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Recent MGB GT owner, This evening I've had a little trouble with her, hadn't started the car all day and in just a quick 5 minute journey i couldn't seem to engage the overdirve as usual in 3rd or 4th gear, anyone else noticed this failure occasionally, could it be that it still needs 'warming up' or anything as such, they're shouldbt be any lose wires or anything after just one day (was working perfectly a day ago)?!?!!?

Again much advise would be appreciated, many thanks

This is a common'ish problem. The O/D unit should not need warming up before engaging for 3rd & 4th...though it's best to warm the old girl up a little before pressing-on into 3rd and 4th O/D. As this is a new car for you, have you changed the gearbox+O/D oil? If not it's worth doing for peace of mind and good for you gearbox+O/D.

Ref' O/D not working, it's most likely to be the switch wiring from gear lever to solenoid. Check wiring under car for condition and good connection + check for connectivity wiith a volt meter. If this is not working then check/buzz out switch on gear lever. If neither of these, whilst oil's drained remove cover plate and solenoid. Disassemble solenoid from its sleeve and clean parts with WD40 as there's often sludgy oil around this area. If O/D was working before chances are it's still working.

Whilst cover/filter off take a look at the oil filter for metal swarf...hopefully it's clear. Clean filter, fit new gasket and re-fit.

I had the same problem some time ago. I replaced the solenoid, no problems since.

Be careful using a voltmeter, particularly a digital type, t could lead to false readings. The solenoid draws current and if there is a bad connection (resistance), then the voltage will drop at the solenoid and there fore not operate.

Old car wiring is best tested with a test lamp, easy enough to rig one up, and possible left in place for a while when driving.

Don't know if you have started fault finding yet, but I would suggest start with the easiest thing first, and check the overdrive switch on the dash board.

They sometimes get dirty and fail to make contact internally. Sometime I have to switch in and out a few times before the overdrive kicks in - probably corrosion inside the switch due to lack of use.