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MG MGB Technical - Overdrive!?!

Had gearbox reconditioned on my 1976 MGB one year ago due to the car not reversing after running (ie. when hot)

The chap that carried out the work said it was because the one way clutch in the overdrive was not engaging properly and preventing the car from reversing.

The problem has now returned and also the engine does not brake the car whilst changing down through the box if the overdrive is not engaged but works fine if the overdrive is engaged until changing from 3rd to 2nd.

Does anyone else know of this problem and is there a solution which does not consist of removing the gearbox again?

Thanks in advance


Tom Ward
Tom Ward

Tom
Was it the o/d or the gearbox that was recon'd? Also, was a new clutch fitted at the same time?

I had problems engaging reverse after coming to a standstill, but then found it would engage smoothly after shifting into 3rd or 1st before trying again for reverse, and also from 3rd to 2nd. I was convinced it was synchro failure.

It's possible the inhibitor switch maybe faulty but my problem was a severely worn thrust bearing and collapsing clutch plate, hence my query above on whether the clutch was done.

If it was your g/b that was recon'd then it maybe time for a o/d recon.

Martin
Martin

Hi Tom

I have an intermittant problem with my overdrive inhibit switch, it SOMETIMES allows the overdrive to engage (or stay engaged) in 1st or 2nd gears (I haven't tried reverse).
I guess that the switch is 'sticky' or needs adjustment. I am planning to wire a dashboard warning light directly to the overdrive solenoid, and maybe use the reversing light switch via a relay to inhibit the overdrive, as a safety measure.

I wonder if it is possible that you have unknowingly engaged reverse while in overdrive at some point ?.

Does anyone know what actually happens in this event ?.

Don
Don

Hi Tom,

It is worth draining out the gearbox oil and replacing it with ATF - this will clean out any gum or residue in the OD that may be maintaining pressure in the OD hydraulics - one source of the problem you have.

If there is no improvement then removing and cleaning the OD solenoid valve, pump valve and relief valve may clear the problem.

If none of this works then it's gearbox out time again.

Don,

Yes - if you reverse with the OD still engaged then the one way sprag clutch is driven the wrong way against the shaft and one of two things happens - either the shaft is snapped or torn off, or the sprag clutch disintegrates - both are fairly terminal!
Chris Betson

Chris
Are there any known harmful affects of ATF in the gearbox and O/d or is this a temporary fill to clean the box up? Would a clean up of the filter on the O/d be required after a short period of the ATF detergent additive doing its work?

I know the manual states categorically no friction modifiers to be used in the gearbox, and EP (the smelly sulphur phos based EP oils) are known to attack copper, but would a modern synthetic with a very high detergency level affect the synchro operation with its super slippery formulation? Having just replaced the oils seals in mine, I was wondering about Mobil 1 or similar? New seals are less likely to be affected by modern synthetics, unlike older seals which will have accumulated a deposit and this will be taken off by the high detergency level and will result in leakages.
TIA

Martin
Martin

Hi Tom, i bought a 69 roadster 5 months ago and have had a similar problem with not getting reverse after driving. When i bought the car, the dealer said the clutch was low, meaning it would have to be replaced in the near future if used daily. Also the overdrive switch was lagging a bit when engaged. A friend suggested a gear box oil change might help,as i am in the middle of servicing for the next mot, i will see what happens afterwards.
Glenroy Thompson

DO NOT use modern synthetics in an OD gearbox.

You can use mineral multigrades as per the engine grades or you can use ATF.

ATF is a lubricant and can be used on its own in but does tend to find leaks! It can also be used just as a cleaner for a short while and then change back to mulitgrade mineral - or you can add 25 - 30% ATF to the mineral oil.

The use of ATF speeds up the response of the OD unit when switching in and out since the oil flows more quickly to and from the pistons.
Chris Betson

I can vouch for the ATF. My OD took a long time to switch off, I mean 5 to 10 minutes, with any type motor oil, and it was clean from numerous changes. I went to all ATF and after 3 cycles it responds intantly and smoothly ever since. Very minor leakage increase but that's a result of the tranny seal quality, and I can live with that. No plans to change it out, although it would be an interesting experiment to see how it reacts now. Tranny sounds fine.
Good luck,
Mark R.
Mark Rotsky

Hi Folks: The prime reason of not using muligrade and the new oils is that the overdrive hydraulic system is operating between 400 to 550 lbs of pressure. These multigrade oils contain high detergents. The detergents at these pressures cause "foaming" that cause the cone clutch to chatter, thus resulting in accelerated wear. In addition you do not want anti friction materials causing the cone clutch to slip. The bottom line is to use non-detergent 30W SAE oil.

Thanks: Rich Boris, 67B roadster
Rich Boris

Tim's symptoms are specifically covered in the Leyland Workshop Manual, the causes given are a worn or glazed clutch lining, or a broken clip on the sun wheel.
Paul Hunt

Thanks for your suggestions, will try changing to ATF to see if that works then attempt some of the other more time consuming ideas if that doesn't work.

Paul, where can I get the Leyland workshop manual as my Haynes manual doesn't list the problem (or several others I have had)!
Tom Ward

This thread was discussed between 09/03/2003 and 11/03/2003

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