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MG MGB Technical - Reverse Overdrive

I have had some overdrive problems, and the archives gives me loads of stuff to try, but I think I may have reversed the car whilst still having the overdrive engaged. The car would not move properly backwards and made a weird wurring noise so I stopped immediately...

Why is reversing in o/d bad and have I done unseen damage to my car? (Everything seems as before)
C Beswick

Somebody explained this in a thread recently. Something to do with components in the O/D coming out of alignment or some such when it goes backwards. If you didn't go far in reverse O/D and the unit still works normally, you have probably got away with it. Best advice to you is get that inhibitor switch fixed pronto so the O/d is disconnected as you move the gear lever across from the 3-4 plane to the 1-2 plane.

Mike
Mike Howlett

The over run gearing takes over when you try to move in reverse with the overdrive engaged, this is the wurring sound you heard. As long as you stopped imediately, no damage will have been done. As Mike already said, get the O/D inhibitor switch replaced or adjusted before driving the car again - it is too easy to forget that you are in O/D and try to back out of a parking stall with it engaged. Good luck - Dave
David DuBois

My Dad cut the tunnel up on his Marina years ago and placed an MG B overdrive gearbox in that car. He removed the OD cutout switch and made it possible to use in all gears. I moved the car backwards once in OD, and it spitted, sputtered and jerked. When I asked him, he told me to make sure it was never in OD again when in reverse. The overdrive was fine. We still have bits and pieces to that car, including the OD gearbox!

~BMC.
BMC Brian McCullough

Normally if you reverse at all with OD engaged you do the damage. If yours still works normally when going forward, OD engaged and disengaged, and is OK in reverse and on the overrun, then you have been lucky. There is a one-way clutch consisting of rollers in a tapered housing. Run this the wrong way i.e. when reversing and it wedges the rollers into the taper either jamming them both permanently together or at least distorting them. Disconnect the overdrive by unplugging the yellow/red from the gearbox loom from where it joins the main loom by the fusebox until you get the lockout switch fixed!
Paul Hunt

On the other hand, your OD inhibitor switch could be just fine and what you have is a broken sunwheel circlip or a loose or worn cone clutch lining, both these will interfere with reversing the car.
Chris at Octarine Services

The car is driving fine, and the overdrive is now working.

All I have done to fix it is change the gearbox oil and clean out the o/d sump, which had some filings stuck to the magnets. (However the problems were intermittent before and I suspect something is intermittently (sp?) wrong with the solenoid. But its fine at the moment and I'm not going to fiddle with it when its working.)

But how can I test if the inhibitor switch is working?
C Beswick

Change from OD 3rd to 2nd, note the revs, then switch the manual switch off. If the revs go up the OD was engaged in 2nd gear hence probably would be if you went into reverse.

If you want to do a more rigorous test on the switch disconnect the yellow/red in the gearbox loom from where it joins the yellow in the main loom by the fusebox and connect a test-lamp or voltmeter on its 12v scale from the yellow/red to the purple on the fusebox. By moving the gear lever back and fore across the gate from the 3/4 plane to the 1/2 and reverse planes you should see the test lamp glow or the voltmeter register 12v in the 3/4 plane but not in the 1/2 or reverse planes. Waggle it back and fore and *every time* you take it out of the 3/4 plane the test-lamp must stop glowing altogether or the voltmeter register 0v. If it tends to keep glowing even a bit or register a few volts, even only one time in 20, the switch is sticking and should be replaced.
Paul Hunt

Find a quiet piece of road. Drive gently along in third gear with O/D switched on. Change down to second gear. Leave the switch on. The gear lever going across the gate should disengage the O/D. After a minute in second, change quickly back into third and watch the rev counter and listen to the engine. You should hear and see the O/D reselect (revs drop) shortly after the gear change - there is always a slight delay. If this doesn't happen, the switch isn't working or the wiring is faulty.

Or drive gently along in second and switch the O/D on. If the inhibitor is working, nothing will happen cos the O/D isn't meant to work in second gear. If the inhibitor isn't working, you will see and hear the revs drop as the O/D comes in.

Mike
Mike Howlett


Right, I've carried out the tests described above and have decided that the switch IS working. So either it intermittently does not work, or I have no idea what was going on with my wurring in reverse gear.

Any other ideas?

Also, is changing the gearbox and overdrive oil and cleaning the filings of the sump magnet enough to make a faulty o/d work correctly?

Thanks for all your help...

Carwyn.
C Beswick

When it happened to me, I was driving in 3rdOD, floored the throttle to overtake AND dropped out of OD, the engine screamed and revved to 6000+rpm like I was in neutral...
thought nothing much of it until, 6 months later, the car ctarted whirring in reverse AND the gearbox started freewheeling in reverse...
it turned out I had broken a brass seal-Oring (but how?) in the front of the ODbox (LH type) and the OD would freewheel in reverse.
Or so said my mechanic. I had to change the whole OD unit, but still have the old one.
Which brings to mind "can I upgrade the old OD in order to have it work in 2nd gear as well?

Maurizio
Maurizio Pescatori

Saluti Maurizio,

So which part of Italy are you in and are there many MGB's or A's around?

I know there is the MG Car Club in Italy but is it located in a major city or are there many chapters.

I frequent Italy once a year on business, my customers are located in Sondrio, Rome, Verona, Palermo Sicily which I must say is the most beautiful, just love Sicily.

My parents were from Abruzzi or more specific near Pescara.

I usually rent a car when I'm there and love to drive like hell when I'm on the autostrata, no fear !

Maybe next time I'm there we can hook up and talk shop see the cars and a little Chianti !

Tanti Saluti,

Dino Zappi
Dino

Carwyn - changing the oil *can* be enough to cure OD engagement/disengagement problems, but whirring usually indicates something more sinister. You *could* connect a warning light on the dashboard to the solenoid side of the gearbox switch - probably yellow/purple - and make sure is always goes out when in 1st, 2nd or reverse even if the manual switch is still on. T'would give you peace of mind, at least.

Maurizio - not easily, and if you did you might break this OD as well. Some Triumphs had a stronger version of the OD and *did* have OD in 2nd. OTOH the factory V8 was modified after only a few were made to limit OD to 4th only as the torque reversals in 3rd were breaking the OD, and you could expect something similar in 2nd on the 4-cylinder.
Paul Hunt

I have found an easy way to tell if the OD is engaged. Put the trans in neutral and attempt to roll the car backwards. If it won't move the OD is engaged. The OD cone clutch sometimes sticks to the brake ring and requires a rap with a mallet to loosen it.
Berry

I have to say this is highly unlikely, IMHO, or OD damage would be very common, which isn't to say that Berry hasn't experienced it on his car. The only way that I could accept that the cone clutch *does* sometimes stick in the engaged position is if the very act of putting it in reverse and driving it frees it. Even then any such sticking should be obvious as failure to disengage when the manual switch is turned off.
Paul Hunt



Despite the fact that the overdrive switch appears to be operating I am still having problems.

The wurring has returned in reverse gear, so that’s twice now and although both times it has not been moved - as soon as I heard wurring I stopped.

On my last run I encountered Maurizios problem where the engines revving like hell but the drive is somehow not engaged. However, flick out of o/d, off the gas and into neutral for a bit and it seemed to work when I put it back into gear to carry on.

Despite this, the o/d works fine otherwise.

I am wondering if the best solution is to keep as I am until the o/d is bad enough to be un-drivable and then replace the whole lot. The car has at least 115k on it and I'd be surprised if its ever had a spanner on it... but I'm also poor and don’t want to spend money I don't have to!

Any other ideas?

PS- Thanks for everyone’s help on this- great as usual, esp. Steve S who sent me a really useful document on repairing o/ds.
C Beswick

My money is still on a broken sunwheel circlip - the wurring is the sungear biting into the annulus because it is unrestrained. Act now and the cost may be for no more than a new circlip - leave it and the reconditioners will not accept your unit in exchange and you will be looking at £250 + surcharge for a replacement unit.

If you are poor and can pull the engine yourself then I can sell a serviced OD unit to replace yours - e-mail me and we can discuss.
Chris at Octarine Services

Paul-I am not sure how common the problem of the clutch sticking is. For me, it occurred after the car (a TR3)had come out of storage for the winter. The first few times the OD was used it would sometimes fail to disengage. The first time it happened, I thought it might be necessary to dissassemble the OD to un-stick the clutch. Then I remembered my MGB Autobook manual mentioned that a sticking clutch might respond to a few blows to the brake ring with a hammer.It worked for me and I developed the habit of making sure the OD was disengaged before using reverse and I found that the easiest way to confirm this was placing the shifter in neutral and attempting to roll the car backwards. If the OD is engaged, the uni-directional clutch will prevent movement. The problem may be caused be weak return springs.
Berry Price

Right. Well its o/d out for a recon then.

Chris, thanks for your offer- I've looked at your great website very interesting. I'm going to pull the B off the road and buy a EuroBox for a bit and then do the work at a later date.

Thanks Again.
C Beswick

This thread was discussed between 24/11/2003 and 01/12/2003

MG MGB Technical index

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