MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Overdrive/reverse gear

I know that there are a 1000 threads on this subject, but can it be explained simply? I have a UK 73 BGT, with the dash O/D switch. What actually happens if after a long touring run with the O/D on, one forgets, and reverses into a car park?...surely the engineers designed the gearbox to allow for this to happen, otherwise some sort of warning light or buzzer would have been put into the car when reverse was selected without switching out of O/D? Some mention was made of an overdrive inhibitor switch..I guess that this is the isolation switch from the powwer feed? If this is in working order, then selecting reverse gear whilst the overdrive switch is left on should not cause any damage?

Looking at the glovebox manual for this car, no special mention or warning is made of making this error, so is it such a big deal?

Can someone enlighten me?
Gary Lock

No problem if the inhibitor is working. Paul Hunt has instructuction for installing a overdrive sequencer relay in his web site.
http://www.mgb-stuff.org.uk/hammerframe.htm
Click on gearbox, overdrive sequencer relay.

Clifton
Clifton Gordon

Gary,

A simple test: try to engage overdrive when you wind it out in 2nd gear. If it goes in, you have a problem; if it doesn't, that means the inhibitor switch is working. That means that regardless of your dash switch, your overdrive cannot be "on" unless the car is in third or fourth gear. Even so, I do have an indicator light on my dash to remind me when I'm in overdrive. Don't wire the light to the dash switch, wire it to the lead between the inhibitor switch and the solenoid. That way, if the inhibitor switch ever fails, the light will remind you that the car is still in overdrive.

FWIW,
Allen
Allen Bachelder

The overdrive sequencer relay is simply a device to counteract my advancing years and prevent the OD from re-engaging automatically when going from 2nd to 3rd and causing a jolt when doing so.

Of course the gearbox engineers thought about the possibility of the driver leaving the manual switch engaged when reversing, which would destroy the OD, and fitted a lock-out switch on the gearbox that disconnects the OD altogether when not in an overdrive gear.

This disconnects the OD in 1st and 2nd as well as reverse, as the OD fitted to the MGB isn't strong enough for the torque developed in these lower gears. The factory V8 originally had OD in 3rd and 4th as well but they soon found the much higher torque of the V8 was causing damage in 3rd, so retrospectively modified the gearboxes to make it available in 4th only. Note this is nothing to do with the '4th only' OD on late North American spec cars which was for another reason altogether (hardly a surfeit of torque!).

Allen's test of switching OD in and out in 3rd and making sure it *does* make a difference, then doing the same in 2nd and making sure it *doesn't* is perfectly valid. However I would never advise connecting anything between the lockout switch and the OD solenoid as a fault could develop which would bypass the lockout switch, cause the OD to be engaged when it shouldn't, which would destroy it. A PO fitted an Overdrive tell-tale to my OD, but it is wired between the manual switch and the lockout switch and so is 'safe'.
Paul Hunt

This thread was discussed between 07/08/2008 and 08/08/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.