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MG MGB Technical - Overdrive Not Engaging

As mentioned in the recent thread on engine oils, my overdrive is not engaging.

My car has never had overdrive, but I acquired a 4-synchro overdrive gearbox over thirty years ago, with the intention of fitting it. It was one of the many things on the 'to-do' list.

Last year, I finally got around to fitting the O/D gearbox, as I needed to get the car back together when moving house.

I didn't have time to dismantle the gearbox before fitting, so I had a look in through the side cover to see if there was anything obvious wrong with it. There was clean oil in the 'box and all looked OK.

I decided to clean the filter/strainer on the overdrive and also replace the solenoid O-rings to hopefully give the O/D a fighting chance.

One of the switches was missing. I can't remember if it was the inhibitor or the reverse light switch, but I replaced the missing switch and checked the operation of both while the 'box was on the bench.

With everything back together, I filled the 'box with oil and then refitted the engine and 'box.

With the car back on the road for the first time in nineteen years, I tried the overdrive - nothing! Checked the oil level and it had dropped quite a bit. Thinking I had found the problem, I topped it up and tried again. Still nothing.

I have checked the electrics and all seems to be OK. I'm getting power to the solenoid and the solenoid is drawing near enough 1A when switching overdrive in, so it looks like a mechanical or hydraulic issue.

Any ideas where to look first?
Dave O'Neill 2

The solenoid can be removed without draining the oil, you can check the plunger moves into the central position when powered i.e. not binding, and the little ball-bearing is in the end-cap

To check the pump and relief valve assemblies the oil will have to be drained. I've not been into the pump but there are several variations in the components for the relief valve

Note the orientation of the plungers and valve body, these can apparently be installed the wrong way round, which unsurprisingly causes OD to not work. After that I think it would need a high-pressure gauge.

only as a quick addition to Paul's post, on this site somewhere is a short article about B O/d problems and he has three videos on Youtube that cover the servicing and others with the o/d.

If I find them later I'll post them.
Nigel Atkins

rushing again, it should have read -

... is a short - John Twist - article about B o/d problems and he has three videos on Youtube that cover the servicing and others with the o/d.

Nigel Atkins

found them hope they give some help.

John Twist article on this site, from a USA slant perhaps but should still apply -

The videos, up to you if you start at 2, 3 or 1, see also (some of) the posts under each video -

104 MG Overdrive Repair Part 1 -

105 MG Overdrive Repair Part 2 -

106 MG Overdrive Repair Part 3 -
Nigel Atkins

Thanks, both.

I will do some investigating.
Dave O'Neill 2

Wish he had done another set on the D type OD.
Bruce Cunha

This thread was discussed between 30/01/2019 and 31/01/2019

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