't move at all and I can't move one way or the
other if I shove the square end of another speedo cable in it.

The angle drive and existing speedo cable spin freely (both were new a year ago as well as the speedo itself being service so I know thats not the problem.

3 questions for you:

So your saying I can remove the bolt holding it in there and
just pull shaft/gear out without disturbing anything inside the overdrive?


Will the overdrive lose all its oil if I do that?


If the gear and shaft seem fine, is there something else inside that could be the cause of the problem?

I'm looking at the moss catalog and I see the 21 and 20 teeth pinion you speak of.

Is the gear made of plastic?

I wonder if the "speedo pinion bearing assy" moss part 2-7311 is whats seized (the bearings themselves) if they seize I wonder does it brake the pinion gear.



Thanks much.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Will the overdrive lose all its oil if I do that?[/QUOTE]
Possibly a little.

Here's page 41 of the Moss catalog showing the expanded figure of how things go together in the OD unit...I think you're talking about items #30-34:

MGB-042.gif
 
yes thats exactly what I'm talking about.

So one can definitely pull that out without
disturbing the overdrive unit?

I would hate to pull it and have something come apart inside and have to remove the engine and tranny.

if its really that simple and losing a little oil is the only consequence that would be fantastic.

Thanks for your help man, I was hoping you would have an answer for me. :smile:
 
As Tony says, it's an easy fix. The gears I've found are plastic and can strip the teeth off in time. Also as Tony suggests, count the teeth on the gear you remove and order one with the same amount. I would also check to see that the cable is free turning with no binds.
 
well the plastic teeth (21 count) appear to be severely worn down, all of them are very flat but sharp near the tips and a few of the teeth are worn further a little concave downward.

looking inside it looks like the feeding gear to this might also be plastic, is that true?

From the exploded view it looks like that huge hub (ANNULUS) is what drives it, surely thats all metal.
 
Easy way to tell - stick a magnet on a stick up inside...if they are, they mght be frozen or worn also.
 
well, i just probed with a magnet.....typical my luck, its plastic and appears to be severely worn, much more than the pinion gear is. Its darn near flat in the middle.

tony are the teeth on the pinion supposed to be razor sharp on the replacement units? I know some gears are supposed to have flat teeth (most of the gears in a transmission for example) but I've seen quite a few plastic gears in my time with razor sharp teeth.

I'm thinking it MAY work if the new ones are really sharp tipped gears.

I don't suppose that other gear could be replaced with the transmission still in the car by removing the drive shaft and the main nut that holds the overdrive output shaft?
 
Been a long time since I was inside an OD - I can look inside some of the ones I've got sitting in my garage....but, I think this is a "remove the OD" job....there may be others more knowledgeable than me though....
 
apparently this company in the uk www.speedycables.com
can retrofit my speedometer to be electric and comes
with a magnetic sensor kit all for around $ 350 USD.

I think I'm gonna go for it. I spent 10 years restoring
this car (last thing I want to do is tear it apart again) and I know its going to cost me a lot more
than that to just pull the tranny and have the drive gear replaced. I'm really [censored] that the rebuilder who is now
out of contact didn't replace it when I had him rebuild it.
 
Back
Top