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MG MGB Technical - Gearbox and Overdrive

I have a 1969 mgb roadster, and when it is up to running temperature or after a long run the car won't reverse. The gear does engage and it has a new clutch fitted.If i leave it standing for 30 minutes it will reverse. The overdrive works intermittently and slow to engage,and won't disengage most of the time and doesn't work on 3rd gear, and i am wondering if that may be the cause. Any help or advice will be much appreciated. Thanks
Glenroy
G Thompson

Glenroy,

Do the forward gears work perfectly when this happens? Or do they slip also? I have a similar problem, but it does happen in forward gears to (worst for reverse though). When I get enough speed to engage OD it is fine. I am fairly certain my problem is a worn OD clutch on the direct drive side. This could be your problem, but we need more info on the cars behaviour in other gears etc. On mine it only happens after 20-30 minutes of constant motorway driving. When I pull off the motorway it is as though the clutch is slipping terribly, but a new clutch made no difference at all! I have since learnt about the OD potential problem which I am now fairly sure is my issue. I am going to try replacing the oil and cleaning out the filter on the OD as this can help prolong it's life with this type of problem (so I am told).

Cheers
Iain
I D Cameron

Thanks for your comments,all forward gears work well, i can tell when the OD hasn't disengage by the sound of the engine,and the gears don't pickup properly.
G Thompson

Sounds like an OD problem, but perhaps a different one to me! Other will no doubt have a few other suggestions. In the meantime a quick check to do first is to make sure the oil level is correct. Low oil level results in intermitent OD operation as oil pressure is used to engage the OD.

Iain
I D Cameron

As Iain points out, the oil level is cruicial for satisfactory operation of the overdrive. I too had this problem but it may be able to be rectified by cleaning filters atc. The grade of oil is also critical and the gearbox does not need anything more than a cheap 20/50. After you clean out the filters and overdrive sump you could refill and run the gearbox with ATF and this will certainly flush out the remaining passages.
Iain MacKintosh

Glenroy, I'm not sure if you mean the car won't reverse due to overdrive being engaged (so it feels like the brake is on) or overdrive isn't engaged but the drive slips in reverse (a different cause).

Have a look here,
http://www.mgcars.org.uk/news/news363.html

If it won't disengage you need first to check whether there is power being supplied to the solenoid which is keeping it on. This would be a rare fault though. More likely dirt is stopping the solenoid/oil coming back from the cylinders when it's turned off. The default position (no power) for the OD is off. It needs power to engage and should return to off when the gearbox shaft stops rotating and/or the solenoid is not powered.

If the drive is slipping in reverse this is due to worn/glazed lining on the 1:1 direct drive band. When this happens it will drive forward as the one-way clutch still locks it but it won't reverse or drive the engine on the over-run in forward gears (it freewheels).

I'd check oil level first. If OK then drain and remove the OD sump plate and filter. You may be surprised how much muck is in there. It may even be possible it's been put back without a strainer guaze as the gasket often tears. If there are any larger pieces of debris in there, like 1/2 pea size brown lumps, that will be brake band and is terminal. If not, remove and clean the solenoid and change the O rings as per the manual, and cross your fingers. Good luck.

Rich

Iain, When hot and running in direct 4th and you lift off, say at 50, does the engine freewheel back to idle revs even though the car is still doing 40 or 50?

Rich

Yes Rich, exactly that but only when the car is cold. After about 5 miles when the oil warms up all is fine. I'm convinced that that I need to clean my filters because it is a slow decay of hydraulic pressure that is causing the problem. If I stop the car during this warm up period the pump stops of course and all works as normal until the next time I disengage if that is within the warm up time.

It's a dault that I can live with nevertheless I must remove the main tubular filter gauze which seems firmly stuck in place.
Iain MacKintosh

Hi Iain, Sorry, my question was aimed at Iain Cameron, I hadn't noticed you mention a problem as well!

Your diagnosis seems good, and maybe removing and cleaning the solenoid and changing it's O rings would also be worthwhile. It could be the problem if it's a bit tardy returning and it doesn't slip or freewheel when hot.

If it freewheels when hot in direct, and slips in reverse, that's usually worn direct brake band and was my problem a few months ago. OD Services in Sheffield did an excellent job and even gave me a guided tour of their premises.

Rich

I would do four things immediately - before driving the car again! Some have been mentioned, but do them all.

1. Check oil level and replenish as necessary.

2. Clean filter screen.

3. Replace solenoid "O" rings. Check the condition of the pigtail wire while you're at it. It often weakens right at the point of entry.

4. Check the lock-out switch and replace it if there are ANY questions about its condition! It CAN be done without removing the gearbox...

Our MG guru here in the colonies is John Twist and I believe he says something like 90 or 95% of all overdrive problems can be solved from under the car. I have had slipping or engine dropping to idle while in gear that was due to $1.58 worth of "O" rings. I've had slipping in reverse that was due to a faulty switch.

Yes, it could be something more serious, but the statistics are on your side. I've thought of connecting a cut-out relay to the reverse lamp switch as an added precaution (such that when the primary side of the relay is energized, the secondary side opens, breaking any possible continuity to the O/D solenoid).

2ΒΆ worth...
Allen
Allen Bachelder

Thanks Rich, that's a very good article. It does however confirm my own thoughts about dirt in the hyraulics. I guess that there is no alternative but to get under there and clean the filters as I've already changed the O rings.
Iain MacKintosh

Rich,

Not quite sure if that is how I'd describe it. Trouble is it happens only after 20 or 30 minutes on the motorway at high speed (say 80 mph average). I don't do this very often! I have a "car day" due in mid July and top of the list is to drain the box, clean the filters and replace the O rings. I'm hoping that this will solve the problem and that the drive will stop slipping. If not I have a second hand OD and box in the shed that will go in at some point to try (after it has been checked over). Reading that article has encouraged me that the problem may be fixed by the O rings though!

My problem is when I come off the motorway, say into the services, and slow down dramatically, when I put the foot down again the engine revs and the car takes a while to "catch up" with the revs. Really like clutch slip! It is OK when OD is engaged though. It is worst for reverse (when parking in said services!) as it almost won't go with the engine revving away, but the car hardly moving! I'm guessing it is linked to v. high temps in the gearbox after long hard run. Could be hydralulics of it all, which will be hopefully sorted by replacing the rings. I'll let you know in due course.

Iain
70 BGT
I D Cameron

OK, I have booked a day off work on the 21st to clean out my OD and replace the O ring seals. Trouble is I'm not to sure exactly whcih seals potentially need replaced. Would someone who knows be kind enough to have a quick look at
http://www.moss-europe.co.uk/MossUK/ProductList.aspx?SubSubCategoryID=MGB_DS007C2-A
and possibly
http://www.moss-europe.co.uk/MossUK/ProductList.aspx?SubSubCategoryID=MGB_DS007C2-B
and let me know which of the seals it would be prudent to replace while I am in there.

Many thanks
Iain

70 BGT
I D Cameron

And one other question. Is the only difference between blue and black labelled OD the speedo drive rate, or are there other internal differences? I ask as I have a spare black label OD in the shed and I'd like to know if I can mix parts if required.

Thanks
Iain
I D Cameron

Hi Iain, For the solenoid I make it the 3 O rings, 37H1939, NKC99A, NKC102A, the ball BLS106, the wire seal 37H1936, and gasket 37H1941. Then for the sump, filter/gasket 37H1942. The pump apparently can be tricky and does not usually need removal. Whether you wish/need to do the relief valve I don't know.

AFAIK the only other difference on the blue label units was a plastic thrust washer inside and I think some get converted back to metal.

I'd give Paul a ring at OD Services, 0114 248 2632,
http://www.overdrive-repairs.co.uk/contact.html
and ask his advice. I'm sure he'll know off the top of his head and will be able to send the parts needed.

I did a lot of research and calling round when mine went AWOL a few months ago. John Twist's article gave hope but apparently many problems are due to wear and the units do need to come out and be serviced. I found marbles of brake band in the sump of mine so it was terminal. OD Services rebuilt it and it really does look and work like new for £200. OK not cheap, but you could spend that on the parts and I didn't want to do the job twice. Rich.

Rich

Thanks for that Rich. Have spoken to OD Services. They thinkg either worn uni directional clutch and/or worn cone clutch (is one of these the brake band you refer to, I can't see anything refered to as a brake band in the diagrams or listed with Moss or OD Services). £130+VAT from them for those two parts (much more from Moss!!) so if that is the case I'll get the reconned unit at £185+VAT. First though I'll try the parts you have recommended above. That'll be less than a tenner, so no great loss if things don't improve!

Is it possible to change the OD without pulling the gearbox? Anyone tried?

Cheers
Iain
I D Cameron

Iain, Yes, there are 2 linings, one on the inside and one on the outside of the sliding member 61, which variously get referred to as clutch or brake linings, as one contacts the annulus 71 and the other the brake ring 59 on the Moss drawing,
http://www.moss-europe.co.uk/MossUK/ProductList.aspx?SubSubCategoryID=MGB_DS007C2-B

My worn one, and yours too possibly, is the inner one which transmits during direct drive. This is supplemented by the one-way clutch which should lock up. This is the default (no power) position of the OD and is operated just by the springs, no hydraulics, so, really, any slipping under direct drive is usually bad news.

Worth a try cleaning things out I suppose. I was going to do that and got as far as removing the sump, found the evidence though, so nailed it back on and took the engine and tranny out. Bad news is you won't get the OD off without taking the lot out as, apart from anything else, the remote has to come off which you can't do under the car. Good news is that it's all very straightforward and quick once it's out.

Rich

Iain,

Yes it is possible to change the OD without pulling the box.

However it still means lifting the engine off its mounts and pulling it forward about 9 inches - which means taking out the rad and support panel on the early cars.

This allows the tail of the box to drop past the fixed crossmember so that you can remove the gearlever extension and the OD unit.

It helps to have the rear of the car off the ground on a lift or a couple of lorry axle stands.

Overall you save only a little time versus lifting the whole lot out and doing the job on the floor.
Chris at Octarine Services

I'm currently experiencing a similar problem with my OD. I've already done the sump & screen cleaning, and new o-rings on the solenoid, but it hasn't fixed it. Is it not possible that the oil pump is sticking? There's an article by John Twist somewhere on this site that, as I recall, points to a sticky oil pump for most intermittent OD problems of this nature.
JFD Dutt

How similar is your problem JFD? Is drive slipping in direct or overdrive, engaging/disengaging by itself etc?

Rich

Sorry guys... I just re-read the symptoms. My problems are a bit different...

Mine does one of two things intermittently:

a) It doesn't engage at all.

b) It slips a bit at low RPM without being manually engaged.

I am actually going to start a new thread regarding this, rather than confuse this one any further. :-)

Thanks,

J
JFD Dutt

This thread was discussed between 06/07/2005 and 08/07/2005

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