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MG MGB Technical - overdrive usa


My question conveys my lack of knowledge of how the OD functions. I have a stock 74B with overdrive. On my drive to work every day I go up a steady but noticeable incline for a couple of miles at about 70mph on the interstate where I have to add a bit of gas…no problems. Is there any reason to disengage the overdrive for the incline? What types of things wear out an OD unit? Thank you. -Jeremy
jjralston

hmmm...my thread title should have been "overdrive usage" I welcome feedback from any location.
jjralston

There is no reason to shift out of overdrive in he conditions you state. Wear normally takes place to the cone clutch during shifting in and out of overdrive. When engaging lift your foot off the accelerator a little, I use the clutch when disengaging the OD. Hard transmission down shifts and hard acceleration in reverse put a lot of excess stress on the cone clutch. Keep the oil in the trans and the filter in the OD clean.
John H

I thought that using o/d in reverse would wreck it. It is one of the reasons why the inhibitor switch is there. O/D is usualy only on 3rd and 4th. FWIW I do not lift off when engaging -it feels smoother under load- but I do dip the clutch when dis-engaging. M
Michael Beswick

Michael, I am not saying to put it in overdrive in reverse. When not in OD the engine power thru the trans is transmitted to the output shaft of the trans thru both the cone clutch and a one way sprague clutch. In overrun when down shifting, and in reverse the one way clutch does not work and all the load is placed on the cone clutch.
John H

As with all downshifting, do so only when you need more power. Ya gotta do it by the seat of your pants. No hard and fast rules about rpms or speed will tell you. But if you put your pedal to the metal and nothing happens, follow your instinct and downshift. If the car is meeting your speed or acceleration needs, just leave everything alone.

About the only driving habit that will shorten the life of anything on your B is riding the clutch, or using the clutch when you could be in neutral - as at stoplights, or in traffic jams. Beyond that, if you're driving smoothly, you'll maximize the life of just about every mechanical component on your B.

In 24 years, I've never broken an overdrive. Even once when my inhibitor switch shorted out and I was unknowingly backing it out of the garage with overdrive engaged, no damage ensued. Sure scared me though - I mean, that is THE classic "no-no" with Laycock units. But these components are pretty tough and they'll generally forgive you for one or two stupid mistakes.

Allen
Allen Bachelder

You use the overdrive exactly as you would any other gear - i.e. engage it when appropriate. If you are going up a hill in 4th and it is getting steeper eventually you will chose to change down to 3rd instead of flogging it in 4th. The same applies to switching OD off on an incline. Likewise when you want to accelerate to overtake, there is plenty of torque in OD 4th, but if you want to get past quicker you switch OD out, or may change down to OD 3rd instead, again it all depends on the conditions.

You don't need to feather the throttle, much less dip the clutch which the drivers handbook specifically says not to do, when switching OD in or out. In fact you get smoother engagement and disengagement when *applying* throttle, which is why the earlier D-type OD had the vacuum switch, which prevented disengagement on a trailing throttle.

Whilst having OD engaged in reverse (if for example the inhibitor switch has failed or been bypassed bypassed by a DPO) *is* a non-no, it isn't an automatic ticket to OD destruction. The OD will only be engaged when the pump has developed sufficient hydraulic pressure i.e. when the car is moving fast enough, which is why people do get away with it. Not something to rely on, though!

Paul Hunt

thanks guys...i'll continue keeping it in overdrive.
jjralston

This thread was discussed between 24/09/2008 and 25/09/2008

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