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MG MGB Technical - overdrive problem

I have a 1973 MGBGT that has developed an Overdrive problem. It only appears after the car is fully warmed up approx. 10 miles of driving. The unit seems to work fine and smooth but after driving for a while and downshifting with the unit off, it seems as if the unit is engaged and I don't get the downshift feel. But when it is doing this and I do turn the overdrive on and off it seems to be engaging and disengaging just fine. Oil level is correct. I don't know if I'm dealing with a dirty filter or sticking pressure relief or what.
Ben Stone

Explain what you mean by 'don't get the downshift feel', which gears?

If you leave the manual switch on and change between 3rd and 4th in either direction then as long as you don't move the lever out of the 3/4 plane OD will remain engaged. Only if you move the lever towards the 1/2 plane will it disengage.

As a 73 the revs for a given road speed should always go up when changing down, even if you change down from straight 4th to OD 3rd although not by as much as changing down from straight 4th to straight 3rd. Definitely if going down from either straight 3rd or OD 3rd to 2nd, very noticeable in the second case.

In 3rd and 4th switch OD in and out again at a steady road speed. There should be a definite change in rpm when going in and out in each gear. Then with OD out in 3rd change down to 2nd, and try switching OD in - there should NOT be a change in rpm then. If there is then there is a problem with the gearbox inhibitor switch. How long has it been doing it? If as long as you have had it then maybe a PO bypassed the switch to get OD in 2nd.
paulh4

I notice this when downshifting to 3rd. and 2nd gear. I have had this car for 20 years and have a 73 roadster with overdrive for 30 which I added the Overdrive to. So very familiar with all aspects of how they should shift and feel. This problem just seems to be something transitional between overdrive and gearbox with both working properly when cold. If that makes any sense.
Ben Stone

Not really. If the revs change and don't change as they should then the OD would appear to be OK. When under power non-OD is through the locked one-way clutch, on the overrun or in OD it's though one of the two clutches in the OD but the symptoms you describe don't sound like those slipping, nor the main clutch slipping.
paulh4

Although Ben doesn't mention symptoms of slipping, the circumstances in which this occurs (after a prolonged drive, in direct drive only, and on high overrun load (downshifting)) are those in which overdrive clutch slip occurs. As it takes 10 miles of driving to get to that stage, and typically the worse this type of slip problem is the less distance needed to trigger it, I am wondering if it is just the very early stages if a slip fault. This would tie up with Ben knowing the car well and so more readily detecting small but unusual behaviour.

So, assuming it is, to follow on from Paul's comments above, the drive connection in overrun is via the inside linings of the friction clutch in the overdrive (referred to in the manual as sliding member).
It isn't usually wear as such, the linings can become glazed and reduce their grip. The manual also lists broken sungear circlip as a potential cause.

It is a relatively common fault (relative to overdrive faults that is), the glazing presumably occurs as a result of many years of momentary slippage when changinng out of overdrive under high overrun loads.

Diagnostic tests:
1. When the symptom appears, check to see if there is a momentary difference from the usual tacho to speedo reading relationship, with engine speed being a bit lower than usual. If it is early stages for this fault it may only be brief, so I suggest you pick a set engine speed to test at and when you start the test drive (gearbox cold) downshift enough times so you know exactly what the engine speed should go to. (or use the gear ratio and maths to calculate it)

2. Once the symptom has appeared going forwards, stop the car, engage reverse and see if it is a bit softer taking up when letting the main (engine) clutch out. (drive in reverse is the same loading condition as overrun forwards).

Cure: engine out and ovetdrive strip. If the linings are glazed a simple scuff can be ebough but for belts and braces fit a new or relined o/d clutch. Check the sungear circlip. Also fit the uprated 37H1928 engagement springs which were factory fitted to the MGC and V8 versions. (That they saw the need to uprate for the heavier engine indicates a possible awareness that the springs on the 4cyl unit might be marginal)

Paul Walbran

Thanks for the comments. I might add that this overdrive seems to engage and disengage much smoother than the one on my roadster. I think I will start with cleaning the two filters and changing oil. I don't believe it is anything electrical.
Ben Stone

Shims in the relief valve control speed and hence force of engagement, plus the usual differences between one example and another in stuff designed and manufactured in that era.
paulh4

This thread was discussed between 23/11/2022 and 24/11/2022

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