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MG MGB Technical - Overdrive slipping?

I have a 1967 B with early type o/drive.
It engages well in 3rd & 4th gear, But if I excelarate
hard to pass a car it seems to drop out of o/d back to 4th. Could this be a clutch problem or an oil pressure problem,or something else. I await your replys
regards Phil.
philip knight

Phillip
First thing to check is the gearbox oil level. This alone could be the cause.
cheers
Ian G Buckley

Don't know how long it has been since you serviced it, but if the oil level is good, you may want to change the oil and replace the o-rings in the box. This seems to help most problems. Worst case senario is the clutch linings are wore out or glazed.
J Arthurs

Didn't the early O/D units use a vacuum switch? The reason I ask is that you mention it only happens when you accelerate hard.
Paul K

If it really does drop out of OD (check by flicking the manual switch off as well) it probably is marginal adjustment of the gearbox lockout switch, it could be loose or should have a couple of shims that you can remove, or could be loose wires. If holding the gear lever hard into which ever gear you are in stops it happening, or if you can make it happen when cruising put applying pressure towards nuetral, it almost certainly is the gearbox switch.

The vacuum switch only bypasses the manual switch, if the manual switch is still on the vacuum switch does nothing.
Paul Hunt

I would want it to revert to non-overdrive for acceleration, so I don't understand the problem.
George B.

Philip - Paul is right on the money, except that if you have a flakey manual switch, you will have the symptoms you describe. I had the same thing happen to me and it turned out to be the manual switch, it would make contact long enough to get into OD, but then open up and as soon as I tried to accelerate, the vacuum switch would drop it out of OD. That is the only situation where the vacuum switch would have any effect, since it's only function is to keep the relay activated after the manual switch is turned off untill you step on the throttle. This eliminates any chance of over running the OD during deceleration. Good luck - Dave
David DuBois

FWIW I have a '67 Roadster on which a vacuum operated kickdown switch got fitted at some point by a PO that acts exactly as you describe. It would be worth checking for a vacuum pipe coming off the inlet manifold or carbs that heads somewhere strange (as in not to brake servo or distributor). The pipe on mine runs from the manifold, over the top of the rocker cover, to a small round switch unit mounted near the master cylinders.

I used the car for a long time with the kickdown, but have disconnected it for the moment as I'm in the habit of accelerating through third, O/D third, fourth, O/D fourth. With it connected I tends to go third, O/D third, third with 7000rpm.

Steve
Steve Postins

Thanks for your replys. I have checked the oil level in the gear box, its fine. So next I'II change the switch on the dashboard, to see if thats the problem.
regards Phil.
philip knight

This thread was discussed between 25/07/2002 and 27/07/2002

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