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MG MGB Technical - Overdrive

I notice when I engage od an increase in noise. It is really more a whirring noise of gears. Is this normal or should there be no increase in whirring.
Rod Merrall

Overdrive should be silent in operation. You certainly shouldn't hear it above the normal MGB noises. Have you checked the oil level in the gearbox?
Mike Howlett

More gears are turning when OD is engaged compared to no engaged. There can be a difference in sound, but it shouldn't be 'noisy'. I suppose there is a point at which increasing noise might warrant an engine and gearbox out and rebuild of the OD if not both, but as well as being a big job it's also very subjective as to when to do it.

I had mine done last gear as the gearbox itself was getting noisy in 1st and 2nd, and it felt like it was on the verge of jumping out of 4th on the overrun. I debated whether to leave the OD as there were no problems with it and risk it needing doing anyway not too further down the road, or have it done at the same time and risk a problem as a result. I had it done, and a few months later the speedo pinion is leaking oil into the cable and contaminated the speedo. A pal had similar with the output shaft and they sent him a new flange and oil seal, I'm hoping to get a new speedo pinion and seals out of them. C'est la vie.
paulh4

As Mike has put check the oil level in the gearbox.

The factory service schedule has to change the gearbox oil every 24,000-miles or 24 months whichever is the sooner, many Bs will have gearbox oil much older, perhaps decades.

So if the oil hasn't been change for a few years then a hot drain and refill with fresh clean oil certainly wouldn't hurt. I t will also give the opportunity to inspect the old oil for anything else that hot drains out with the oil.

If the gearbox oil is perhaps many years old then a hot drain plus a flush and clean might be useful.

At the same time a clean of the filter(s) is advised.

Below is just as an example, page from a later copy of the Driver's Handbook.




Nigel Atkins

Thanks. The gearbox has fresh oil and filter so that is not the problem. The increased sound is not over intrusive so will leave for now.
Rod Merrall

Rod,
I'd a feeling we'd been through the oil business before but my memory isn't the best. As you've done fresh oil and filter did you take up Mike's suggestion of checking oil level?

Did you also hot-drain/change (flush/clean) the rear axle oil and/or recently check the axle oil level?

Sounds, noises can be difficult to pin down in a moving car and you can think it's from one thing to find it's actually from another. And once you hear it you can start to fixate on it. Unless the noise is bad the fact that you can hear it might suggest that you don't have other noises too bad so your car is at a good level of running.

This week I got an intermittent rattling noise and thought it'd be my 'navigator's' metal water bottle rattling against the seat frame or runner but it wasn't, and I had to apologise for suggesting it was. Next I thought it might be the windstop fixings come loose, one was slightly loose but it turned out not to be that. It was the obvious, the last thing I'd fitted to the car, another attempt at a fixing for holding the exhaust silencer box. I took it off this morning, a ten-minute job, only took about 45 minutes and 11 tools to remove a few fittings and replace with nuts and washers, always a total 'joy' for me to work on my car.

It would have been a bit easier and quicker if someone hadn't pinched my 1/2" ratchet ring spanner a few years ago, from when I was fitting a fixing for holding the exhaust silencer box!
Nigel Atkins

Thanks Nigel. Yes I checked oil level and pretty sure increased sound coming from overdrive since it only appears when od engaged and disappears when disengaged.
Rod Merrall

Fair enough Rod. As I put it might be a good sign that you can hear a difference as there are so few other loud enough competing noises.

I don't hear the whining of the rear axle until I notice it then it seems very noticeable until I forget about it or other louder noises overtake it.

I can remember once hearing the swishing/whirling type of noise from the gearbox, but it wasn't, at the moment I forget what it turned out to be. Same with a sinister noise from elsewhere turned out not to be but I forget now what it was.

The old advice was "turn the radio up". :)
Nigel Atkins

This thread was discussed between 16/11/2020 and 23/11/2020

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