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MG MGB Technical - Overdrive wont disengage?

Hi, I am running an 5 brg. engine and all syncro overdrive box in an MGA Coupe - have just had the overdrive box repaired - new sliding member installed (snap ring came loose, also overdrive face was about 70% worn, normal drive face showed signs of slipping (black lining)).
Now overdrive oil pressures are 50 and 450 psi. Overdrive engages straight away when switched but does not disengage despite oil being warm and switch being operated on off multiple times, even jerking the accelerator does not disengage. To disengage sliding member, requires the car to be put in reverse and backed up - with a slight clunk, it frees itself from the brake ring. Oil is Penrite HPR-30 (20W-60). Any thoughts on how to rectify?
Mike
Mike Ellsmore

The Workshop Manual emphasises that there should be no pressure at all with the overdrive disengaged. Whilst I can't imagine 50psi keeping it fully engaged when the operate pressure needs to be in the order of 450psi, it's not helping. That could be caused by problems in the relief valve or passageways.

The Manual also says that with failure to disengage being caused by a sticking clutch it can be caused by insufficient bedding-in. Reversing to release it is not good, that requires that the one-way clutch be forced in the 'wrong' i.e. locked direction with as much force as is required to release the sliding member from the brake ring, which could damage the one-way clutch. You could try repeated engagement under maximum acceleration, although that leaves you with the problem of disengaging it each time.

Other than that it could be weak return springs in the actuating cylinders, in which case return to rebuilder is called for.
PaulH Solihull

Beyond what Paul has stated, your operating pressure might be just high enough to force the two friction surfaces together so that they are becoming one. Try draining the oil and refill the transmission/OD with ATF. It's a thinner fluid and may allow the the two friction surfaces to disengage from each other. This seems to have worked for a lot of people and some have even left the ATF in the transmission/OD permanently with no ill effects. RAY
rjm RAY

Thanks guys, I am going to take it for a spirit run this weekend and a motorkhana, if this doesn't work I plan to change oil to ATF. I will report back what happens.
Mike
Mike Ellsmore

One cause of retention of pressure and failure to disengage can be rotation of the centre bush in the housing of the earlier LH units. This is the bush through which the mainshaft fits and via which oil is fed into the mainshaft. It also provides the thrust face for the sun gear. Normally it is secured from rotating by a roll pin. If the bush rotates, the oil feed into the mainshaft is cut off, and as this is also the bleed off from the solenoid and the centre of the relief valve, some pressure is retained in the hydraulic system as a result and the overdrive stays engaged.

I found that Penrite 20W-60 is much thicker than other 20W oils, caused me some problems in both engine and gearbox. In cold weather (which you guys in Oz probably don't get) it was like treacle in viscosity, and caused a major pressure drop across the oil cooler on a cold frosty morning. And the need to replace the engine bearings.
Paul Walbran

Paul,
I cannot see an roll pin in the Moss LH overdrive parts list - what is the Moss P/N for this pin and bush?
Mike
Mike Ellsmore

Mike
I think the bush and housing came as an assembly, as my original BL microfiche doesn't show either bush or pin either.

The overdrive with the roll pin was originally most simply identified by the oil filler position on the box. Late had the oil filler on the side, early had it on top with a dipstick. With the passage of time there is potential for mix'n'match exhange units of course

To clarify, this is one cause of the symptom you are experienceing but not the only one - I mentioned it because it had not been mentioned earlier in this thread. But is sure took a lot of tracking down when Garth Bagnall first found it over here in the early '80's.
Paul Walbran

Mike-
I have to agree with Paul that 20W-60 oil is too thick for use in an Overdrive unit and that it could be at least a part of your problem. The Overdrive is operated by an electric solenoid that is controlled by a switch, usually mounted on the steering column or on the dashboard. An Overdrive inhibitor switch is invariably fitted within the electrical circuit in order to prevent the engagement of Overdrive in reverse and the two lowest or all of the lower speed gears. If you engage reverse gear with the Overdrive unit engaged, then you will likely do damage to the Overdrive unit. If after doing so your Overdrive unit still works normally when going forward with the Overdrive unit both engaged and disengaged, and is OK in reverse and on the overrun, then you have been very lucky indeed. The Overdrive unit has a one-way clutch consisting of rollers in a tapered housing. If you run this the wrong way, i.e., when in reverse gear, then the rollers will be wedged into the taper, either jamming them both together permanently or, at the least, distorting them. Disconnect the Overdrive unit by unplugging the yellow / red wire from the transmission loom from where it joins the main loom by the fuse box until you get the Overdrive inhibitor switch fixed.
Stephen Strange

Mike-
I tried to do an edit of my previous posting, but the time limit expired!

I was trying to include that your pressure reading should be 400 PSI to 420 PSI, not 450 PSI, which could be caused by the thicker 20W-60 oil.
Stephen Strange

I've never had a problem with 20W/50 in the roadster gearbox, or 90wt gear oil in the V8 gearbox, and the latter at least was kept outside and regularly used in temps below freezing. The factory specified 20W/50 as being suitable down to -10C or 15F. It may well be that using a lighter oil, or ATF, appears to solve the problem, but that only means it is a work-around for some problem inside the box. Nevertheless, if it *does* work, then it saves removal and dismantling looking for the real problem, which may be very difficult to spot.

"Disconnect the Overdrive unit by unplugging the yellow / red wire from the transmission loom from where it joins the main loom by the fuse box until you get the Overdrive inhibitor switch fixed."

This is not an electrical problem, but a mechanical one, from Mike's description of it releasing itself with a clunk when forcing it in reverse. This *is* jamming the one-way clutch to free the sliding member, and so is *not* to be recommended. If it were electrical the OD would always be disengaged with the car stopped, and only engage again when going fast enough in any gear, forwards or reverse, and only then if the manual switch were left on with the inhibitor switch faulty, or both switches faulty.

I have the Laycock manual for the LH unit but I can't see any reference to a roll-pin for the sun-wheel thrust and oil transfer bush in that.

PaulH Solihull

Agreed, 400 to 420psi operated. Given 50 and 450 psi either the gauge is over-reading or the passage for the excess from the pressure relief valve is partially obstructed.
PaulH Solihull

Paul-
I agree, that oil passage for the excess from the pressure relief valve needs to be carefully inspected, and the freedom of action of the pressure relief valve checked as well!
Stephen Strange

Paul-
I'm afraid that there is an electrical problem involving the inhibitor switch circuit. He shouldn't be able to engage Overdrive in reverse gear! He needs to deactivate the Overdrive until the problem is resolved, otherwise he may ruin his freshly-rebuilt Overdrive unit.
Stephen Strange

Maybe I misunderstand the original post but it sounds like the O/D is electrically switched on in forward gear, engages, then sticks in the engaged state and can only be unstuck by reversing. So it's not electrically engaged in reverse but rather mechanically bound. I agree though that it should not be used until the problem is resolved.
Rob Edwards

Rob-
I can see how the poor wording of my posting could be misinterpreted. I agree that his being engaged in reverse gear is the consequence of a mechanical (or hydraulic, if you want to be more specific) problem.

Mike-
You should check to see if the pressure relief valve assembly has been properly installed. It is located in the top left hand corner. Remove both the ½” BSP plug of the pressure relief valve and its sealing washer, and then withdraw the pressure relief valve assembly. In the following order, remove the filter screen, the spacer tube, the low-pressure valve assembly, and the spring of the pressure relief valve. Keep them together in that order and in their original orientation. Now, remove the plunger of the pressure relief valve. Examine both the plunger and its seat for pitting, scoring, and excessive wear, and then replace any worn or damaged parts. Examine the O-rings of the pressure relief valve body for signs of deterioration and replace them as necessary (Moss Motors Part # 290-930 and 290-925). Check the spring of the pressure relief valve for signs of collapse or weakening. Its free length (uncompressed length) should be 3 cm. If the spring is fouled by dirt, or has become weak, the pump will not generate the 400 PSI of pressure that is necessary in order to operate the Overdrive unit. Clean the filter of the pressure relief valve. Reassembly is the reverse of the above order. Verify that the pressure relief valve is installed in its correct order and that its parts are in their correct orientations (the factory parts list provides an excellent illustration of this). I once put a piece in upside down and it took me hours to figure out why the Overdrive unit would refuse to engage.
Stephen Strange

When I ran the Penrite 20W-60 in mine, there was no problem with the operation of the overdrive. (But first gear did seize to the mainshaft ... )

IIRC the V8 and MGC have a higher relief valve setting tha the 4 cylinder version of the LH.
Paul Walbran

Paul-
You're correct. The MGC Overdrive unit runs an internal pressure of 510 PSI to 530 PSI. I know because I've got an MGC.
Stephen Strange

Quite so Rob, explained better than I did.

Incidentally the LH OD *did* have a roll-pin, but it was a modification, which is probably why it doesn't appear in some reference material. I've just seen both types dismantled at Stoneleigh, and Sheffield Overdrives told me that originally the bush was glued into the casing but could work loose and rotate. The roll pin is inserted along the length of the bush such that half is in a cutout in the casting and half in a cut-out in the outer circumference of the bush. With the sun-wheel against the bush I can't see how this roll-pin would come loose.
PaulH Solihull

On further thought if you can park it overnight with it stuck in engagement, and it's still stuck next morning, then it's nothing to do with excess hydraulic pressure where it shouldn't be as that would have dissipated hours before. That leaves mechanical stiction.
PaulH Solihull

Paul
You are quite right, memory getting scrambled obviously. The roll pin is the fix to stop the bush rotating. The sign my kids put up over the dinner table ("decreased ability to think and remember") says it all ...
Paul Walbran

This thread was discussed between 14/02/2013 and 17/02/2013

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