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MG MGB Technical - Proper Operation of the Overdrive?

Hello All,

Finally had a chance to get the '63 B out on the highway, and use the (D-type) overdrive. It's so nice to cruise at 60 MPH at 2,700 or so RPM.

One problem, I actually have no idea how to properly operate the overdrive - mainly when it's OK to switch it on and off. I would appreciate a primer.

Thanks, Greg
Greg Van Hook

Greg - When you want the O/D on, flip the switch down, when you want it off, flip the switch up. There is no special considerations, such as declutching, letting up on the accelerator, etc. except is coming out of O/D if the original vacuum switch is still in place and operating properly. The vacuum switch was in place with the D type O/D because it was thought that coming out of O/D while decelerating would cause it to over run the one way clutch. The vacuum switch stops the O/D from deactivating when there is a high vacuum present (ie. when the car is coasting). Simply blipping the accelerator will drop the vacuum level, allowing the O/D to drop out. I just flip the switch and use the brakes to slow down or let the car coast down and and then when I depress the accelerator, it will come out of O/D. The O/D will activate when you are in 3rd or 4th and deactivate when in 1st or 2nd if you forget to switch out when coming to a stop. This doesn't hurt a thing as it will switch back in as soon as you shift up - it will just cause a bit of a jerk when it does reactivate. You will see a lot of things written about needing to feather the throttle or declutch when going into or coming out of O/D, but according to Chris Betson of Octarine Motors in England (who does a lot of O/D work), nothing special is needed, just flip the switch and continue driving, letting the O/D unit do its thing. Cheers - Dave
David DuBois

I think there are plenty of opinions on how to use overdrive in the "subjects beaten to death" section of the archives......
Flabbergasted

Use the OD gears just like you would any other gear, i.e. when it is appropriate. That might not sound very helpful, but just like when you change manual gears at different rpms according to what you are doing, so it is with OD. 4th OD is basically selected when you have reached your cruising speed, say 40 or above, and switched out again when you want to overtake or speed up, but it will accelerate reasonably in OD 4th anyway. 3rd OD is very useful for twisty sections, or long inclines, where 3rd is a bit too high and 4th a bit too low. But the D-type isn't so useful here as OD 3rd is closer to straight 4th than on the LH-type, which is closer to mid-way between 3rd and 4th.

Like David says, just switch it in and out as required, there is no need to feather the throttle and definitely don't declutch. Unfortunately even after 20 years I still forget to switch OD off when changing down from 3rd to 2nd and don't like the thump when it comes back in automatically when I go back up to 3rd again, usually inappropriately, so I then have to switch it out and get another thump. So I developed a circuit to lock OD out under those conditions until I turn the manual switch off and back on again.

Paul Hunt

This thread was discussed between 09/04/2009 and 10/04/2009

MG MGB Technical index

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