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MG MGB Technical - Loss of overdrive

A bit of advise/knowledge please

1980 mgb roadster. With overdrive

How much oil should there be in the gearbox to allow the overdrive to work.

I’ve lost my overdrive and have checked/drained the gearbox oil to see if low gearbox oil is the problem. I got just under 2lts of oil out (full is 3.4lts). My question is is that enough oil to operate the OD or is low gearbox oil the problem. The OD was serviced, filters cleaned etc 200 miles ago.

(Yes I know the gearbox should be full – I’m looking for a potential reason of the problem and what to eliminate low oil)

Any assistance welcome

Brian
Brian Davis

It could be an electrical issue.

The wiring from the switch is known to fail where it flexes at the bottom of the gear lever.

ISTR my brother having a problem with his ‘78 GT when it was only 3 or 4 years old. In his case, one of the wires to the interlock switch broke off. We managed to fix it through the aperture on top of the tunnel.

It would be worth checking that you are getting voltage to the solenoid connection at the gearbox, with ignition on, overdrive switch on and 3rd or 4th gear selected.
Dave O'Neill 2

Dave

Will check the wiring but the immediate concern is the gearbox oil, I know that was low (drained today and refilling tomorrow when I get more oil).

I've had the overdrive stop working before when the gearbox oil was low so need to eliminate that (also why the oil is low when it was refilled with a new front oil seal not so long ago).

I'm still trying to determine how much (or little) oil I need to make the overdrive work.

Brian
Brian Davis

I seem to remember that when I filled my gearbox, the level needs to be checked again after the car has been run, as the level does drop.
Dave O'Neill 2

As a fault diagnosis you probably should have tried adding more until it trickled out from the fill/level plug.

Whilst some does remain in the gearbox with that amount drained it does sound as if the level was too low to begin with. However as the level reduces the symptom usually becomes erratic engagement and even drifting in and out.

Also as Dave says after a drain and refill even to the correct level initially it will drop quite a bit when run and OD switched in and out so needs more oil.

As far as electrical tests are concerned (which should be done before anything else in the even of non-engagement) this is one of the rare occasions where you need to check the current (as advised by the Lucas Fault Diagnosis manual) and not voltage, you could have voltage at the solenoid but no current through it. Find the bullet connector in the engine bay where the main, rear and gearbox harnesses connect together and part the one that has white/brown from the main harness and brown from the gearbox harness, and insert an ammeter. Turn the ignition on, engage 4th (or 3rd), operate the manual switch on the gear lever and you should see about 1 amp flowing. A bit less with the engine off but a bit more with it running and charging.
paulh4

This thread was discussed between 03/04/2026 and 04/04/2026

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