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MG MGB Technical - Overdrive in Reverse

I have a question that was brought up during our conversation at lunch last week concerning the overdrive engaged while driving in reverse gear. We all agreed that it will damage the overdrive but none of us know why it will damage it or what it will damage. Can anyone help us out with this?

Thanks....Norm
Normd55

Yup - the tail end of the mainshaft runs in a one way sprag clutch which slips when the unit is in OD because the gearset makes the output shaft turn faster than the mainshaft. It locks in direct drive to carry the power straight from the mainshaft to the output shaft.

If the OD is engaged in reverse the sprag clutch will lock while the gearset is trying to turn the output shaft faster than the mainshaft - result either destroyed sprag clutch, ruined mainshaft or even snapped mainshaft.

DON'T DO IT!!!
Chris at Octarine Services

It's like having two gears engaged at once in the main box, if that helps with the forces that can be involved.
P Hunt

Isn't there a fail safe to prevent this happening though?...assuming everything is in working order?
Gary Lock

Well of course, the lockout switch on the gearbox. But the question related to what is damaged and why if it *was* engaged in reverse, which can happen if people bypass it for example to get OD in all forward gears.
P Hunt

You can use the reverse lights as a safety to disengage the overdrive, and still safely have over drive on all forward gears. Why? I'm thinking about those lucky supercharger owners who might want a "sport" or "economy" mode. Might be a bit of fun.
Peter Sherman

I did this(reverse light switch controlling a relay to cutoff the power to the overdrive)back in '93. I wrote an article about it for the AMBGA's magazine. It pops up on the internet from time to time. And yes, I do have a supercharged B. RAY
rjm RAY

Of course the inhibitor switch should prevent power getting to the solenoid in reverse - BUT - the switch can fail "on" (unlikely but possible) so that the solenoid is still energised if the OD switch is not "OFF"

Always check the OD switch is off before reversing = just in case!

Also it is possible for the cone clutch to jam in the OD position - in which case it matters not a jot what the electrics are up to - the OD will not disengage and you must not reverse, even under push power!

Finally the reason the OD only works in higher gears is that it is likely to be damaged by excess torque in 1st & 2nd - a supercharger is one good reason to limit it to 4th gear only - not add 1st & 2nd!
Chris at Octarine Services

I would not even consider using OD in the lower gears with a supercharger. My B is supercharged and my OD is crisp and operates perfectly with instant changes under normal driving. If I do a flat out power run and flick in the OD at high revs you can feel the change take just a little longer. To me this means a bit too much wear and tear to do it all the time. I use the OD the same as normal for normal driving but if I/m in a hurry I only use it in top gear and then usually lift the foot a fraction on change up. I think if the torque of the SC can slow the change in high gear it would be a lot worse in the lower gears. It would work but I/m sure it would shorten its life.
Denis
Denis4

Is the overdrive really that weak? I had always considered them quite robust.
My intention is to fit a supercharged MGB motor and overdrive gearbox to a 1961 MGA. If I can ever find a car that is! Despite the fact that I would need to alter the transmission tunnel a little. I had wanted to keep the drive train "MG". Now you are making me think I would do much better to fit a toyota supra gearbox. I've got one of these this in the MG V8 and it is a really nice gearbox. Smooth and precise, really adds to the driving pleasure. It is also very light and compact, so no tunnel mods. Stronger easier lighter better. The overdrive box is looking like a really bad move.
Peter Sherman

"a supercharger is one good reason to limit it to 4th gear only - not add 1st & 2nd!"

Which is why V8s were modified to remove OD from 3rd, and why 4-cylinder cars didn't have it on 2nd, which some Triumphs with a different, stronger unit did have. OD 2nd *would* be useful on an MGB as it is quite a long stretch to 3rd if you are climbing a steep hill. Personally I would *never* consider changing the MGB gearbox with it's two 'electric' ratios making six in all to a five-speed, OD 3rd is very useful for hill climbing and twisty bits, and the electric OD anticipated paddle changes, surely considered the knees of the bee, by very many years. Flicking the OD out for overtaking and flicking it back in afterwards beats fiddling about with clutches and levers any day, I wouldn't consider changing even the V8 even though it is only on 4th.
P Hunt

I don't use overdrive in first, but it comes in handy for in town driving in second, when you're maintaining a constant speed on a flat surface. I've been using this setup for well over 15 years, with a '72 LH overdrive, and I have never experienced any problems. These units are extremely robust and can put up with a lot of abuse. That being said, I don't abuse mine, but do drive the car using the power that it has. Now, any more testimonials? RAY
rjm RAY

In town on a flat surface you are using little horsepower and the OD unit will cope easily. Flat out up a hill is a different story entirely. The unit itself is strong enough - it was used in the MGC and the V8 - but - fitted with stronger springs and higher oil pressure to cope with the increased torque.
Chris at Octarine Services

Chris,

Is there an electrical lockout preventing the od from engaging in 1st and 2nd?
Steven 67GT

There is a switch that inhibits the overdrive from engaging in 1st and 2nd mounted on the top of the transmission. I bypassed this and installed my own inhibitor in the reverse light circuit. I've been running my LH overdrive like this for 20 years with no problems. But, I do not apply anywhere near full throttle in 1st and 2nd overdrive. This has made the system work for me. As Chris mentioned, the torque that is produced my multiplication in the lower gears can damage the overdrive sun and planetary gears if too much power is applied. RAY
rjm RAY

The OD MK2 gear box is plenty strong enough if used as intended, even with the extra power of a supercharger.
Its just that it was never intended to be used as an eight speed. My treating the OD with caution is in extreme usage situations and its still working great with just on 34000miles with the SC. Maybe I/m being a bit careful but with the torque of the SC engine I find using the OD just as an OD gives performance enough.
Denis
Denis4

It's more a case of the switch only allowing OD to be engaged in the appropriate gears than being a lockout as it is normally open and the linkages operate the switch to *complete* the circuit. This is 3rd and 4th on most cars, 4th only on V8s to protect the unit, ditto American spec after about Feb 77 but for completely different reasons.
PaulH Solihull

This thread was discussed between 13/07/2010 and 20/07/2010

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