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MG MGB Technical - Overdrive hiccups

I have 1975 BGT with, I think, an od problem. Od works fine but when disengaging the car runs a bit weird then there is a bang and it is OK. Yesterday I did not use the od but on one occasion on the overrun downhill it seemed to snatch. I think it may be a problem with the oil relief valve or oil pump in the od or dirt in the passages which prevents the od from fully disengaging. If this is the problem I am minded to leave investigating it until the winter. If I still run the car but do not use od would this damage the od?
R E Merrall

Should have mentioned I use 20/50 engine oil in my gearbox which was Halfords own brand. Is that likely to contribute to my problems?
R E Merrall

Oil should be fine. I would first get your overdrive checked with a pressure gauge. Good MG garages should be able to do this for you at reasonable cost. Gauge can be read in cabin. 450 psi overdrive engaged, 0 psi as soon as you disengaged. If not you have a problem, most can be fixed without removing gbox. Check out John Twist University Motors Youtube videos for a how to.
Mike
Mike Ellsmore

It is possibly the solenoid plunger not releasing fully My page http://www.mgparts.co.nz/advice/technical-notes/overdrives/ describes a more advanced phase of what could be the same problem.
But equally it could be something else. As Mike says, a pressure gauge reveals all!
Paul Walbran

Check the wiring harness to the O/D especially the bullets, also the switch. many "hiccups" are caused by intermittent electrical problems.
Allan Reeling

I would also consider changing over to a SAE 30 non-detergent oil. Your 20w50 might be foaming up due to additives.
DLD

The bang is worrying. As the problem is on disengaging it's less likely to be an OD electrical problem, if it was OD re-engaging when it shouldn't it shouldn't be any different to switching it in and out manually. I've had the problem Paul talks about but all it caused was a gentle switching between OD and non-OD and the change in retardation and rpm that caused while on the overrun.

Where is the bang from? If from the exhaust, and the tach drops and jumps back up again at the same time, then it sounds like the action of switching out OD is momentarily interrupting the ignition supply in some way. You say 1975, is that chrome or rubber bumpers? In both cases an unfused white feeds the overdrive manual switch, and if the wiring has been chafed or the switch (particularly the dash switch) is faulty it could be momentarily shorting out the white and killing the ignition that way. That would need sorting quickly, as the short could stay on one day and cause a harness fire.
paulh4

I always regard recurring (minor) faults as a warning, so I would suggest action before winter! It may be more difficult to get it pressure tested reasonably locally nowadays. Before doing anything I would check the levelin the gear box: and then drain the gear box followed by removal of the o/d plate. See what comes out from that last bit of oil and how much muck there is on the gauze plate. (Buy a new gasket before you start). New oil (Halfords will be fine) can't hurt. I did the above on mine and the bits that came out showed the clutch was on the way out.

If you are not using the o/d, for it to "snatch" would imply there is power getting "past" the switch. Tho possible, switches are more prone to turn off when on as opposed to on when off!
Michael Beswick

Thanks for advice. The car is rubber bumper Paul and the bang seemed to come from the od area. I had just come off the dual carriageway and switched the od off at low speed and after a few minutes the car seemed to hesitate a few times then the noise. I thought perhaps the od had not fully disengaged and suddenly did so. I have new o rings is it worth replacing those and cleaning out the orifices?


Rod
R E Merrall

Hesitation is also an indication of ignition problems, so I'd still be looking in that area.
paulh4

This thread was discussed between 29/08/2016 and 31/08/2016

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