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MG MGB Technical - Overdrive failure in '72 B

Several times over the past couple of years my o/d has stuttered for a period of weeks then failed to engage. Replacing the solenoid o-rings, twice now, has cured the problem for a month or so, then I get a repeat of the problem. I have a third set of o-rings waiting!
The sequence of events is; o/d engages fine for several weeks; o/d engages/disengages relatively randomly, usually at lower speeds e.g 50 in top, again for a few weeks; o/d fails to engage at all.
It all looks like a lack of pressure/internal leak past the o-rings/ball valve etc. I am about to strip the car for its 'makeover of a lifetime' and have been contemplating a refurbished o/d, but my funds manager cautions against recklessness and she shall be obeyed!
As far as I know the o/d is original, so pretty old now and, oh yes, I have checked the oil level (20/50 Duckhams). I see there is some interesting debate about which oil to use.... so maybe I should try something different??
Any fresh ideas?
Richard Coombs

Hi Richard.

It might be worth connecting a (maybe temporary) dashboard indicator lamp to the electrical connection of the solenoid to be quite sure that there are always volts on it when there should be. A permanent dashboard indicator for 'overdrive engaged' has some merit.

Intermittant electrical connection doesn't really fit your symptoms (unless you were disturbing the corrosion of a dodgy connection when working on it), but it might be worth eliminating it.

Don
Don

Hi Don,
Thanks for the idea/reminder. It'll certainly be worth testing the connections although, like you say, the symptons don't really fit.
Richard
Richard Coombs

Whilst some people claim that changing to ATF for example has 'fixed' their OD, I think all it means is that maybe ATF is better at dealing with a certain problem, but the problem will still be there. I've also heard that ATF can be helpful as a flushing agent, but only for a short period then replaced with the correct oil afterwards. Intermittent disengaging and failure to engage can be caused by maladjustment or wear of the lockout switch on the gearbox, but wouldn't normally be 'fixed' by changing the solenoid O-rings. It also needn't be a problem with the O-rings but something else, it's just that new O-rings may be slightly better at doing what they are supposed to do when new, this reduces soon after fitting, but should still be able to do their job for a lot longer than that.
Paul Hunt 2

If your o/d is dropping out and you are happy the problem is not electrical it would be worth checking the pressure release valve spring. It should stick out of its cap by at least 3mm.

I suggest it so as to eliminate it. If your problem was that as the once the oil warmed up the o/d started to drop out then the pressure release valve would be prime suspect.
David Witham

Sounds like marginal oil pressure to me - new solenoid rings allow it to work but as the wear in they allow more oil to bypass them till there is insufficient to operate the clutch.

Take out the relief valve and clean it, also take out the pump valve and stretch the spring a little ( I think David is confusing the pump valve with the relief valve)
Chris at Octarine Services

All,
Thanks for the suggestions. I shall definitely have a poke around at the pump etc.
My only other thought re the solenoid was that maybe the ball wasn't seating correctly. Should I give it a tap or is this a bit risky?
I also see that others have measured the hydraulic pressure in the o/d. Can someone tell me how/where this is done?
Richard
Richard Coombs

I had the exact problem you have a couple of years ago. Replacing o-rings helped for a short time. Finally, I fixed it when I had the engine and trans out for a clutch replacement. I took the OD apart down to the alloy body. Next, I cleaned it with soap and water blowing out all the passages. Also, I took apart the various hydraulic parts, cleaned them and reassembled them with new rubber parts. I have not had a problem since.
Ross Brown

Hi Ross,
I too am about to take out the engine for a new clutch and was coming to the same conclusion re the o/d. I'll do just as you did.
Thanks for the reply and the encouragement.
By the way, did you re-fill with 20/50?
Richard
Richard Coombs

Yes, I used 20-50. Before I took the thing apart, I tried 30W non-detergent and ATF. These oils did not help. In fact ATF seemed to leak from somewhere.
Ross Brown

This thread was discussed between 22/10/2006 and 27/10/2006

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