MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - OD switch. Which way

The Overdrive in my 67 GT has been working intermittently for a while. I have eliminated electrical issues and changed the oil. Transmission has about 7,000 miles on it since a rebuild.

I am reasonably sure that it is the switch on the top of the transmission. My dilemma is to tighten it or to add a spacer?

Unit typically works at initial start up, but then goes in and out intermittently.

My thought would be as it gets warm it is moving just to the edge of being not working. So I should move it in.

Thoughts?
Bruce Cunha

Tighten if loose or remove a washer if it is tight in there.
Chris at Octarine Services

Not trying to score points, you say you have eliminated electrical issues, but an intermittent switch would be an electrical issue.

The easiest way to confirm or eliminate the electrics is to connect an ammeter in series and position it where you can see it when driving. If that drops to zero when OD drops out then it is indeed an electrical issue, then voltage tests at the gearbox switch will confirm whether the fault is before the switch, after it, or in it.

I've had a problem where the 'spring' connection on the end of the solenoid had lost tension and replacing the cover gasket in an attempt to cure an oil leak further reduced the pressure on the spring to the point where the connection was broken when things warmed up.

However that was on a 4-synch LH-type OD where the current is about 1 amp when the solenoid is powered or zero if not. As a 67 you could have the earlier D-type, which has a two-stage solenoid that takes 17 amps when first powered, dropping to 2 amps once operated - less easy to find an ammeter that will handle that. Also if the vacuum switch and relay is still installed there are two sources of power to the OD, so the ammeter has to be in the correct one to measure solenoid current. Said relay and vacuum switch, and their connections, can also be a source of ODs dropping out.
paulh4

Bruce,
I have a similar problem with my LH overdrive after a couple of rebuilds. It won’t come out of overdrive after it is warmed up and engaged at high rpm (ie 5,000 rpm). Engages no problem but won’t disengage after it has been used for few kilometres at high speed (on the race track). Only way to disengage is to back up with overdrive switch turned off. Both overdrive rebuild experts don’t have an answer. Overdrive Repair Services say occasionally there have “stickers” but say they normally come good - I am still waiting!
Disengage springs have been checked for length/spring rate...ok. New sliding element and brake ring last rebuild but still same problem .....won’t disengage at high rpm (engine is not standard, around 110nhp at rear wheels)
Help
Mike
Mike Ellsmore

Thanks All. Chris, great to hear from you. Should have a better picture of it this week. Going to drop the cross member and lower the trans so I can get to the switch. Hoping I don't have to pull the center console.

Started by checking the dash switch with a continuity checker. It worked every time. Checked the vacuum hose to the dash switch for leaks. Used a vacuum pump to check the dash switch (works fine)

With the car off the ground and a VO meter hooked up to the solenoid it works fine. I did swap out the solenoid, as I had a spare. It did not fix the problem.

Uncomfortable letting it run all that long with the transmission running while it is off the ground to get it hot.

On the road, it works fine for 4 or 5 miles and then works intermittently. If I shift into OD, it does not engage immediately, but will suddenly kick in. Does the same when I switch it off. It may or may not kick out.

Accelerating or quickly letting off the gas does not change it.



Bruce Cunha

Bruce,

With the extra information you have now given, my focus would switch to the oil pressure in the OD unit.

Sounds to me like you may have a sticky ball valve - slow to engage as the oil warms up and then slow to release

My first line of attack would be to drain the oil and refill with ATF to clean out the transmission. You can run safely with ATF in the box but it will find any leaks!

Mike,

I'd do the same with yours - I have had 'sticky' rebuilt OD units in the past.

Chris at Octarine Services

Bruce - confused here. You mention a vacuum hose which implies a D-type on a Mk1, but it doesn't go to the dash switch, but a vacuum switch on the bulkhead by the relay.

The vacuum switch system keep OD engaged until you accelerate, or select a non-OD gear. You would have to accelerate for a second or so for it to switch out in 3rd or 4th.

You also say you hope you don't have to remove the centre console. But even North American spec only got that with the Mk2 i.e. the LH-type OD without vacuum switch and relay, other markets didn't get it until the 72 model year, before that it was just a speaker box. With those only the tunnel carpet needs to be pulled back to expose a large removable panel on the Mk1, and a smaller one on the Mk2. The large panel gives easy access to the OD switch from above, and even the smaller one with the rear cross-member lowered and pulled across gives adequate access from above. With the later centre arm-rest and cubby only that needs to be removed, and the tunnel carpet cut round the small panel as well as the gear lever hole.
paulh4

Bruce,
sorry if I've already suggested the following to you before, as I've a vague memory of something a while back.

Following on from what Chris has put, the factory Workshop Manual has to drain and refill g/box & o/d oil and clean the o/d filter every 2 years (or 24k-miles whichever is the sooner).

Checking the oil level would be at 6 and 12 month services and it'd be sensible to also check before and after very long or very arduous journeys.

Also when engaging and disengaging o/d you keep the load on it by not lifting off the accelerator pedal.

These John Twist videos should give you general information from drain, clean and refill onward, apparently there are omissions in them but as I say the general idea.

Personally, as I'm totally non-technical and non-mechanical I'd always start with checking the gearbox oil level, then if required a change of oil and clean of filter if not done in the last two years and a clean and flush oil before refill with standard oil in your case, then 2 years time and onward it's just drain, clean filter, refill on future changes.

104 MG Overdrive Repair Part 1 - https://www.youtube.com/watch?v=XPSgUeHAy8E

105 MG Overdrive Repair Part 2 - https://www.youtube.com/watch?v=LCqPu77EWEE

106 MG Overdrive Repair Part 3 - https://www.youtube.com/watch?v=FHw_o5y0HWE

There's also a page on this site, and MGA Guru(?), with John Twist text on o/d faults

- plus of course the knowledge of the other posters here.
Nigel Atkins

Bruce,
my apologies, on rereading your original post I see you put you've changed the oil - I missed that bit.

Hence Chris suggesting the flush cleaning, you didn't mention if you'd cleaned the filter.

Sorry I've a bad memory, did you do a long journey in your B a couple of years ago?
Nigel Atkins

My concern isn't so much for the filter but for any dirt / gum / sealant around the ball that controls the oil pressure and hence both engagement and disengagement of the OD.
Chris at Octarine Services

Hi Chris,
sorry it was another rushed post, I meant the two things to be separate. I should have made it two sentences not one.

I was wondering if Bruce just done just a straight drain & oil refill only without cleaning the filter (or cleaning flush).

Nigel Atkins

Only about 7,000 miles on the OD since rebuild. Change the oil about every 3,000 miles and always clean the filter.
Got to the switch today and it is tight. Washer in place.

Talked to the rebuilder and he recommends checking the adjustable rod that the solenoid pushes on. I found a reference to this in my Haynes manual and it says it can cause intermittent engage/disengage if it is not correctly adjusted,

A bit hard to get to with the transmission in, The plate is close to the tunnel wall. It looks doable. I have to build the test rod and bend it so I can get it into the test hole.

Should get to that tomorrow.

Paul. Your referring to the plate on top of the tunnel behind the shifter? That is what I was referring to. Probably will do that if the adjustment does not do it.
Bruce Cunha

So today, I rechecked conductivity through all the switches and took the tunnel cover off to check the connections on the gear switch. One of the wires was a bit loose, Check conductivity on that switch while in 3rd and 4th. All good.

12 V to solenoid and solenoid was snapping the piston in very well. Ohms of the solenoid check out for both the pull and the hold coil.

Checked the adjustment of the engage/disengage rod. It was off. (I did call the rebuilder and he said it can move out of adjustment with use/wear)

Adjusting this is done by adjusting a self locking nut on the end of the solenoid piston rod. But, you cant get a wrench into the box where the rods are. You have to pull out a plug that is in front of the piston rod. It comes out easy. You can then push the piston rod forward and adjust the nut.

But then you have to put the plug back in. The transmission mount is about 1 1/2 inches away so getting your fingers in to get the plug started is a PITA.

I think this can be added as one of the worst jobs to do on the B.

I will put everything back together tomorrow and hopefully all is now right.
Bruce Cunha

Well done on finding the problem and sorting it, PITA it might have been but at least it was inexpensive and reasonably easily sorted.

Treat yourself to a long, fast drive.
Nigel Atkins

Sounds like I will be moving on to Chris's recommendation of flushing the OD.

Took the car to GOF west at Lake Tahoe. That is about 60 miles. OD was working just fine until I go to Tahoe. Then it started acting up again.

On the way home, it would only occasionally go in. When it activates, the unit works fine. It goes out of OD fine.

Home and put the car back on stands. Hooked up a long wire from the dash switch wires so I could activate the OD while under the car. Solenoid is clicking every time I put the wires together.

Will drain the oil and clean the filter tomorrow. In running ATF. Any type better or worse? How long should I run it through the OD.
Bruce Cunha

If it's not too late - standard ATF.

I'd think 50-100 miles of driving, engaging and disengaging the o/d as much as is sensible would be more than enough, more important to get all oils fully warmed up.

As often discussed, to engage and disengage the o/d should be under load so you don't lift off the accelerator pedal and don't need to dip the clutch pedal.

Buy about double the amount of ATF you need to fill to allow for top ups and the excess you can use during the end of the ATF draining as a further static flush of the box whilst the box is still very warm.

As Chris has put ATF is on the thin side for warmer weather and more likely to creep everywhere hence more cleaning as a flush.

Get the existing oil (and later ATF) as warm as possible before draining to be able to get as much existing oil and muck out as possible.

Leave the existing oil (and later ATF) to drain as long as possible to get as much existing oil and muck out as possible.

On refill of fresh (engine) oil, start with about two thirds of stated quantity of oil (refills never take the full dry fill quantity stated), top up very slowly, better to need to add a little more than overfill.

Either go for a drive or have the gears moving to make sure the oil disperses fully then leave the oil to fully settle and check and top up as required.



Nigel Atkins

Did you check the adjustment of the solenoid? When it starts playing up measure the resistance on the wire leading to it and it should be about an ohm or less. Disconnect that wire from everything else i.e. the relay and vacuum switch to avoid confusion.

ATF seems to have different viscosities like engine and gear oils, I'd be inclined to use a grade slightly lighter than the oil you normally use.

As for how long is a bit like a piece of string, i.e. if it is going to do the job of flushing it will do it in its own good time. Say several trips of at least 60 miles without encountering the problem, then drain, clean, and refill with your usual oil. If it happens again at the same point that it did before then obviously it hasn't worked. You could try leaving in ATF longer, and I have heard of people running with it all the time when that was the only way to solve the problem.
paulh4

If it runs fine on the ATF ( I use Dexron III ) and it doesn't leak horrendously then I would just leave it in there.

The gearbox will be happy on it too - I run the LT77 box on my V8 on Dexron without any issues and gear shifts are faultless even when it is really cold.
Chris at Octarine Services

I must admit I thought it was a lot warmer in California than it appears to be from a quick search, I was thinking more about the oil in hotter rather than colder weather.

As an aside I've found with the few LT77 boxes I've had that they require a bit of experimenting with the oil to suit the particular box if you want to use it during the hotter and the colder weather of the whole year.

Whatever oil works best for box and owner is fine but it always also pays to check the oil level in the box and change the oil regularly (and thoroughly).

Always surprises me that some owners will be changing the engine oil very regularly (sometimes more frequently than required perhaps) but don't even check the oil level let alone regularly change the oil in the gearbox and rear axle (not meaning another here).
Nigel Atkins

The unit has about 7000 since rebuild and I have changed the oil twice in that time (about every 3000). Currently running 20/50 per my rebuilders instructions.

Going to drain and check the filter to see if there are any signs of metal particles. Will let it drain for a day and replace with ATF.
Bruce Cunha

Sorry Bruce I've just seen I put >>another<< instead of - not meaning -anyone- here.

I did previously (belatedly) notice you'd put you'd changed your oil in your original post.

Towards the end part of the drain of the existing (engine) oil whilst the oil is still hot/warm I'd tip some warmed-up ATF in to the box so that it drains through to the open drain hole (and removed filter opening?) hopefully pulling a little more existing engine oil/debris out.

I'd do the same on the ATF drain too.

After a long period of draining it'd be interesting to measure the final amount of refill (engine) oil it takes to fully refill the box (after checking level after a quick drive).
Nigel Atkins

Nigel,

I have drained many OD gearboxes, 3 & 4 synch, I always drain them level on the bench overnight to start with and then stand them on end, bellhousing down, overnight again before putting them level, taking off the side cover and tipping them over to get the last drops of oil out before dismantling.

The second drain produces about 1/2 pint of oil - as well as in the OD unit there is some trapped in the lower part of the gearbox itself.
Chris at Octarine Services

Cheers Chris.

That's my point really, that the residue that's left after an overnight drain let alone normally left on a quick (and cold) drain and refill is a lot more than many owners would expect. Then of course the residue will be previous oil with whatever muck (sorry can't think of correct word(s)) it has suspended in it and possibly other muck settled in the box and o/d.

Obviously the more thorough the oil change and how long it's left to drain the better the ratio of fresh to existing oil and less accumulative build up from dirty oil.

Same applies to coolant changes, only it's easier to take the system apart more for draining and perhaps siphon out more residue (on a Spridget at least).

1/2 pint left in from an overnight drain out of 5-6 pints is probably a lot better than from engine fluids.
Nigel Atkins

Excellent points. Flushing with ATF would appear to be a valid exercise.
Bruce Cunha

This thread was discussed between 23/08/2018 and 19/09/2018

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.