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MG MGB Technical - Overdrive mystery

I’ve been unable to work out the following overdrive problem in my 72 GT. When the car is warm say after 5 miles, operation is perfect but from cold it’s a different matter.

The overdrive engages perfectly but when I disengage it, the car freewheels and the engine idles if you release the accelerator. Accelerate at this point and direct drive resumes, decelerate and freewheel occurs again. Selecting overdrive at this point makes no difference and direct drive is maintained. The only way out of this is to stop the car and drive off again whereupon normal drive and overdrive is resumed only to go through the whole process again when you want to deselect overdrive. This is clearly due to hydraulic pressure not being released somewhere until the transmission warms up whereupon it is OK.

I have removed the sump changed O rings and cleaned the filter. The transmission fluid is even a “cheap” 20/50 that I wouldn’t put in my lawn mower and would rather it wasn’t in my MG but I had to go down this route to ensure that I hadn’t got a viscosity problem.

Any ideas out there ? just don’t tell me to take the overdrive out because I’m not doing it and reluctantly will live with this annoyance until something else needs to be done.
Iain MacKintosh

The planet wheels are not locking to the annalus . You are still getting direct drive Ok because this goes via a uni directional clutch at the rear of the planet and sunwheel assy. As you say there are two chances to fix it , have the unit out and overhauled or kill or cure in place with a high detegencey oil , or even a flushing oil . There are all sorts of ways this could go wrong . If you do this clearly you will need to change the filter again afterwards .
S Best

Undo the plug next to the OD sump cover and drop out the relief valve - make sure it is clean and the ball valve on top is operating corectly - you should be able to move the ball using a paperclip bit it should not rattle when shaken.

Failing that, drop out the solenoid and make sure the piston moves freely (don't lose the ball!).
Chris at Octarine Services

Ian, My 73 MGB is doing almost the same thing. When I first start out, when the car is cold, first gear is fine, second gear is fine, but shift into third and step on the gas the engine rev's but the car does not go anywhere. My car is not as bad as yours, as if I let up on the gas and let the rpm's drop down, the transmission will re-engage and then everything is fine, until the car is cold again. This is with a new O/D transmission, with less then 1000 miles on it. Nest week, the shop is getting ready to pull the transmission back out and replace it. They said that the cause is the O/D clutch and there is nothing that can be done, short of rebuilding and replacing it. So, since you do not want to pull the transmission, it sounds like you will have to lve with it. We have been fighting mine and trying different solutions (i.e. oils, etc.) for over three months. Good luck.
Robert Browning

Try draining the 20W50 and filling with automatic transmission fluid. Dexron for GM products works fine. Sometimes a bit of dirt will block something critical and the high detergent ATF flushes it through after driving 20 or 30 miles. Worked twice for me, one with similar to your complaint and another that wouldn't engage at all.
Anthony Henderson

Thanks all, it does seem that there is dirt somewhere in the system so I'll remove the relief valve and solenoid again and check its OK. It seems that dirt is preventing oil returning from the cylinders so flushing may be a good idea. I'll try the ATF but unless that sorts it out I think I'll live with it....for the moment at least.

Iain
Iain MacKintosh

It would be worth checking the solenoid throw , there is a hole in the lever , and in the gearbox case. You should be able to pass a drill bit through both when its in the right position .Realy need the car on a ramp , or at least realy high on axle stands to do this . It's all in the workshop manual .I seem to recall that you can prevent the pressure falling if it thows too far.
S Best

Whilst perfectly valid this relates to the earlier D-type OD, a 72 should have the later LH-type.
Paul Hunt

Yeah, It is the LH overdrive that is fitted. I'll have the relief valve and filter out and check it again. Perhaps also I'll run ATF to flush the unit and clean the filters again. If this doesn't fix it I'll live with it until something else goes wrong. Pity I didn't pull the overdrive apart when I rebuilt the box a few years back but then it was OK. Oh well there's no fool like an old fool!!

Iain MacKIntosh

I just got back from 300 miles on my newly installed OD. It's freewheeling in 2nd and 1st. Also, it sometimes will drive in reverse, sometimes it won't. If the clutch on the direct drive side of the anulus is gone, that would explain it...I guess. But, get this, The speedometer is reading less and less and less. What's up with that? Maybe Palo Alto Speedometer didn't do a good job recalibrating my speedo ($175 ouch), but it's kind of strange that they would both start happening at the same time.
~Dan
Dan Neu

According to the Bentley manual "Slip in reverse or free wheel on over-run" is caused by:
-Worn or glazed clutch lining.
-Broken circlip on sun wheel

So much for that OD. Anybody know where I can get parts or get it rebuilt. I'm in Palo Alto, CA

I still have no clue about the speedometer. I'm taking it back to PA Speedometer.
~Dan
Dan Neu

Call Jim Griffin & Son, British Auto Repair, San Bruno, CA 650-589-7619. Jim has retired. his son carries on. Not cheap, but good.
Dan Robinson

Is it the oil???

I used Castrol GTX 20/50 which I assumed was similar to the recomended Castrol XL. Any thoughts?
~Dan
Dan Neu

GTX specifically or various makes of 20W/50 for temperate climates, or thinner multi grades for colder climates, *is* recommended for the gearbox. Castrol XL isn't mentioned, at least in the 1976 edition of the Leyland Workshop Manual.
Paul Hunt

I'm getting occasional slipping in reverse as well, but normal operation otherwise. I would imagine that the broken circlip would be an all-or-nothing proposition? Would intermittant slipping point more to the clutch lining?

Cheers!
Rob
Rob Edwards

If it is only in reverse it sounds more like a problem with the one-way clutch.
Paul Hunt

Hmm. That doesn't sound cheap.....

Cheers!
Rob
Rob Edwards

The way they lump them together, slip in reverse or free wheel on over run, makes it sound like it could be either or (clutch ring, circlip). They never mention the one-way clutch. I plan on replacing then both. It doesn't look like that difficult of a job. The circlip and one-way clutch are available from Moss, but the clutch ring (sliding member) isn't. Does anyone know where I can get one.
~Dan
dan neu

Try ORS or me!
Chris at Octarine Services

Hey...I experienced this odd sensation, too, a few
weeks ago.

With the OD "off", traveling about 25-30 mph, upon
letting up off the throttle to come to a stop - it felt
like the engine was freewheeling as I didn't feel
the normal sensation of deceleration from engine braking. It felt as though I had slapped the shifter
into neutral at the same time when let up off the
throttle.

Everything would become normal again when accelerating away from a full stop. I even tried
blipping the throttle a few times while in gear,
but it all seemed OK as the car would buck
forward and then pull back, as normal.

It would freewheel only did when hauling the car
down to complete a stop. Warm or cold.

Anyway, it all seems to have gone away on it's own
(for the moment...knock on wood) but I don't like
mysteries such as this and I can't help but think
that this will come back to bite me later down the
road (for me, it'll be a dark, deserted, hilly road).

A complete trans oil change and filter cleaning
and check of the soleniod seems to be in order.

Out of curiousity - I was wondering if the OD can
be unbolted and removed alone - or does the whole kit-&-kaboodle (engine & trans) have to
be hauled out? Oh joy.

After over 30 years of being around these cars
- it amazes me they are still full of big surprises.
Daniel Wong

Oh heck, I seem to have opened a can of worms here. I decided to live with my faults (and the car's) until something more serious goes wrong. Reason for this is that the whole darned lot has to be dragged out as there is no way the overdrive can be taken out without removing the engine and transmission.

Unfortunately I haven't flushed the unit as I am currently rebuilding the brake servo in any spare time I can find but will report if this works.
Iain MacKintosh

This thread was discussed between 16/02/2004 and 23/02/2004

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