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MG MGB Technical - D-Type overdrive advice please

I fitted an appropriate 3-synch overdrive gearbox to my 1965 MGB nearly thirty years ago. I bought the gearbox when I was working in the UK for a year in 1984, and had an overdrive specialist in London at the time check and rebuild the overdrive unit for me. (No major dramas. They did fit a new overdrive clutch). I had the gearbox itself rebuilt by my local garage soon after I arrived back in Sydney.

The overdrive has been mostly faultless since. I've had to replace two dashboard switches and one failed solenoid in those thirty years. I've never fitted the vacuum inhibitor harness, vacuum switch and relay, (never realised they existed back then) though I have now managed to acquire all these items (and plan to ultimately fit them, just for the sake of originality).

My problem is that over the last two months the overdrive has become sluggish to engage under load especially in hot weather. No problem engaging under a very light throttle. Disengagement remains instantaneous.
Is it possible that the new (about five years old), presumably Asian made solenoid is starting to fail? Are there any other suggestions? Dirty filter? Needs oil change?

And while I'm at it, the overdrive has tended to slip when engaged under load in third gear for a few years now. I've simply adjusted my driving style to suit; I tend not to use third overdrive too often anyway.

The Mk I D-type overdrive gearboxes are very uncommon over here in Australia, so there's very few with much experience with these earlier overdrives in my part of the world. I'd be grateful for any advice.
T Aczel

oil changes in the service schedule are every two years/24k-miles whichever is sooner so if you've not followed this then this would be the first place to start

the engine oil in the gearbox breaks down quicker than if gear oils where used so the engine oil in the gearbox needs changing more frequently

at the services you'd also clean out the o/d filter

have the gearbox oil as warm as possible before draining and leave to drain as long as possible, both will help to get as much muck and old oil as possible

don't forget you have two drain plugs

the o/d should slip in and out effortlessly and (almost) instantly, the clutch does not need to be dipped or accelerator pedal lifted off fully it works best under load

see this article, plus if you need them I can list the John Twist video about o/d -

Nigel Atkins

Overdrive is engaged by a solenoid moving a plunger that shuts off a valve. That valve when open maintains a low pressure for to lubricate the OD and most of the oil from the pump goes straight back to the sump. When the valve is closed it causes the pressure in the system to increase and moves the pistons to engage OD. So a weak solenoid could well cause both a slowness to engage, as well as slipping by not holding the valve fully closed at the higher engaged oil pressure.

Fistly you can check the windings by measuring the current. When first powered they draw 17 amps which is the pull-in winding plus the hold-in winding. This should equate to about 0.7 ohms. Once operated the current should drop to 2 amps (or 6 ohms) which is just the hold-in winding. A current lower than 2 amps for example from a bad connection would cause weak solenoid operation, although if in the external circuitry that is more likely to fail to engage OD at all. A current higher than 2 amps could indicate the windings are partially short-circuit and giving less magnetic pull.

However the solenoid could also be binding, so that might be the first thing to check before replacing it. Unlike the LH-type solenoid the D-type has a linkage that has to be adjusted correctly. The valve ball and seat could also be dirty or damaged allowing leak-back.
Paul Hunt

What type of oil are you currently running in your transmission/OD? I've been using non detergent 30W oil since I replaced a balky solenoid and the OD never responded so well. MY OD transmission has been in my '67 B for close to thirty years with only a solenoid failure in all that time. I believe that your problem is being caused by low oil pressure acting on your actuating pistons. Change the oil and clean the filter. The Bentley manual describes, in detail, the process for checking and adjusting the solenoid linkage. RAY
rjm RAY

Assuming there is sufficient oil in the box then slip in OD could be due to incorrect adjustment of the solenoid operating lever, the pump non return valve not seating fully or a worn or sticking relief valve.

The first thing I would do is adjust the solenoid lever as per the manual - then I would drain the gearbox and fill with Dexron auto transmission fluid and run for a week or so, then drain and refill with engine oil. The ATF is highly detergent and will clean up the valves etc.

If you still get slip after that then get the OD operating oil pressure checked and if that is in spec ( 400 psi) then is is likely a worn cone clutch lining or possibly if the clutch was replaced by a bonded unit then one of the linings may have become unbonded!
Chris at Octarine Services

Thank you all for the comments. They're all helpful.
I've always run engine oil in the gearbox and the overdrive, but generally multigrade 20/50. It's definitely been more than two years (and possibly four or five) since the transmission oil was changed and filter cleaned. So that sounds the obvious place to start. A run with ATF as Chris suggests sounds very worthwhile too, especially if the oil and filter do prove to be less than pristine.
Thanks Paul for the solenoid current figures. I was aware that the solenoid was a two stage device, and drew more current in the initial engagement than the subsequent running once engaged. To have the actual figures to check is an excellent bit of information.

As for checking the O/D specs "as per the manual", two out of my three MGB workshop manuals only refer to the later LH overdrive. (I was starting to get anxious!) Most Australian Mk II MGBs built here had overdrive fitted. The D-type, Mk I MGB overdrive gearbox was however only offered here in Australia in the last few months of Mk I assembly, January to August 1968. Abingdon had gone over to the Mk II by then. (As with a number of other examples I could list, it appears that the UK parent company offloaded superseded models and components to the Australian colony to clear the decks for the newer models back home. Another example: they were still shipping MGA 1600 cars for assembly here for months after the MGA 1600 Mk II had been introduced in the UK!)

I'll get going on looking at the suggested issues in the next two weeks, and will report back. I hope it's not the o/d clutch; that'd be a serious pain on this side of the planet.
T Aczel

As far as the linings go a pal with a supercharger broke his up and found chunks of it sitting on the filter screen above the sump.

Paul Hunt

keep checking back here as previously Chris has offered a downloadable PDF of the workshop manual that includes your o/d
Nigel Atkins

Lol - but that means visiting the website ....
Chris at Octarine Services

Sounds like after 30 years the clutch linings are wearing out; especially when slipping is experienced. The "D" type cone clutch is rather small in comparison to the LH type; approx by 1/2. Time for complete rebuild & functional bench test.

The dash mounted O/D switch is not designed to deal with heavy solenoid current; that is why the relay is used in the activation circuit to pass the large current gulps. Moss sells a sub-harness from relay to main harness junction & overdrive connections. The dash switch harness was built in from the factory.
Rich Boris

The main reason for the relay is that it is used in conjunction with the vacuum switch to keep OD engaged if the manual switch is turned off on the overrun.

However it's quite possible that the load on the manual switch without the relay is damaging the contacts, and limiting the current, making it harder for the plunger to completely close the valve to develop full pressure. You can check that by comparing the voltages on the input and output sides of the manual switch with OD engaged. If you see the same voltage i.e. 12v then the switch is fine. If you see less on the output than the input then the manual switch is limiting the current. But as I said earlier in the current tests if it's bad enough to affect the hold-in winding I'd expect it not to be able to pull it in at all, as any resistance has a proportionally greater affect on the higher current required by the pull-in winding.
Paul Hunt

The D type design always used the relay on cars with overdrive. The vacuum switch was an option; which also is wired to the relay.
Rich Boris

Don't think so Rich - the vac switch is essential to protect the D type OD unit from overload on the overrun and was always fitted with the relay.
Chris at Octarine Services

T Aczel,

I have a pdf copy of the MBG Manual (BL 14th Edition) which gives the 'D' Type overdrive fault finding details and also the wiring diagram, including the vacuum switch.

If you send me you email address to richardl.wale at, I can email it to you. It's 23Mb, so if your ISP can accept large attachments, let me know if you would like it.

Richard Wale

workshop manual can be downloaded from here

D.K. McNeill

there you go, info from Workshop Manual on your o/d
Nigel Atkins

Thank you everyone.
I've downloaded the workshop manual using the link provided by D K McNeil. The D Type overdrive section (and much of the remainder) looks the same as in the one of my three Australian published workshop manuals that does describe my type of overdrive.
I'll start to investigate shortly and will report back.

As for the vacuum inhibitor circuit, I noticed very early on that disengaging the overdrive on a closed throttle, especially at higher road speeds seemed to be rough treatment for the driveline, and have avoided doing this.

I've covered over 220,000 miles in this MGB in my 44 years of ownership, (and a total probably well in excess of 250,000 miles; I suspect the odometer was wound back before I bought the car) and it's held up pretty well, body corrosion issues aside. The car sees much less use now than back when it was my everyday car for about my first 15 years with it. Even so, I'd guess that the overdrive and gearbox have now covered about 80,000 to 100,000 miles since fitted. The gearbox is definitely tired now too, but only since I stupidly lent the car for my secretary's husband to drive one evening. No crunching or whining, but I can no longer engage second gear on the move without double declutching; second just won't go in. Slides in nicely though with double declutching. So the gearbox too is coming up for work.
T Aczel

I'm not sure why your D o/d should vary from what's in the manual other than it's a slightly different unit to the ones fitted to Bs (?) - perhaps someone else will know of any variances

I think a good drain and flush use clean of g/box and o/d with ATF then refill with engine oil and clean of o/d filter might help 2nd gear a little and the o/d if only by a flush cleaning plus you can see if there are any parts in oil from either or o/d filter

have the gearbox oil and later ATF as warm as possible before draining and leave to drain as long as possible as both will help to get as much muck and old oil and ATF as possible

you might also consider a different weight and range of oil for the age of your g/box and o/d and for the heat(?) of NSW

this plus the other things mentioned will help, even if not fully cure
Nigel Atkins

No variation Nigel. I'm certain my D type overdrive is the same as all the others. However, perhaps incorrectly, it didn't occur to me to check the manuals (one of which had the information on my overdrive), and I just elected to ask of the collective wisdom of those on this site. However it has been great getting the input from all who've contributed. It's given me a logical sequence of steps to work through.
T Aczel

sorry I thought the workshop manuals didn't describe your type of overdrive

I only know about basic servicing work (and not much about that) but it's surprising how often these basic things are overlooked and how just attending to them can bring good improvements

best of luck, I hope things improve for you
Nigel Atkins

BHA 4282 was the BMC part number of the vacuum switch/electrical relay that was standard fitment with the D type overdrive. #9 at the top of the page. RAY

rjm RAY

The vacuum switch and relay were two separate components, BHA4282 is just the vacuum switch. The relay is a bog-standard single normally open contact type BMK685/6RA 33302B/Lucas SRB111. The relay is above the vacuum switch, to the right of the heater in the attached.

Paul Hunt

Thanks Paul (and as before, everyone else too).
Image of the vacuum switch I was, I think, fortunate to find and purchase.

T Aczel

View from the other side attached.

Disappointingly I'm not going to have the opportunity this weekend to start investigating my overdrive, but hopefully next week. I will certainly report back what I discover.

T Aczel

Leave it to Paul to uncover my oversight. #38, part number BMK 685 in my above scan, shows the factory relay. RAY
rjm RAY

Very fortunate I'd say, I don't think I've only ever seen one for sale, and only once seen one offered following a request to a forum.

FWIW this describes the switch including calibration. It talks about 7 in Hg. as being the switching point, i.e. at anything above that the switch would be closed and OD held engaged. However that represents quite a large throttle opening, from what I've seen on a vacuum gauge, I would have expected less throttle to trigger disengagement.
Paul Hunt

I've just realised that even with the vacuum switch and relay installed there is a good chance of the manual switch taking the very high current operate load of the overdrive solenoid under some circumstances.

The vacuum switch is said to be closed when the inlet manifold vacuum is greater than 7 in.Hg. This is significantly less than the typical level when idling, and so means that the vacuum switch will be closed when idling, let alone on the overrun.

If you happen to be in an overdrive gear with a closed throttle but overdrive isn't currently engaged, and operate the manual switch, the manual switch will power the solenoid through the vacuum switch at the same time as it operates the relay. There is a finite time before the relay contacts can close, and until they do it will be the manual switch that is supplying the 17 amps to operate the solenoid.

Really there should be a diode in series with the vacuum switch to prevent that happening, or the relay should be a 'dual make' or 'split-charge' type to separate the vacuum switch and overdrive solenoid circuits when the relay is released. But neither were available at the time.
Paul Hunt

Please forgive my innocence at all things MGB, but as you all know I have just got a Hybred GT (1967 plate 197? shell).
I have a 3synco box with overdrive (which one I am not sure),I have not got a Vacuum Switch or Relay, but when I drove it home the overdrive appeared to be working, Dropping the rev's by about 500.
Years ago I had a Sunbeam Rapier With overdrive and in those days it was common to dip the clutch when engaging the overdrive and as I remember the gearbox had an inhibitor switch which automatically disengaged the overdrive when you came back down into second gear.
Do I understand that the MGB overdrive dose not do this and that damage can result from improper use of the overdrive switch, or am I reading this wrong.

Confused Graham

GJ Barker

some believe that dipping the clutch helps the o/d which it might if the o/d isn't working properly but the (later at least) o/d should slip in and out effortlessly and (almost) instantly, the clutch does not need to be dipped or accelerator pedal lifted off fully as the o/d works on load
Nigel Atkins


THe 3 syncbox will have the D type OD unit.

These are fragile compared with the later LH type fitted on the 4 sync boxes.

To protect them the vac switch was fitted to ensure the OD was not used to drive the engine on down changes as this can damage the unit.

All ODs are designed to give a smooth and cushioned change of gear and dipping the clutch is NOT needed.

Driving without the vac switch fitted is fine as long as you switch out of OD without lifting off the throttle - basically just leave your foot alone on the throttle and the engine will automatically speed up as it changes down out of OD.

Chris at Octarine Services

OK, first update re my overdrive, and unfortunately things so far don't look too promising.

In the end, not being able to get access to my neighbour's hoist (I hate grovelling under cars these days), I had my very capable and British car sympathetic (especially Jaguar) mechanic, who's helped me look after my two MGs for a quarter of a century, check out the overdrive.

The oil drained out looking pristine. The filter looked almost completely clean too, but was given a thorough clean up nonetheless.

The solenoid snapped in and out perfectly. (My mechanic knew the current figures for the solenoid off the top of his head (though he quoted 16 amps as opposed to Paul's 17 amps).

The linkage clearances also checked out to spec.

Refilled with engine oil, there was no change on the test run, and as before, slow to engage the overdrive under more than a light throttle load, and worse as the oil warmed up.

So the oil has been drained again, and refilled with ATF.
I'm currently waiting to do a test run with the ATF, but the rain has set in around here, so the test will have to wait a few days. However I must say I'm not feeling too hopeful.

Any more involved assessment would need the engine/transmission pulled out for access.

My mechanic, who fitted a five speed conversion to my MGA, is inclined to change the MGB over to a five speed gearbox too. I however like the feel of my MGB's gearbox, and the neat overdrive function, and after forty four years of ownership, the car remains in a very original unmolested state. I'd prefer to keep it that way.

For the interest of others, I'll attach a photo from my (supposedly retired) mechanic's workshop. That's my MGB in the background. The XK120 in the foreground has been converted to a five speed gearbox and four wheel disc brakes.

As always, Id be interested in any suggestions, thoughts and advice.

T Aczel

While I realise I'm getting off the original topic query, I should have added that it is the lack of good quality replacement parts for the early three synchro gearboxes that makes my mechanic lean towards a five speed conversion. Both my overdrive gearbox, and my original four speed three synch gearbox have the steel ring compatible gears, and there are I believe, no decent quality replacement steel synchro rings available for these gearboxes. I have obtained a genuine late four hole layshaft, and can source an original genuine BMC cluster gear for this shaft, there's still a bunch of bits that I'm likely to need for a rebuild that are likely to be unavailable or of a poor standard. (I read of someone obtaining a set of brass synchro rings that were out of round!)

And since I've taken things off the topic, for those interested, I'll attach another photo of the Jaguar XK 120. It's the same car seen in the earlier photo, despite the apparently different colour.
I'm told the car, despite the spectacular appearance, is actually not a pleasant car to drive. At least an MGA or MGB remain a very pleasant car to drive, even in this day and age in today's traffic conditions.

T Aczel

Ill trade my A for your jag ( if its yours any )day!

Steven Devine

Thanks Steve, but the Jag is not mine, and I've already got one of what you're offering!

And while it's my fault that we've drifted off the topic, I'd like to get back to the issues that led to the post, namely advice regarding my overdrive, and secondarily my gearbox.

T Aczel

Tom, maintain the mystic of the overdrive transmission! Your problem sounds like my problem with a slipping cone on my LH OD unit last year. A new cone from Overdrive Repair Services fixed the problem
Mike Ellsmore

"inclined to change the MGB over to a five speed gearbox"

I'd rather have the six-speed gearbox of my MGB ...
Paul Hunt

and I'd rather have the torque of my V8 that makes gear changing pretty well redundant - can do 10 to 90 mph in just third gear ....mind you the weight of the clutch inclines me towards as few gear changes as possible!
Chris at Octarine Services

This thread was discussed between 14/02/2014 and 01/03/2014

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