Testing overdrive solenoid

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Testing overdrive solenoid

Post by Bumpa »

While the body is away being painted I am looking at the engine and gearbox from my 1971 B roadster. This afternoon I thought I would check the overdrive (LH type) and applied 12v to the yellow/red wire with the negative from the battery connected to the gearbox case.

When I moved the gear lever from the 1-2 plane to the 3-4 plane I expected to hear the solenoid click, but it didn't. I checked the inhibitor switch for function with my meter and it works properly, passing current in the 3-4 plane but not in the 1-2 plane. All the wiring seems to be intact and working.

Obviously the gearbox has no oil in it - would this prevent the solenoid from working? Am I correct in thinking that the solenoid should click audibly? Will I have to remove the solenoid and clean it?

Overdrive is a mystery to me and I am loath to disturb it. As far as I know the O/D worked the last time the car was used, but that was probably 5 years ago.
1969 MGB GTV8 3.9 Conversion, 1977 Triumph Dolomite 1850HL, 1971 MGB roadster undergoing restoration.
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Peter Cresswell
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Re: Testing overdrive solenoid

Post by Peter Cresswell »

The only test you can do easily is the solenoid. You can't really hear the solenoid action on the LH Overdrive with it installed as it is inside the casing. As the gearbox is out of the car and drained of oil in your case, you can try energising the solenoid as you have done and at the same time use a screwdriver as a listening probe by putting the blade on to the cover and your ear on the end of the handle. You should then be able to hear the solenoid operating. Although the solenoid operates immersed in the oil (oil certainly pours out if the cover is removed when the gearbox is full of oil!) it should operate without oil in the gearbox.
In order to do other tests you need a lot of specialist testing equipment, so it is a job for an Overdrive Specialist. A google search will lead you to them along with their contact details, but there are not many in the UK - I know of 3!
Before you put the gearbox back in the car remove the filter (the rectangular plate) and clean or replace it. You can get kits to replace the various seals on the valves in the Overdrive system but you might be better taking it to one of the specialists to have it checked over and any worn parts replaced. There is a friction cone clutch and the linings on this wear and break up, and if the seals on the valves are worn or perished they can leak pressure past them - the Overdrive works with quite high pressures of 400psi + so the seals are important. They can also replace any worn bearing and oil seals as well more easily than you can as there are a lot of special tools needed to strip it down.
Now is the time to make sure it is fully functioning as once it is back in the car you can't remove the gearbox and overdrive without taking the engine out with them.
The Overdrive units are pretty robust and not too much goes wrong, and when it does it is usually electrical.
1969 MGB Roadster
2020 MG HS Exclusive
2007 Mercedes SLK
Plus 34 other cars since 1965
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