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MG MGB Technical - Overdrive removal

A friend has a a problem with his overdrive and is considering getting it refurbed - I have always taken the engine/ gearbox out in the past, but is it possible to pull off the O/D by removing the cross member/prop and angling the gearbox down ?

His O/D problem is really strange he advises it seems to engage when it feels like it - even with the electric disconnected !
The link Nigel posted noted worn O rings in the Solenoid - is this possible ?
richard b

It's only possible to get the gearbox out on its own on a 3-synch, and then only if you don't have OD! But even then the process described is so convoluted I wouldn't even bother. Having been involved with it both ways I wouldn't pull both together either, engine first then gearbox.

What year is it? 3-synch OD has two voltage supplies so both have to be disconnected to completely isolate it. Engaging by itself would be very unusual, and dangerous as if that happens in reverse fast enough to develop hydraulic pressure it wrecks the one-way clutch.

Bad O-rings in the solenoid variously cause leaks and non-operation, but debris from the smallest one could have got into the outlet and blocked it, which will develop pressure. That'e easy to remove and check, you don't even have to drain the oil.

The same could happen with dirt in the pressure relief valve, but that needs the sump removing to access, which needs draining.

Worth checking both before going to the trouble of getting it out.
paulh4

Sorry should have noted 4 synch Chrome bumper 72/3.

Think we will try the O rings first in the solenoid.

Then the seals on the relief valve - don't think you have to remove the main filter to get to it but need to drain the oil.

I was questioning just pulling the O/D off the gearbox - I seem to remember the shaft it fits on is quite long though - so that may be an issue even with the crossmember released.
richard b

Richard,
did you see the JT vids they cover a '73 B o/d over three vids, lots of useful info (solenoid in part 2).

104 MG Overdrive Repair Part 1 -
https://www.youtube.com/watch?v=XPSgUeHAy8E

105 MG Overdrive Repair Part 2 -
https://www.youtube.com/watch?v=LCqPu77EWEE

106 MG Overdrive Repair Part 3 -
https://www.youtube.com/watch?v=FHw_o5y0HWE
Nigel Atkins

Even if you did get it off, you wouldn't be able to refit.

We have found that the bush in the centre of the main housing can lose its locating roll pin and turn, this blocking off the oil feed to the mainstream. As a result oil pressure build in the system. It is very unusual but does happen.

I strongly reccommend that you get the hydraulic pressure monitored before removing the overdrive if you can, it provides a mine of information that will inform where to look for the fault.
Paul Walbran

"don't think you have to remove the main filter to get to it but need to drain the oil."

Ah yes the relief valve assembly is outside the sump. I removed mine as part of an oil change which does need the sump and hence main filter cum gasket to come off. I thought I've read somewhere that the oil doesn't need to be drained to get at that, the WSM says to, but they often include steps that aren't always required. If the oil does need to be drained whilst you could do that by removing the relief valve plug there are about a dozen small parts in there, you wouldn't want to lose any in the container of drained oil.

As well as eight small screws and nuts that join the OD to the gearbox, and the long shaft, that shaft has to engage with two sets of splines in the OD and the pump. Together with the tapering tunnel and the fixed cross-member I doubt it is even remotely possible to remove and refit in-situ. Maybe if you removed the fixed cross-member (I have seen a claim that someone's is like that but no pictures), took the rad out, engine off its mounts and pulled right forwards... But I think the remote gear-change assembly has to come off as well.

As Paul says, if the solenoid and relief valve are clear then a pressure test would be my next step, although if you were planning to send it away to someone like Overdrive Repair Services anyway then they should find and fix anything that is wrong.
paulh4

Reporting back,

We sourced new o rings and my friend had a local garage he uses to intall them, having given them the details off these threads.

He reports having had a couple of good length drives that the problem seems resolved.

Many thanks to all who responded, its a failure I had never encountered before.
richard b

This thread was discussed between 30/04/2019 and 14/09/2019

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