Overdrive HELP

Technical MGB discussion
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Posts: 1
Joined: Sun Jun 16, 2019 3:05 pm
Forename: Max
Surname: Linder

Overdrive HELP

Post by Swissmaxie » Sun Jun 16, 2019 3:40 pm

Hi all,
I am a bit worried about my overdrive.
The O/D did not work and I traced it to the solenoid.
Ordered parts inc filters/o rings and set about replacing.
All was well until I dropped the gauze filter. Sitting on top were parts of a thrust bearing.
See picture
I reassembled everything as I had to drive home and to my surprise the o/d works fine, ( a bit slow to engage at first but it hasn't been operated for 20 years)
Now works as it should, no noises or bangs.
I have checked the microfiche and looked at the only part that it could be and it looks totally different.
Has any one got any thoughts?

Thanks Max

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Peter Cresswell
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Forename: Pete
Surname: Cresswell
Location: Stone, Staffordshire

Re: Overdrive HELP

Post by Peter Cresswell » Mon Jun 17, 2019 11:51 am

Hi Max,
It is impossible to diagnose potential problems of this nature, so I'll offer some thoughts for you to consider
Although you don't give the year of your car or the type of overdrive you have, from the fact you ordered a gauze filter I assume the Overdrive unit is the LH type with the internal solenoid rather than the D type with the external solenoid. However looking at the workshop manual pictures, both types have a thrust washer associated with the unidirectional clutch that looks like the one in you photo. This raises several questions:
1. Where is the remaining piece of the thrust washer. You don't say if the O/D unit stopped working during your ownership or if it was not working when you bought the car. It might be that a previous owner found the missing piece and the big bit you have found was lurking away somewhere inside the casing.
2. I am guessing here but I think it is possible that the solenoid failed and it stuck in overdrive. Since the thrust washer is associated with the unidirectional clutch if an attempt was made to reverse the car it might have shattered the thrust washer at that point, and you have now found some of the debris that ensued. This is obviously a key part that limits movement of the unidirectional clutch.
3. You now have the problem - 'was the thrust washer replaced?', and how can you be sure? The answer is you can't be sure unless you dismantle the O/D unit, which isn't easy as a number of special tools are needed so it is usually considered a job for experts, and you have to remove the engine and gearbox/OD unit from the car to remove the O/D unit from the back of the gearbox. So all in all not a simple job of a few minutes!!
4. Continuing to use the car might result in irreparable damage being done, but the fact that operates without any odd noises might be a good sign but you will always have the worry that it might break at any moment.

I know of three people who can repair MGB Overdrive units (or offer you expert advice) in the UK. See:
I have used OD Spares and found them to be very good, but they were also the most convenient for me.
A replacement overdrive unit from any of the major parts suppliers is around £450, and a 'good' secondhand gearbox with O/D is usually around £300 but carries the risk that it might not be as good as the seller thinks and it is a lot of work to take the engine and gearbox out again to return it. I either case if you have the LH -Type then make sure you replace it with one that has the same colour label on the solenoid cover. This is either Black on earlier cars or blue on later cars (which had the Triumph style of instruments). If you the wrong one then the speedo will read incorrectly by about 20%.
Incidentally the part in question has the same part number for both the D-Type and LH-type of unit, and costs less than £20.
Good luck!!!
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Vic Butler
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Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Overdrive HELP

Post by Vic Butler » Mon Jun 17, 2019 4:44 pm

If the car is reversed with the overdrive engaged, the uni directional clutch will be destroyed and the overdrive will spin round in its housing.
As you said it still works then that hasn't happened.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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