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MG MGB Technical - LH overdrive rebuild

Hello all - I replaced a number of components in the overdrive on my 76 mgb including the unidirectional clutch. When I try to engage the overdrive, the car labors and no rpm change.

Question - how would the car act if I put the overdrive clutch in backwards? Car works great except for the overdrive.

Thanks for your input.
MRW Welty

I think it would jam up - if it's even possible, much like it is supposed to if it is engaged when reversing.

But what do you mean by 'labouring' - does this suddenly happen and only happen when you try to engage OD?

If there were any additional mechanical load being applied when you engaged OD I'd expect the rpm to drop along with the speed of the car.

Have you reassembled the relief valve correctly? It should be as per the attached. The plunger can be fitted either way round and the wrong way stops OD engaging, but there would be no other effects on the running of the engine i.e. you would just carry on at the same speed and revs as before as if it had not been engaged.

There is a faint possibility that there is a short between the manual switch and the solenoid, which could be dragging down ignition voltage, but that would result in other symptoms like smoking wiring, and would need other defects in the wiring to be present.


paulh4

Thanks Paul - only when I shift into overdrive then it bogs down. I was thinking might have resistance in the sliding member but acts like I’m braking the engine. Stops when overdrive turned off. When I installed the one way clutch it could have gone in either way, and I installed via the diagram. I assumed if power train works normal without OD engaged the clutch was in correct but perhaps that clutch only functions when in OD. Mark
MRW Welty

That's right. In normal drive the clutch is locked and that is what gives the drive to the rear wheels, even though you also have a sliding clutch that is also engaged and locking the input to the output. That provides the drive in reverse, and engine braking on the overrun, which is when the one-way clutch would otherwise be free-wheeling.

With OD engaged the sliding clutch locks to the fixed brake ring and the epicyclic gears come into play, which drives the output shaft faster than the input shaft. In this condition the one-way clutch is again freewheeling, and the drive and engine braking comes solely from the sliding clutch being locked to the casing.

As it bogs down when you engage OD the sliding member must be doing something. If the one-way clutch were the wrong way round with OD engaged it would be driven into the locked position, and to get any forward movement the sliding clutch must be slipping against the fixed brake ring. This would jam it up, and the friction from the sliding clutch on the brake ring is normally what would destroy it.

The only other thing I can think of is that the epicylic gearing is binding badly, and that is slowing you down. That would change the revs relative to the road speed, but unless you can force the car to do the same road speed both engaged and disengaged it wouldn't be easy to see if the revs were altering.

I'd recommend a call to Sheffield Overdrive Repair services here in the UK if there isn't anyone in the US. There is another place at Coventry somewhere I understand, but I don't know the name.
paulh4

Is it Overdrive Spares at Rugby?

http://www.odspares.com/
Dave O'Neill 2

Thanks Paul - I will take the relief valve apart to confirm it’s correct. One more thing - if the one way clutch were in reversed, could the car operate with the clutch in free mode and the friction cone only driving the car? And when OD was engaged the locked clutch would hold and the cone would slip against the fixed member causing the bogging down? I hope that’s not an option. Mark
MRW Welty

You almost certainly have the one way (sprag) clutch the wrong way round and it will be bright blue by now....
Chris at Octarine Services

Could be OD Spares, but I'm sure the people that did my V8 gearbox said Coventry.

Yes the clutch would drive the car in non-OD mode, but could well slip under high acceleration. But the bigger problem has to be damage to the tapered slots in the one-way clutch, and heat damage to the sliding clutch and the brake band.

I don't think there is any point in looking at the relief valve from what you have said.

The one-way clutch is obviously a Murphy's Law job if it will all fit together either way round.
paulh4

The sliding clutch will lock solid in OD or direct drive. That is not the problem.

With the one way (sprag) clutch if it is driven with OD engaged the wrong way the rollers just slide on the inner race but with a huge amount of drag.

With it in normal drive the sliding clutch locks the annulus to the input shaft so there is no relative rotation of the sprag clutch.

So - either drive it without using OD and you will be OK - or get the OD unit out of the car again and rectify the issue. It is possible to drop the unit off the back of the gearbox by lifting the engine and pulling it forward, but I find it is just as easy to pull the whole thing out - saves grovelling under the car to undo the OD.
Chris at Octarine Services

Makes you wonder why it is even there, then.
paulh4

Thanks everyone for your help! The saw comes out today to make the fixed crossmember a removable bolt on piece. Good news is I’ve only attempted OD a few times and less than 100 miles on the car since reassembly. I’ll let you know what I find inside.
MRW Welty

It is there to provide engine braking on the over run while OD is selected.

When driving forward in OD the input shaft is rotating faster than the annulus because of the .8 to 1 reduction of the planetary gear set, and the sprag clutch freewheels.

On the over run the annulus is driven by the road wheels and is going faster than the engine which is idling so the sprag clutch engages - without it there would be no engine braking effect.

Putting the sprag in back to front is effectively the same as engaging reverse while OD is on - a big NO NO!
That's why we have the pesky interlock switch.
Chris at Octarine Services

Hello all - Here is the latest - Picture one shows the roller clutch on the correct orientation I believe.

The second picture is the issue. When overdrive is engaged, the planetary gears have been rubbing on the sliding member and it appears to be worse in the center as it has worn a slight taper on both items. When I manually push the sliding member, it seems to go in freely and seat well against the fixed ring. Ive looked at all the diagrams, read the articles, but still can't explain why. I assume the sun gear is fixed on the shaft as it doesnt appear to move freely. Could that be an issue? I appreciate the help.





MRW Welty

Is that as you pulled it apart?

The sprag clutch should be retained in the annulus by a large circlip.

You need to remove the front housing and release the four sliding member nuts to slide the clutch off to release the sunwheel.
Chris at Octarine Services

That does make more sense, as thought earlier.
paulh4

Hello. I’ve determined that the sun wheel is not turning and fixed or somewhat fixed to the shaft. I am assuming this should turn freely which it does not.
MRW Welty

The sun wheel is splined to the shaft - it won't turn without the shaft turning.

you need to take the whole unit off the gearbox to sort it out
Chris at Octarine Services

Hi guys - Paul you were correct on the binding and the sprag clutch was in correct. Chris, the clutch was held in with the washer and clip. I have attempted to remove the front half of the overdrive, and it’s loose in that I can move it slightly, but it cannot be pulled away from the transmission. The sun gear/wheel does not move on the shaft in anyway. There is a locating ring in front of it and who knows how to get that off. The sun wheel should free wheel or the bottom gears of the planetary could not lock when in OD while the shaft turns inside of it to power the larger gears. If the sun wheel is seized on the shaft would that prevent removal of the front assembly? The oil galleys supplying the sun wheel could be plugged as there were damaged parts inside when I bought the car after 35 years in storage.

Any thoughts are appreciated as the manuals don’t address this

Thanks

MRW Welty

Thanks Chris - the metal ring is shown in my second picture it is located half under the sun wheel gear and a little to the front in a slight depression in the main shaft and is called a spring ring, positioner, etc depending on the overdrive picture diagram you look at.
The mgb shop manual stated the sun wheel was freewheeling on the shaft which is where I got that. I’ll try to pull the gear off some how.
MRW Welty

The sunwheel is definitely splined into the sliding member - but there is nothing holding it to the mainshaft - it won't rotate or pull out without removing the front casing.

My guess is that it is seized in some way to the mainshaft. If you can get a puller on the case you should be able to pull it off.
Chris at Octarine Services

The whole overdrive should just pull off with all the bolts round the joint undone, it should just be a splined shaft between gearbox and OD. It's putting it back that is tricky, as the shaft has to fit through two sets of splines, and if you turn the output shaft in one particular direction the two sets get out of line.

I'm wondering if the pump shaft - or something - has got caught behind the cam on the gearbox shaft somehow, although in theory that shouldn't be possible for the pump shaft at least. As OD did seem to be engaging when driving the pump must have been reciprocating, i.e. on the cam.

Or perhaps the two sets of splines in the OD are being forced in opposite directions and gripping the shaft. Careful levering?
paulh4

Simultaneous - almost replies.
paulh4

Looking at the second pic again, I think the "ring" is actually the sunwheel bush that has come loose and jammed up.
Chris at Octarine Services

Thanks Paul and Chris - Here is the answer to the laboring cause and no OD. With heating, WD40, and a large pipewrench, I was able to get the Sunwheel to move a touch, and then after 30 minutes of lube and turning, I was finally able to get the Sunwheel and whole assembly off the main shaft. the Sunwheel is not fixed to the mainshaft in any way and is meant to freewheel. When OD is engaged, the sliding member is pulled to the brake ring which stops the sliding member AND the Sunwheel from rotating (while the main shaft inside continues to rotate and moves the end spline on the planetary gears) while the smaller planetary triple gears are also stopped on/by the Sunwheel.

So in use, when the brake-ring was trying to stop the Sunwheel from turning (but couldn't) it caused the friction between the sliding member and the brake-ring and thus the laboring (or braking). The oil galleys in the main shaft were plugged causing the seize.

The brass thrust washer is broken, but the Sunwheel and bronze bushing are still in good shape. The clip in my photo is called a spring clip and fits in the recess in the main shaft although I'm not sure of the purpose - D type diagrams show it used but most LH diagrams do not. (its about 1/2 wide band of stainless)

A nice time to work on a frustrating project while the world is on hold. Thanks again for listening and for all your help,

Mark

MRW Welty

Well done for getting it sorted, especially with most of the work being done under the car!
paulh4

With OD engaged the planetary gears rotate around the fixed sunwheel and drive the annulus - that is where the 0.8 gear reduction comes from!
Chris at Octarine Services

This thread was discussed between 16/03/2020 and 23/03/2020

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