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MG MGB Technical - to overdrive or not to overdrive

Ok, this may be a stupid question. I have a 72 MGB, and from the looks of it I do have the overdrive transmission. On the right hand side of the steering wheel I even have a switch to engage and disengage the overdrive. My question is: Is there a way to actually check if the OD is working? Is there a light that comes on while driving, should I feel the additional power or see it on the tachometer? I'm a newbie so any help will do!! Thanks
Dylan

Dylan,
Welcome!
Easy one to check. Just drive at a steady speed in top, take a note of the tacho reading and when you flick the switch to engage you'll see the revs drop by a few hundred. Disengage and they'll rise up again. If you're rally lucky it will work on third gear too.
If you see no change report back and you'll get heaps of help from the helpful people on this board.

Steve
Steve Postins

Dylan - As a convenience to the manufacturer ALL MGB's of your year have an overdrive SWITCH incorporated with the wiper switch. Very few, however have an overdrive itself. You can check if you have it and if it's working via Steve Postin's post. You might well have one, though, that's not working. You can double check and look at the tranmission and see of there's an extra "box" with a flat plate at the end of the transmission. Consult your Moss or Vicki Brit catalog for a "fer instance" on how an O/D looks different than stock. In all likelihood, you probably don't have one, but you could be lucky.

BTW - an overdrive does not give "extra power" it just provides more mechanical advantage, permitting lower lower rev's at a given road speed. This permits better use of your powerband for different road speeds. Me? I like to use it to keep up with Michigan freeway traffic in the 80 mph range, with a reserve.

There is no light or other indicator than lower rev's (readble, as you say, on the tach) and the switch arm position.
John Z

If its original overdrive, there should be a small sticker on the steering column near the switch. It should have Direct and Overdrive with arrows pointing to the relevant switch position.
Marc Deaver

Welcome to the madness. Yu may have to actually look under the car and view the tranny. If it the tailend tapers back to the driveshaft its non OD. if it has a lumpish box on the end where te dive shaft meets its OD. Al te cars had the OD switch widin, it as just a matter of it actually having the OD unit to connect to. You can also check the wiring. Look in the engine bay by the charcoal canister. You should find a yellow wire coming out of the wiring harness. This is the wire coming from the switch. If its connected to something you probably have OD. Disconnect it, turn the ignition on and put the car in fourth WITHOUT the engine running. Connect a test light to the wire and see if it has power. Some have wired an LED indicator to the circuit and mounted it on the dash or console as a visual reminder the OD is on. Good luck let us know what you find.
wiliam fox

I seem to recall that there is an "o" in the serial # for those cars that came with overdrive original, or maybe I'm just dreaming.
Greg Bowman

The O/D switch came on most all cars irregardless if an O/D transmission was actually installed or not.
This was done as a factory production cost saving measure.

There's no dash light or any obvious visual indicator to tell you whether if the O/D is engaged.

Only the tach, your ears, and the seat of your pants, will let you know whether if the O/D is engaged,
or disenaged, while running.

Tuck yourself under your car and look towards the tail end of the transmission casing for a bulbous
housing (about the size of a healthy cantaloupe) wearing a flat "Laycock" I.D. plate. Non-O/D trannys
have a tail housing that is tapered and narrow, and there's no Laycock I.D. plate.

Some car owners without O/D transmissions will utilize the unused O/D switch as an additional
anti-theft device (ie: for shutting off the electric fuel pump, or cutting the feed to some other circuit).

Since you are new at O/D issues: some advice...

Whatever you, do NOT use Reverse gear while the O/D is engaged - or else all hell will break loose.
I mean this, literally. The O/D solenoid on some cars can sometimes be slow or sticky to actually disengage the O/D, so do not rely solely on the switch position into thinking that the O/D is not
active. Be sure to glance at the tach and feel for the O/D to disengage via the seat-of-your-pants,
and by listening for the change of noise pitch from the engine and tranny. Teach this to all who
will drive or borrow your car (...with any luck, they'll be so discouraged that they'll rather stick to
driving their own minivan or SUV).

Do not use regular transmission 80W or 90W gear oil, or anti-friction additives in the transmission.
Only use the appropriate weight engine oil. 40W to 50W in warmer climates. 30W to 40W in medium
to cooler climates. I've used multi-grades without any hangups.

Some folks choose to press the clutch pedal when engaging the O/D. Although this is not absolutely
necessary, I'm one of those folks who does this. I find that it's imparts a lesser jolt along the entire
drivetrain (possibly making the u-joints, and clutch disk springs, and many of the other spinning parts live longer). Besides, it just feels more civilized to you and your passenger. I do not press the clutch when disengaging the O/D, however, as I want to insure feeling and hearing the O/D disengage, but
I usually wait until I'm around or below 30 mph to do this.

If you are handy, install an in-line fuse somewhere along the electrical feed to the O/D soleniod.
The closer to the switch, the better, but having a fuse anywhere along the ciruit is better than
having no fuse at all. There are other threads on this BBS giving instructions on how to do this.
Daniel Wong

Dylan-
Greg is on the right track. If your car was built with an overdrive unit, Then the number plate on the engine (on the block below #2 and #3 sparkplugs) should read 18V-585-Z-L. If it says 18V-584-Z-L, it's a non-overdrive model. However, after 31 years it's always possible that the engine, transmission, or both have been replaced, so follow Wiliam Fox's advice on what to look for under the car. Whatever you do, never put the car into reverse with the overdrive engaged. You'll ruin the overdrive unit. If you find that you actually do have an overdrive unit, come back to this BBS and we'll be only too happy to help you get it back into action again.
Steve S.

coyuple of oferdrive comments: When I first got my C-GT the od lockout awitch was not connected and for 6 years I had od on all four forward gears. There were numerous times I reversed in od and, while I know this will destroy the od - nothing happened! No fire, no smoke, no failure no appearance of the Prophe Jeremiah. Now 60+K miles with the od connected properly there have been no problems. the factory rally teams ran od on all four gears but they got a new od between rallys.
John
John Simmers

Dylan,

Here is an image of a motor & gearbox with overdrive. Note the rear section, where the drive shaft connects, below the the shift lever. This is the overdrive unit that all have described.

http://www.mzaff.com/Images/1977roadBack/bayReassy/bayReassmble06.jpg

-mike
Mike Zaffarano

You have to be going fast enough in overdrive reverse to build up enough oil pressure to move the cone clutch before the damage will occur. Sounds like you were lucky, it certainly isn't a recommendation to anyone else to ignore a non-working gearbox override switch.
Paul Hunt

Again I agree with Paul, a non working od switch is something to be replaced. My point was that a couple of accidents will not kill the system.
John
John Simmers

Be a bit careful about switching off your Overdrive when going down steep hills. It is better to use the clutch. I was once moving down a hill at speed and reached out causually and switched the O'drive switch off. Unlike Daniel I did not find the "jolt" reassurring in the least bit, infact at the time I thought I might have left the transmission on the road behind me. (he's right about the other stuff though)

Having said that, O'drives are great, effectively they give you 6 forward gears and heaps more go at high speed. Great for long distance driving, you can just change back a fourth between Overdrive third and Overdrive fourth.
I seriously missed having a O'drive on my second car.
If you don't have one and ever need to reco' the gearbox, do get an overdrive.
Peter

Concerning not reversing when the overdrive is engaged.
Wouldn't it be possible to use the reverselight switch/wiring-loom(?) for disengaging of the OD? Anyone have any ideas?

/Jesper
Jesper

Jesper - why, when there is an OD switch on the gearbox for the purpose? I suppose you could have a 'belt and braces' break-contact relay on the revrse light switch as well, but how would you know it would still be working when the main switch failed, which is almost unknown? I suppose you could have a warning light ... but then you wouldn't need the 'belt and braces' relay ...

The only warning the manual makes about disengaging overdrive is that it should not be done as such an engine speed so as to put it into the red zone on the tach. Other than that it specifically says the clutch should not be used, neither is there any need to feather/increase the throttle pedal. I do seem to recall someone posting from Oz that they had repaired 'dozens' of overdrives with glazed clutches because people didn't de-clutch when switching out, but not from elsewhere. Don't see how it gives 'heaps more go', tho.
Paul Hunt

I suspect the old rally Healeys used the reverse switch to prevent od on reverse. Od on all four forward gears is neat, but the BMC competitions dept. got new od's for each event. My feeling is that od on all forward gears is possible (and for a brief period is FUN!) but get a new od every thousand miles. I don't believe the od can take the pressure of 8-speed operation for long periods. Any real data out there?John
John Simmers

RPM vs peak power Paul, the O'drive cars have a higher top speed. Perhaps with a really good set of extractors and a worked head it makes no difference?
Peter

The problem with OD in the lower gears is the higher torque reversals can be too much for the clutches, which is why the factory V8 had OD limited to 4th only on all but the very first few cars.

Higher top speed yes, but 'heaps more go' to me sez getting there faster, hence my doubt.
Paul Hunt

I may be talking out of turn here as my 'B' is a V8.
I'm sure Paul Hunt will tell me if I am wrong. The O/D gearbox has a filter on the bottom on the V8 but I'm not sure if this is the same on the standard 'B' box.

The O/D on the V8 seems to go in quicker as the revs build and the car will really lurch forward when it is engaged, although the revs drop at the same time.

Coming out of O/D can cause the car to really feel as if the brakes have been momentarily applied unless the throttle is hammered at the same time and I tend to change down to 3rd before disengaging unless I need the direct 4th power of any reason.

As far as I know MG's have never had any sort of indicator to tell you when you are in or out of O/D.

Martin

Funnily enough my V8 came with a warning light - the same size and shape as the handbrake and seat belt warning lamps used on later cars but saying 'OVERDRIVE' instead i.e. it is a pukka item from somewhere but not an MG. Unlike the roadster where I can just look at or feel the 'shepherds crook' it is useful on the V8 which has the column switch. Either you are much more sensitive than I or your OD is much more violent than either of mine for some reason. Yes, you can feel it going in and coming out (said the actress to the Bishop) but it has never bothered me. Having said that what *does* bother me, particularly on the roadster with its lower power, is going up a gear only to realise I left the manual switch on, so I devised a circuit that, once you have gone into a non-OD gear, locks the OD out until the manual switch is turned off and on again.
Paul Hunt

The Overdrive light is a nice idea, if I still had an O'drive I'd put one in. Probably not strictly necessary but not a bad addition none the less.
Peter

Having worked on many overdrive boxes over the years, I have found the most common reason for an overdrive not working is a low oil level in the gearbox. The filler is 'conveniently' located under a rubber bung, behind the radio/centre console on top of the transmission tunnel. If you find you have an O/D box and it doesn't work, try this first as it only costs a pint or two of oil (engine oil).
Dave
dave

Hi,

bit of a newbie here - only had the car two months and it is my first car ever.

I know this may cause some debate but here goes what is the "best practice" when engaging the overdrive.

I got a 1974 Uk Spec MGB GT with overdrive on 3rd and 4th and the shunt forward when engaging seems a bit clumsy like it could be wearing / damaging something.

I have to explain to my mates what overdrive even so it's pointless asking them and my dad isn't too sure so...

What sort speed should i be doing when engaging, is it necess to clutch ??? You know the really obivious sort of things that you just know till someone tells you.

Would really appreciate the help, cheers guys!
Simon C

The handbook says not to de-clutch or vary the throttle when engaging or disengaging, just don't disengage at very high revs if it would put the tach into the red area. Like the question 'how do I know when I'm in love?' the answer to when to engage it or not is equally unilluminating - when the conditions are appropriate, just like at what revs you chose to change up or down. For example if I am pootling along at 40-odd I'd engage it then. But if I'm accellerating up to 70 or so I wouldn't engage it until later. I might knock it out to overtake at 60 or 70, but leave it in down to 30. It's generally used for cruising, but if on a long climb or a twisty bit I might well use OD 3rd rather than 3rd or 4th.
Paul Hunt

Personal experience seems to be that use of O/D (in or out) on a trailing or neutral throttle produces the sort of teeth-rattling snatch that is not only probably detrimental to the drivetrain, but is definately detrimental to the neck (not a transmission component). Personally, I flick the switch with impunity while accelerating and de-clutch at other times. I've found that just a "dab" of clutch (enough to let it slip a bit) is adequate to take all the stress out of the process.
Tim Cuthill

I recently purchased a used tranny and Overdrive unit from Mechspec for my 68 GT. I will probably install it during the summer. My old tranny had some unusual noises and so given the price and labor, I just bought the overdrive tranny. Since I will be making a new installation, I will replace the clutch housing, plate and bearing, the rear main engine seal, service the flywheel, new speedometer cable, install new casing seals on the tranny and OD unit. I will also install an indicator light on the dash. I have a 68 with 70 dash which gives me the opening for a rheostat on the dash. My car does not have the wiring for the rheostat, so the hole has a plug in it. So any suggestions for maintenance or replacement while I have the engine and tranny out? What about using synthetic oil in the tranny?

thanks
Steve

Steve,
I don't know for certain but I would be wary of synthetic oils in O'drives. I am led to believe antifriction additives do weird things to the O'drive clutch. I figure a sythetic oil is just one giant 100% "additive". (I used to use Penrite.)
Peter

Steve, stick to normal oil... there's no need for synthetic and it'll probably stop the 'box working properly.

When switching my overdrive off I like to give a quick heel-and-toe blip to avoid the lurch. With practice it's possible to be completely backed off the throttle, switch the overdrive out and blip, and not feel it at all. When switching in I'm usually at WOT and hence get a kick as the OD engages... I would like to be able to lift momentarily at this point, but the time delay varies depending on speed and oil temp and my Brain can't seem to predict that yet :o)

Dylan, good luck with your car.

ttfn,
--
Olly
Oliver Stephenson

Hi Guys,

Just a few comments on the points made in this thread.

Engaging / disengaging the OD smoothly - treat it like you were changing gear using the gearlever, i.e. increase the engine revs to match on disengaging and ease off the revs when engaging.

Glazed clutches - all clutches are "glazed" except when brand new! Fact of operating in oil..... the cone design still works however!

V8 OD - like the Cs OD unit uses stronger springs and a higher oil pressure so will engage faster and more positively.

Inhibitor switches - seem to be OK is service but every one I take out of a car fails my electriacl test which is that it should make good contact in 9 out 10 operations, most only make in 3 or 4 times out of ten, this is why I always supply new ones with my gearboxes and kits.

Oil - never use synthetics or anti friction additives the OD will work with anything from EP 90 to ATF fluid with 20/50 engine oil being the normal recommended lubricant - ATF or a mix of ATF is good for speeding up engagement or dealing with low ambient temperatures as well as cleaning out dirt, be warned though, it will seek out leaks with a vengeance!
Chris Betson

Inhibitor switches - fails in what way, Chris? If you are saying they are failing closed then I am not being rude when I say there must be something wrong with your test method. I very often leave my manual switches on when coming out of an OD gear but have *never* had the OD remain engaged. Nothing wrong in providing a new one as you describe, though.
Paul Hunt

Paul, they fail to close the contacts reliably - I simply connect a test meter across the contacts and repeatedly press the operation "button". If they don't make good contact 9 out of ten times I scrap them = I scrap most switches!
Chris Betson

I recently did a resistance check on an Inhibitor switch and a reversing light switch and both had over 50 ohms resistance across the terminals. They were working in their respective circuits but I suspect they may fail at any time. I diassembled one of them, cleaned the contacts and it now reads less than one ohm. FWIW, Clifton
Clifton Gordon

Chris and Clifton - a meter passes a very low current which can fail to break through an insulating film which a heavier current i.e. the lights or solenoid will. In my telecom days certain switches had 'contact wetting' currents passed through them for this reason, cleaning the contacts would work, but only for a while. In tests for something else I found that the overdrive would not engage with 35 ohms in series although it would with 20 ohms.
Paul Hunt

Paul, Thanks for bringing up points I either don't remember or wasn't taught when I went to a one year army missile radar maintenance school almost 50 years ago. I guess mine would show some resistance after going to the trouble of disassembly, not easy, and cleaning. In other words it seems I wasted my time worrying about it. Regards, Clifton
Clifton Gordon

Granted that the switch may make more reliably with a heavier current but my reasoning goes thus - if some old switches make contact as reliably as a new one using the meter then they are as good as new and can be reused.

If they don't then I don't want a customer saying "I put all this together and in the car and now you say I've got to get it out again to change the switch?!"
so they get a new switch.....
Chris Betson

Chris, Paul, Clifton,

Wish you guys had had this discussion two weeks ago before I installed my o/d gearbox! It seems to be mechanically ok but get intermittant operation. I've wired a telltale into the wire from the inhibit switch to the solenoid, and I find I can make it wink on and off by wiggling the shift lever, and so far I've not observed the light on but the o/d disengaged. So it seems pretty clear that the inhibit switch is at fault.

That being the case, is there any hope of replacing the switch without the engine and gearbox coming back out? Or should I resign myself to the inevitable?

Cheers!
Rob Edwards

Rob, You can replace the switch by lowering the rear of the transmission so it rests on the built in crossmember, but it's still a tight fit. I had an OD problem, the OD would not engage in 3d gear but worked fine in 4th. I could get it to work in 3d by pushing and holdong the shifter to the right. My switch had two plastic washers under it, I removed one and it works fine now. While I had the trany lowered I decided to check the resistance across the closed switch contacts and noted some resistance, but Paul's comment explains why it was probably ok. Some people have cut holes in the tranny tunnel to access the switch. Regards, Clifton
Clifton Gordon

Rob - that is *usually* an adjustment thing as Clifton describes. Lowering the crossmember, pulling back the tunnel carpet and removing the access plate gave me enough room on my 73. Had to support the gearbox to avoid trapping the speedo cable, really needs lowering and raising on a jack as it is heavy.
Paul Hunt

Clifton, Paul,
Cheers! Sounds like there's hope after all! I was really dreading pulling and reinstalling the engine and trans less than three weeks after I had last done it. I can certainly try it as you both suggest, though. Nothing to lose by trying!

Cheers!

Rob Edwards

This thread was discussed between 14/01/2003 and 27/01/2003

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