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MG MGB Technical - Overdrive slips in reverse

My son bought a late 1977 MGB GT yesterday, Saturday, and today we changed the engine and gearbox/overdrive oils.

The overdrive works perfectly in 3rd and 4th as it should, but today we tried to reverse the car up our drive, which is fairly steep, for the first time and it started to go up and then stopped as if something is slipping, with a whirring sound from the gearbox area. It will make the same sound if you try to reverse against the handbrake.

There is no sign of clutch slip at all going forwards, the overdrive engages and disengages perfectly at any speed or throttle opening.

Is there any electrical interconnection with the overdrive and the reverse light switch? The reverse lights do not work, but we have not checked either the switch or the bulbs yet.

Richard
Richard Wale

Hello Richard,

It is not made to work in reverse..

If it tried to engage, this means bad connections in OD wiring.

Cheers,


Jean G.
Jean Guy Catford

A shorted line to the O/D solenoid or a lockout switch that is stuck shut, keeping the O/D engaged all the time. This situation needs to be corrected before driving the car in reverse - the O/D will be damaged if it is continued to be riven in reverse. Cheers - Dave
David DuBois

I don't think it is engaging in reverse, but will check the wiring and operation later today. On a level(ish) surface it reverses quite normally, the problem seems to occur when the engine has to work a bit.

Going out in it today with the overdrive smoothly working, brought back memories of a TR2 that I had many years ago with o/d on 2nd, 3rd and 4th - was superb!!

Richard

Richard Wale

What oil did you put in the gearbox/OD?
Dave O'Neill 2

As the other posters say, the car should never be driven in reverse with the OD engaged. If the lockout switch is working properly, the OD is electrically isolated with the gear lever in reverse. But it doesn't sound like you're trying to reverse with the OD engaged.

Sounds like the cone clutch in the OD is perishing. You'll probably find that it slips in forward on overrun, as well, especially as the gearbox gets hot. It'll only get worse. Fix is to rebuild the OD. Sorry for the bad news!
Rob Edwards

Dave,

Comma mineral 20w/50.

Rob,

Thanks for the tip about the overrun.

I am going to disconnect the o/d solenoid connection completely and try it then and also will check on the overrun when I go out later today.

Richard
Richard Wale

Decided to try the overrun first. It drops revs on overrun in direct 3rd and 4th, in o/drive it is fine. This is on a cold engine/box, no need to warm it up to find out!

As I've never had an overdrive problem before, TR, Rapier, Healey 3000 and Humber Sceptre in the 60/70's, I had no idea about what to check when we tested the car before buying, other than to try it in 3rd and 4th with and without power.

I know, should have asked here first!!

Now, PLEASE tell me we can get the overdrive off the back of the box without taking the engine out? We have a overhead chain hoist, large and small trolley jacks and a good, deep and long inspection pit to work with. I know the manual says engine and gearbox out, but is there a way around this?

Finally where is the best place for overdrive rebuild/exchange?

Richard
Richard Wale

Did you disconnect the power at the OD solenoid yet? The only way to get at the OD is to remove the engine and transmission. Chris, at Octarine Services, is the man to see. He often frequents this board. RAY
rjm RAY

Ray,

No, I checked that it only works in 3rd and 4th, which it does and then the 'overrun' test, which it failed!

I have just come across this series of 3 videos showing what can be done with the o/d still in place to fix slipping when trying to reverse and slipping on the overrun - have to try this before buying a new unit I think?

http://www.youtube.com/watch?v=XPSgUeHAy8E&NR=1&feature=fvwp

Richard
Richard Wale

When mine started playing up, I cleaned the screen and valve. I hadn't seen Twist's video so I didn't know to look for the sorts of things that he found in the valve in the video, but I didn't find anything that struck me as obviously wrong. In my case it made no difference on the OD performance. It's certainly worth a try, though -- if it works, it cheaper & easier!
Rob Edwards

There is an article in Classics Monthly this month which deals exactly with your problem i.e. an MGB overdrive slipping in reverse and quite clearly shows the overhaul procedure although whether you would want to tackle it yourself is another matter.
Iain MacKintosh

John Twist's videos are, indeed, a godsend. I would follow his instructions before taking the car in to a service center. The LH overdrive units are extremely robust and their faults are almost always repairable without having to remove them from the vehicle. Their hydraulic system is well designed, but foreign matter will cause them to fail. RAY
rjm RAY

<<The LH overdrive units are extremely robust and their faults are almost always repairable without having to remove them from the vehicle>>

RAY, I hope you're right; we will find out in a few hours time!! We have a new set of o-rings and are going to use the guidance from the Twist videos later today.

Richard
Richard Wale

Richard, May the OD gods shine down upon you. I've been running the same 4 synchro trans/od for over 25 years, even having bypassed the factory cut out switch for 1st and 2nd gear, and have only had a single problem. A solenoid failure. It was only 38 years old, but the factory warranty wouldn't cover it! RAY
rjm RAY

The OD gods had other ideas unfortunately. We dutifully removed all the hydraulic parts as per the Twist videos, cleaned them and started putting them back, when we noticed the end of a rampant spring ring lurking around inside the overdrive.

With some wriggling we managed to extract it, and saw that the ring was somewhat misshapen, but complete. Not sure which ring it is, but will re-arrange its shape to roughly circular to give an idea about the size.

Completed putting it back together and it runs as it did before - slips in reverse under load, but have not risked trying the overdrive.

Have decided to get an exchange OD unit as we don't know what the real problem is.
Richard Wale

Richard,

Maybe I'm dreaming, but I thought I remembered someone saying that the OD could be removed with the trans still in the car. RAY should know, and he said you couldn't, but has someone else heard that you could?

Charley
C R Huff

Manor garage at Wantage are pretty good with o/ds. (Richard is the man) They swapped out my solenoid when one of the bolts was frozen in, the hardest one to reach of course. As noted above only 40 years use and no goodwill payment. I could have written but it mighr come back "gone away"
Stan Best

Exchange overdrive ordered from Beehive - nice little job for us in a couple of weekend's time. Oh, and some work on the NS sill and maybe some work on the engine, whilst it is out, and .......!

Definitely not a back to bare metal restoration this time, but then I said that when I started the Minor bodywork in 1992, and 6 years later it was just that, plus a 1380 midget engine, Toyota 5-spd box, Metro 4-pots calipers etc. etc.

Richard

PS Anyone going to Silverstone on Monday with MG's on Track? We will be there - ssh(in the Minor).
Richard Wale

Drive still goes through the OD regardless of whether OD is engaged or not. Forward motion with OD disengaged the drive is through the one-way clutch, but for everything else i.e. forward motion with OD engaged, and in reverse, drive is through the clutch plates. In reverse the chances of slip using the friction of the plates alone is reduced as the design is such that the plates are forced more tightly together. It's one of the reasons why OD when going forward is only available in the upper gears, where torque at the OD is reduced, which would otherwise cause the clutch plates to slip in lower gears.

If the OD solenoid *does* happen to be engaged when reversing, firstly you have to be going fast enough for sufficient oil pressure to engage the OD ratio, and when it does that effectively engages two ratios at once which locks up the gearbox and destroys the one-way clutch.

But you should always confirm the operation of the gearbox switch (which only enables OD in the upper forward gears) when first driving a new to you MGB, as some people have been known to bypass the switch in order to get OD in lower gears.
PaulH Solihull

Paul, Having bypassed the factory lockout switch, many years ago, I occasionally leave the OD switch in the on position when taking off in 1st gear. The OD engages at 15 MPH. You would have to be going pretty fast, in reverse, to obtain the pressure needed, but I'm sure that it's possible. I wired a cutout relay, into the back up light circuit, as insurance against this happening. RAY
rjm RAY

This thread was discussed between 13/03/2011 and 18/03/2011

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