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MG MGB Technical - Reverse gear engagement

I have a 1975 BGT. Recently reverse gear has, on occasion, been difficult to properly engage. I think it is engaged but it won't reverse. It is as though the clutch is slipping, with the revs rising but no movement. The clutch is OK and I have no problem with forward gears. It only seems to happen when the engine and gearbox are hot. I did a 200 mile run today and had the problem a few times. Eventually I seem to get it engaged and it reverses fine. When I got home I let the car cool down and reverse works fine. Any ideas on what the problem might be?


Rod
R E Merrall

Take the shift gaiter off and see if the stick fouls the gearbox cover, when going to reverse.

Herb

OH when replacing the chrome ring, the short screw goes to the front.
H J Adler

Do you have overdrive?
Dave O'Neill 2

Dave

Yes and it works fine

Rod
R E Merrall

"Do you have overdrive?"..."Yes and it works fine"
Do you have the OD engaged when you are shifting into reverse? If so that is your problem - the OD doesn't like reverse and all it does is to make a ratcheting noise. Cheers - Dave
DW DuBois

Check also that you have the plastic bush in the end of the selector shaft, (or on the end of gear lever.) It often gets lost and the lever therefor requires more "throw" to select a gear and Herb's problem develops!! (It's not catching at this distance!)
Allan Reeling

For a start the gearbox should have a lock-out switch so overdrive can't be engaged in reverse even if the manual switch is still on. However that gearbox switch could have been bypassed of course. Drive along in third or fourth gear, switching the manual switch in and out waiting a couple of seconds before moving the switch each time, and the revs should go up and down as OD switches out and switches in. Now do the same test in second gear, and the revs SHOULD NOT change. If they do then your gearbox switch is not doing what it should.

Overdrive will only engage if the overdrive input shaft is spinning fast enough to raise the oil pressure high enough to engage OD. If you can't even start the car moving in reverse, it is something else.

If you do manage to go fast enough in reverse to engage OD, then it will wreck the one-way clutch, and that is as likely to give problems in forward gears as reverse.

But the crux of the matter is in the diagnosis section of the WSM, which says if it slips in reverse then it either has worn or glazed clutch linings or the sun wheel has a broken circlip. It can also happen if the oil ways and pressure relief valve are partially blocked, which can prevent pressure from fully releasing when disengaged, which can leave the clutches mid-way between engaged and disengaged, which will result in no drive in reverse.
paulh4

Thanks for the rather depressing diagnosis! The gear lever bush looks OK though it is split, is this how it is designed?
R E Merrall

Split is good!!!
Allan Reeling

I'm afraid I do not understand the influence of the overdrive in reverse gear. I had assumed the od was non operational in reverse with the drive from the gearbox going straight through as it were. Can someone explain how it works. If the problem is as Paul explained I take it that is an engine/gearbox out job.


Rod
R E Merrall

Rod, I don't know if you have found Paul Hunt's truly amazing web site, but if not go here http://www.mgb-stuff.org.uk/gearstext.htm to find his section on the transmission. Your car should have the LH type overdrive, so you can ignore the section on the D type. Keep scrolling down, there is a lot of info there. But if the O/d needs attention, then it is an engine/gearbox removal I'm afraid. It's the only way.
Mike
Mike Howlett

Yes, glazed linings or broken sunwheel circlip.
It's not about trying to engage overdrive in reverse (though doing this will certainly have this effect) it is that direct drive in reverse or overrun is via the linings. And getting worse as the gearbox gets up to temperature is a dead give-away.

You can confirm this by seeing what happens on overrun - does the engine speed drop significantly (= faster than the car is slowing) when you lift off the accelerator? And then does it play catch-up again, sounding similar to a brief episode of clutch slip, when you put the power on?

If all this is happening, it's overdrive-out time unfortunately. When rebuilding, we always use the heavier MGC/V8 spec engagement springs (37H1928), which virtually eliminate this problem.
Paul Walbran

Rod - yes the intention is that the drive goes 'straight through' in reverse, just as it is in forward gears when OD is not engaged. It's not that OD is bypassed in reverse, drive still goes through the OD, but straight through with no gearing alteration. The problem comes about if - for any reason - OD is allowed to engage in reverse, which changes the gearing. That may sound like it shouldn't matter, but if it happens the one-way clutch is trying to apply one gearing, and the friction clutches are trying to apply a different gearing. It's very like accidentally selecting two gears in a manual gearbox - you get an nearly irresistible force and a nearly immovable object, and one of them has to give way. In the OD it is the one-way clutch.

"does the engine speed drop significantly (= faster than the car is slowing) when you lift off the accelerator? And then does it play catch-up again, sounding similar to a brief episode of clutch slip, when you put the power on?"

Now that is something else - in non-OD at any rate, typically the solenoid plunger not allowing the valve ball to come back far enough to fully release hydraulic pressure. That can come about from a swollen O-ring on the end of the plunger, easily replaced with the OD in-situ, or fit an additional thick gasket with a hole cut out for the plunger between the casing and the existing gasket.
paulh4

Paul, the latter effect you describe certainly does happen, first came across it 35 years ago and it's one of those very pleasant 10 min miracle cures that gives much relief to the owner!

However, slip on direct drive overrun also happens when the linings are glazed. It can be distinuished from the swollen plunger o-ring cause by three characteristics:

Worse when hot
Accompanied by loss of drive in reverse (again when hot)
Swollen o-ring cause usually cycles in and out with a period of a few seconds, whereas glazed linings stays slipping for as long as the car is on overrun.

The amount of slip can vary from minor (like that of an automatic transmission between overrun and drive) to complete (engine drops down to idle on overrun). We have had customers come to us having (elsewhere) renewed the (main) clutch due to this effect but of course it hasn't cured the problem.
Paul Walbran

Can't say I have noticed the speed dropping off but I will check next time I am out. Is this with the o/d engaged?
R E Merrall

Yes. RAY
rjm RAY

No, it's in direct drive. As far as the overdrive components are concerned, the load in reverse under power is in the same sense as overrun going forwards - both unwind the one-way clutch, leaving the OD clutch linings to take the entire load.
Paul Walbran

If the clutch is the problem will it do any harm to run the car until the autumn. It drives forward perfectly and od works well, and I would rather do the job when the car is off the road.
R E Merrall

Another question. Is repair a diy job or does it require special tools?
R E Merrall

It's a pretty specialised job, it's a matter of determining what is wrong as well as fixing it. I'd advise sending it away to someone like Overdrive Repairs at Sheffield who are all ex-Laycock for a full overhaul, another one at Rugby but I've not used them.

But if it only happens intermittently and when hot it's more likely to be something simple than the one-way clutch, or even the friction clutches. I'd start by doing the solenoid O-ring or extra gasket thing.
paulh4

Thanks for all the advice I'll keep you posted
R E Merrall

This thread was discussed between 22/05/2016 and 26/05/2016

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