MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Slipping O/D

Hello!
To all you O/D experts out there -- I've mentioned here before that my O/D has begun occasionally slipping in reverse. It does it very, very infrequently and the O/D otherwise works flawlessly. No slippage in forward gears, O/D lockout works properly, O/D engages and disengages properly. I haven't driven it since Novemember or December.

Since:1. I'm currently working far too many hours to think about fixing it, 2. The weather is turning nice, and 3. It's killing me not driving it (mainly because it seems the only time I have away from work is while driving to and from work), would I be doing irreparable harm to it to drive it?

I'm assuming it will need a rebuild, so if driving it only hastens its inevitable demise and causes no additional damage (such as catastrophic failure), then I'm willing to hasten the rebuild -- as long as its failure will be gradual and progressive. And I have the non-O/D box that came in the car, so the first chance I get to spend any real time on the car I will swap gearboxes and can get the O/D rebuilt at my leisure. (Recommendations gladly accepted.)

I also realise that without seeing the inside of it, this exercise it little more than speculation....

What do you think, Sirs?

Cheers!
Rob
Rob Edwards

I speak only with experience of LH type overdrive.

I was advised by my (non-mg specialist) garage that I would need to rebuild and replace my o/d. However, I cleaned out the overdrive sump, changed the filter, changed the gearbox oil and replaced the o-ring seals in the solenoid unit. Then it worked.

However, the same procedure applied to a different car with the same symptoms failed to fix it. I think it might have fixed it, but it had been used faulty for too long and another more serious fault had developed.

So advise is check oil and o-rings and don’t use o/d too much while its playing up

Hope this helps.
C Beswick

I was always told that putting the car in reverse while overdrive is engaged will seriously damage the overdrive unit. Is this not correct?
Steve Simmons

That is correct. As I mentioned in my original post, my lockout switch is functioning correctly so I am therefore direct drive whilst reversing, not overdrive....

I would try the oil change/oring renewal/et al if I had the time, but I can't even find the time to do that.... :-(

Cheers!
Rob
Rob Edwards

when's the last time you actually changed the oil in the O/D & Trans Rob?
CJD Dark

I last changed its oil when I noticed it slipping -- about November or December. I used mostly Castrol 20w50 with a quart or less of ATF for a little more detergent action. I drove it like that for a short while, and when it didn't seem to help, I stopped driving it.....
Rob Edwards

I had problems last year with the O/D slipping in forward gears while disengaged which caused overrun. I know that the manual gives other reasons for this, but before going down that particular road I made sure that I completely cleaned the filters and replaced the oil with 15w50 twice!! The second time a couple of months later just to make sure I had cleaned out all the crap! It hasn't slipped since. 20w50 would have done the same thing, but crucially in my experience so far, I believe it is better to add no additives of any description and certainly not Hypoid oil. The O/D just doesn't like them! Failing that I guess it may be a mechanical failure in the unit.

cheers

Paul
Paul Eades

"certainly not Hypoid oil. The O/D just doesn't like them"

FWIW the factory V8 uses Hypoid oil in the gearbox and OD.
Paul Hunt

Hi Paul (Eades),
I'm assuming that it is indeed a mechanical/wear problem in the unit, and I'm also assuming that it will need to be repaired, probably sooner rather than later....

I guess the question that I'm asking is, in a expert's opinion, would an O/D unit displaying the symptoms I'm seeing be more likely to die a sudden, violent death or just continue to get worse? Again, I know this is all just guesswork!

Cheers!
Rob
Rob Edwards

As a minor "expert" I would say that it depends on the cause of the slipping -

if it is due to a cone clutch lining coming loose then it could go on for years,

if it is due to wear on the cone clutch lining then it will get progressively worse over time,

if it is a broken sunwheel circlip then you could well have catastrophic failure when the sunwheel jams up the planet gears and annulus or the broken clip gets into the gears.

The safest course is to drive it normally but don't engage the OD - this will keep the gearset locked and minimise damage.
Chris at Octarine Services

Having specialized in rebuilding British Overdrive units, the best bet is that the inner brake lining on your brake plate which should grab the annulus and lock it in direct drive when the overdrive is not engaged, is bad. This is what would cause the slippage. The one way clutch in the annulus only allows the annulus to overrun when the brake plate is pulled away from the annulus by oil pressure and locked into the outer brake ring. You may be able to just get a new brake plate if you aren't afraid of tearing it down and replacing it. If not, you might as well replace all the o rings in there as well, there are quite a few.
Jake

Make sure the electric switches are working and set up right. These two switches are the cause of most issues. The number of washers affects the engagement. Good luck.
Bill Haglan

I'm with the rubber seals first- They should only cost about a quid so start there. I don't beleive that dodgy oil could be a probem, providing that there is enough oil in the box...

Good luck!
Tom

Thanks all!

Sounds like the news is quite as reassuring as I had hoped. Still, if I can drive for a while with the O/D in direct drive and not create too much risk of fragging it, I might risk it.

Someday, when I'm not working 70+ hr/seven day weeks, I'll attend to it properly. 'Til then, though, I've barely enough time to put the top up & down, much less undertake serious repair....
Rob Edwards

I assumed we were talking about a 'B' 1800, Rob didn't say and I always understood that the 1800 O/D shouldn't have Hypoid oil 1. because it affects the operation of it, and b. because some of the components can be damaged. Is the V8 O/D different? Perhaps Paul from Solihull could confirm that as he knows a lot more than me, by the way his web page is very useful and worth checking out.I am certainly no expert, but changing the oil worked for me that's all I can say. Mine's a 1973 MGBGT by the way. I just thought it would be a simple first step to try before having to spend piles of dosh on taking the whole thing to bits!! Hope you sort it out though Rob!! Best of luck.

cheers

Paul E. :-)
Paul Eades

AFAIK the V8 OD and grearbox is the same basic design as the 4-cylinder, except the ratios are different of course and the V8 OD uses higher pressures.

To the best of my knowledge the 4-cylinder doesn't *need* hypoid whereas the V8 does because of the greater torque. Having both, the V8 does have a 'heavier' gear lever particularly from cold in winter. Hypoid in the 4-cylinder may therefore result in similar effort being required to change gear and possibly slightly different OD engagement and disengagement characteristics, but no 'damage' as such. To the best of my knowledge.
Paul Hunt

As I understand it the potential problems with additives are that friction reducing ones may cause the belts to slip , and prevent the drive from locking up . I have used fully synthetic 15W50 with good results , and since ATF is designed for use in epicyclic boxes wich work the same way as an overdrive they should be fine . I imagine you are using EP90 in a V8 gearbox , this will certainly slow things down , and I can imagine it not wanting to engage in cold weather until you have driven a few miles and warmed the oil up .
It will not be the interlock switch , this will only cause problems when the o/d is selected , causing it to drop out as the engine/gearbox twists with torque reaction.
S Best

The Hypoid used in the Triumph & MG V8 boxes is GL4 which has less sulphur than GL5 which will attack the yellow metal.

Hypoid 90 is actually the same viscosity as 20W/50 - gears oils are measured on a different scale.

Laycock (and their successors, ORS) say that the OD will work with every oil from ATF up the hypoid 90.
Chris at Octarine Services

Thats really interesting re viscosity of Hypoid , so would it work in an engine in an emergency ?
S Best

Yes it would work as a lubricant, but it does not have the additives that engine oils has to combat acid and other contaminents found in the engine.

Chris at Octarine Services

This thread was discussed between 03/03/2004 and 09/03/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.