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MG MGB Technical - Overdrive operation question

I don't have overdrive on my current 73B and didn't have it on the 67B I had many many years ago either. As such I have no experience using it and am curious as to how it works.

When you make the change into overdrive with the stalk switch it makes the change on the overdrive unit on the transmission. That's pretty clear and simple.

What I've read though is that it only works in 3rd and 4th (and perhaps only in 4th on some cars?). So, what happens if you're driving down the highway at 60 or so in 4th with overdrive engaged, and then get off the highway. As you approach the end of the exit ramp you're down shifting to lower gears. If you don't take the car out of overdrive does it go into "normal" second on it's own? Do you cause damage to the overdrive unit/transmission if you forget to take it out of overdrive before down shifting to a lower gear and/or driving in a lower gear with it engaged?

Again....just want to know how it all works.

Thanks,
BH
BH Davis

Hi BH. There is an isolator switch oc the gearshift linkage which only allows overdrive in 3rd & 4th. this is because the torque in 1st & 2nd gears is too great for the o/d unit to cope with for any length of time. The switch arrangement also prevents o/d engagment in reverse gear.(sudden death for o/d unit). When down shifting into second the o/d is automatically disengaged and when upshifting back into 3rd will re engage unless the stalk switch is used. Jim
j soutar

Watch your RPM if you downshift from third to second in overdrive as you will actually be shifting down two gear ratios.
J Heisenfeldt

Your question has already been answered above but I'll add that not only will shifting from 3-OD into 2nd take you by surprise, but forgetting to disengage before accelerating will startle you pretty good. You engage first, then second, all is well, and then you go into third gear. For a split second everything is fine, but then then OD engages again. The RPMs drop and the car lurches, and until you realize what's happening, you think the engine is dying. It's happened to me on many occasions and I still never learn! :)

Don't worry, you can't damage the OD by shifting through the gears.
Steve Simmons

Thanks to all of you. I appreciate the lesson.

BH
BH Davis

I had a similar question. Is it better to disengage the OD and let it shift back to regular 4th when coming to a stop, or downshift, stop and then shift out of OD?
BEC Cunha

Bruce - "Is it better to disengage the OD and let it shift back to regular 4th when coming to a stop, or downshift, stop and then shift out of OD?" - Yes. Either way will work fine with no damage to the transmission or O/D. Cheers - Dave
David DuBois

"you will actually be shifting down two gear ratios"

More like one and a half, as OD 3rd is somewhere between straight 3rd and straight 4th, and OD 4th is like a '4th and a half'.

The only problem with not turning the manual switch off either before you change out of an OD gear, or afterwards, is that when you go up through the gears again when you select an OD gear OD will re-engage on its own, usually inappropriately i.e. at lower revs than you would choose to, and cause a thump in the transmission as it does so. So you then think 'bugger' and switch it out, only to get another thump as it does so. The thumps don't cause damage, it's just annoying. After some fifteen years this was still happening to me (Ok, I was doing it to myself) so I constructed an additional lockout circuit so that once you have come out of an OD gear, OD couldn't re-engage again even in an OD gear until you had reset it by turning the manual switch off and on again. If interested see http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'Overdrive Sequencer Relay'.
Paul Hunt 2

Paul,

That's pretty slick! It's just like a magnetic contactor on a machine starting circuit. The power keeps the electromagnet in the contactor closed until you hit the stop button. As soon as the stop button opens the circuit you loose power to the electromagnet, and hence power is cut off to the motor.

If you are unaware of industrial machinery, this design was come up with as a way to insure machines didn't start up on their own again once power was restored after a power failure.

BH
BH Davis

About ten years ago I put an OD into my 74B and absolutely love it. I'm not sure if the OD switch is really doing its job or not. It will enter OD in 3rd or 4th and will downshift to OD3rd from OD4th. However I do stay away from alot of downshifting especially when coming to a stop in order remove many shifts and try to save the clutch. This can be where my problems are. If I drop it out of gear and coast in to stop and don't take it out of OD, then when I go to start it will begin in 1stOD! I can then shift to 2ndOD ussually and on. If I stay in 2ndOD very long it seems to drop back to non OD shortly by itself. Ussually I just drop it back myself. In ten years I have made errors, parked the car in OD and slipped it into reverse to get out of the garage. My drive is very short and downhill so once I start it rolling I drop it out of gear right away and coast thru it. I have never caused any damage to the OD box and I never recall it being in OD reverse. I honestly don't know If it operating correctly or not. Again I do love the unit. Am I just lucky or is it operating as would be expected?
Bob Ekstrand

I have got overdrive on all of my 4 cyl. B's and when one of the gearboxes lost oil and made the OD inoperable during a journey, i immediately learned what i missed.
The OD is only working on 3rd ond 4th on all of the 4 cyl. cars and is controlled by a switch incorporated into the gear selecor mechanism. The version that is switched with the steering colume mounted whiper switch behind the wheel is the best choice for quickest operation, as the dash mounted switches or the late Triumph version (1977 on) in the head of the shifter knop are very slow and uncomfortable in comparison.
It will not cause a problem at all, if it is not disengaged when shifing down to 2nd, you will immediately do it when selecting 3rd again due to the poor response caused by the different ratio with OD on during excelleration.
As long as the electrics work well and regular changes of the od gearbox oil is done, the OD is one of the largest benefits your B can accept and you will love it too and learn what you have missed all the time before.

Ralph
Ralph

Bob Ek

It sounds as though the OD interlock switches on your gearbox aren't funtioning correctly. The OD should not engage when starting off in first, second or reverse gears - the interlocks are designed to prevent this. I believe that you have been lucky not to have suffered some damage when the OD was engaged in reverse. FWIW.

Barry
BJ Quartermaine

I am one of those who just can't resist downshifting when slowing down.

My uncle used to build Indy Racing engines and as he said. Brakes are a lot less expensive than clutches and transmissions to fix, but god I love double clutching and downshifting.
Bruce-C

Like Barry, I think the only luck Bob has experienced is not wrecking his OD - yet! If the switch is sticking closed in 1st and 2nd but not in reverse, then it can surely only be time before it sticks in reverse as well to wreck the OD. OTOH maybe a PO fiddled with the switch operation so it *is* closed in all forward gears, but not in reverse. Whilst that is less likely to cause OD failure than a sticking switch, the reason why the factory limited it to 3rd and 4th is that in 1st and 2nd the torque reversals can be too much for the unit which will also damage it. Other BL cars with a stronger OD got in 2nd as well, whereas conversely the V8 with the more powerful engine had it changed to 4th only (the North American change to 4th only on later cars was for completely different reasons).

It also sounds like Bob has never used the manual switch, in which case he is losing performance if he accelerates with OD engaged in every gear.

UK motorway Police (when we still had them) never used to brake but changed down to reduce speed, as even when travelling at less than the speed limit so many other drivers were paranoid about overtaking a Police car they they slammed on their brakes as well, causing shunts.
Paul Hunt 2

This thread was discussed between 07/11/2007 and 11/11/2007

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