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MG MGB Technical - Overdrive help

Hi everyone,

78 "Rubbernose" roadster, UK original. 4 speed manual with LH-type overdrive. Until a couple of years ago this was my daily drive. Since then I have had it on and off the road while doing various restoration jobs (repainting, some mechanical work, etc).

A little over a year ago (October 2010) I had the gearbox rebuilt and a new clutch installed. The original overdrive was not touched (although it was probably removed and reinstalled during the other work).

December 2011 I was finished all the restoration jobs and it became my daily drive again. Felt like a new car (much better than my 2006 Subura Impreza).

Last week the overdrive started playing up. At first it slipped out of overdrive (then back in) while I was driving at a steady 100km/h. The rest of the journey it slipped in and out randomly. On the journey home it failed to work at all.

I immediately put it on the ramps/stands and checked the oil level. I spent days chasing a similar problem a few years ago only to discover the gearbox was nearly empty and a bit of oil fixed everything.
It was quite low this time so I topped it up to the level where it starts to overflow the inlet hole.
Checked all the electricals and made sure there was power getting to the solenoid (only when in 3rd/4th gear). While I was there I re-sheathed the wiring inside the gear shifter as it was looking a bit shabby.

Still not working. Not even intermittently.

I put it on the ramps again and completely drained the oil and replaced with fresh stuff. The old stuff contained had a lot of milky white fluid as if some kind of additive had been used. It all smelt like oil so I couldn't tell what it was.
I recall when driving it (since the gearbox and clutch overhaul) after driving a very short distance I would always smell something a bit soapy almost like a dishwashing liquid. In hindsight I'm guessing that milky fluid in the gearbox could be the culprit.

I have flushed the gearbox with 2 x 5L bottles of fresh oil and saw no more milky fluid. Then I filled it up with a 3rd fresh bottle. As far as I can manage, the oil is fresh and the electricals are working properly.

Still not working.

How do I go about inspecting the overdrive unit further? My mechanic is still on Christmas holiday. I have a suspicion he might have used an additive and that could be what caused the problem, but it was more than a year ago so I won't be looking at it from a "warranty" point of view.
Is there anything I can do myself to try and solve it?

Any help appreciated always.

Dave,
Sydney - Aus
D O'Brien

"...I won't be looking at it from a "warranty" point of view"

Should be, certainly partial if not complete. You are telling me that the gbx was rebuilt and installed by a paid shop, and after 14 months of not being used there is low or no oil in it, and the oil isn't oil??? Should have saved that liquid. These cars will go for 30 or 40 years without leaking that much oil, so if it wasn't full it wasn't filled, or the assembly was really bad.

Given that it had previously run out of oil, and that OD clutches seem to have a finite if long lifetime, the actual OD may need a rebuild. "No oil" or "not Oil" certainly could cause an otherwise good OD to fail, but that is for discussion; bad work or failure to fill is not, year or no year.

Been working on these for 15 years before yours was born - in a similar case I would consider this a no question "I pay at least the labour on the redo." Maybe why I don't have any money!

Meantime, might be worth checking the filter and check valve as described in the WSM.

FRM
FR Millmore

It's also worth checking for continuity through the solenoid. Mine failed with an internal open circuit, and after only 30 years in service. The D overdrive has more chance to fail here having a pull in and maintaining coil with switches. Anf yes additives in an o/d gearbox are a no no, the clutches rely on friction to work.
Stan Best

Thanks for both your replies.

My regular mechanic has been working on this car for about 10 years and he is extremely honest and trustworthy. He has made a few trivial errors in the past and immediately took responsibility every time.
I happen to know he sent the gearbox to another place for the rebuild and I know they only offer 3 months warranty on their workmanship. It was passed for registration immediately after that work (December 2010) then I rarely drove it for the following year and it just passed registration again in December 2011.
My mechanic and I might come to an agreement, but the 3rd party gearbox guy is likely to say "3 months warranty. Not my problem you didn't drive it for a year."
I will still mention it to him when he returns from holiday, but I wasn't holding my breath and meanwhile I might as well try and fix it myself so I can drive it while the weather is so nice.

When I say it was low on oil, I used a 500ml oil syringe to add some fresh oil until it started to overflow. It took almost 2 syringe loads so that's almost 1L. The overflow contained a little bit of that milky stuff so that's why I decided to completely drain it and refill after checking the electricals.

I will check the solenoid, although I replaced that only a few years ago when I tried everything else before checking the oil.

Lucky I have the WSM, although it seems to assume you have removed the gearbox and overdrive before the instructions for dismantling the overdrive unit. I remember when I replaced the solenoid years ago it was an absolute pain getting that little ball bearing to sit on top of the plunger while I pushed it back in. I don't have the equipment to remove everything so if I can't work on it in place I will have to wait for the mechanic.
D O'Brien

The only real way to check the electrical circuit on an OD is to put an ammeter in series, and you should see about 800mA with the ignition on (and manual switch on and in an OD gear), closer to 1 amp with the engine running. On an MGB with gear-lever manual switch (I assume) this would be inserted in the white wire that goes from the main harness to the gearbox harness via a handy bullet connector. It makes sense to do this before doing anything else.

This sounds to me like the classic worn or loose gearbox lockout switch, which the above will reveal, but then again the gear-lever manual switch wiring is prone to damage as well. Usually this manifests itself as a short, which burns the wiring, as it is unfused from the factory. Fit an in-line fuse at the above bullet connector as a matter of course. Also in the fuel pump circuit which is also white in the main harness to white in the rear harness, both my cars and several I have worked on came to me with the pump wiring damaged.
PaulH Solihull

Hi,

Have you seen this on youtube:

http://www.youtube.com/watch?v=XPSgUeHAy8E

Jules
J Wale

I would have to agree with Paul's assessment. It sounds like a problem with the 3rd/4th gear lockout switch. When these wear out, they will cause the power to be interrupted to the OD solenoid. Replacing the switch, or removing one of the two fiber washers underneath it, will usually cure the problem. RAY
rjm RAY

I note that youtube is part 1, but starts off by digging in to the unit itself. I would *never* do that without confirming the electrics are working correctly beforehand. It might be nothing more than a loose spade or bullet!
PaulH Solihull

I started dismantling what I could on the overdrive yesterday after draining all the oil once again (and buying a new batch of "classic" mineral 20w50 to refill later).
As I pulled the solenoid out I noticed what might have been my problem all along (or one of the problems at least).
It turns out I hadn't tested the electricals completely. When I tested earlier there was power getting to the bullet connector at the solenoid so I assumed it was all good. I didn't realise the wire connected to the solenoid was actually broken at the tiny solder joint connecting it to the fine wire in the coil.
I am far more handy with electronics than I am with mechanical things, so fixing that broken solder joint was right up my alley. On the bench the solenoid is working properly now. Fingers crossed this is all I need to do.

Hopefully whatever that milky white fluid that came out of the gearbox wasn't the cause of my problems. If so, the electrical problem was fortunate because it helped me discover something that could have ended up a lot worse.

I will find out shortly when I take it for a test drive. If it's no good I will be back on the ramps pulling the rest of the overdrive apart.

BTW that YouTube video was brilliant. I am confident about tackling the other parts now I've seen what is involved.
D O'Brien

All fixed. Just in time for a perfect Sunday drive. :)
D O'Brien

This thread was discussed between 20/01/2012 and 22/01/2012

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