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MG MGB Technical - Overdrive - ooops!

Hi all - help please.

1967 3 synchro B roadster with an early overdrive unit? (according to my local MG Garage. Our overdrive has developed a bit of a problem. When the engine is cold or only just warming up performance of the OD is normal in 3rd & 4th. However once warmed up the unit will cruise in 3rd but if I open the throttle the revs increase as if the overdrive has dropped out. release the throttle and overdrive cuts back in . also in the overrun in OD 3rd the car free wheels - OD 4th is not affected at all and operates normally.

suggestions please guys.
Regards
P L Hoath

Sounds like the same thing mine does. I haven't pulled mine down yet, but apparently those are classic symptoms of a worn out or glazed cone clutch, or a broken C-clip. Either way, it's rebuild time. :-(
Rob Edwards

You should have a vacuum switch mounted in the engine bay (mounted near the clutch master cylinder on mine, round and black with a rubber pipe attached to the manifold). On my car it was set-up to drop the car out of overdrive when you stamp on the throttle, just like an auto kick-down. Someone may correct me but I think it was to stop too much torque going through the not so strong early overdrives.

It should work in 4th too. Not sure what you are describing with the free-wheel thing. No engine braking? Can you give some more detail?
Steve Postins

Yes -- no engine braking. Just like if you had depressed the clutch. Mine does it in reverse as well....
Rob Edwards

Hi all
Yes the free wheel bit is as Rob suggested and is no dynamic braking on the engine in OD 3rd. I have checked the vaccum switch and this appears to be OK - my thoughts are that it is the cone surface and this would also explain the effect in 3rd and not 4th - due to the extra torque

engine oil levels are also OK and it is 20/50 no additives.

this looks like a major rebuild coming up at the moment unless some one knows better - have a had suggestion on the other BBS that it might be the inhibitor switch and will try to suss this later today.

Any more suggestions gratefully received.

Regards Paul
P L Hoath

The inhibit switch would prevent the overdrive from operating, but it would not cause the temperature dependancy nor would it cause the free-wheeling....
Rob Edwards

Check the gearbox oil level is correct first.
Richard Evans

still haveing problems - gearbox inhibit switch functioning OK. but still slipping in OD3rd. Gearbox oil levels OK

Any more ideas before I start taking things apart.
Regards
Paul
P L Hoath

I suspect you have an oil pressure problem in the O/D (and perhaps a damaged cone). Fouling, worn pump or leaking engagement pistons in the O/D could all cause it. Blocked screen in the sump as well.
I'd give these a good check before tearing it apart. Find a way to test the line pressure.

Mike!
mike!

This thread was discussed between 31/07/2004 and 02/08/2004

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