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MG MGB Technical - Mgb Overdrive queston

I have a '70 mgb roadster equipped with overdrive. Problem is that I can't seem to keep the overdrive engaged. It "slips" out of overdrive frequently when cruising at hwy speeds. It's necessary to let off the accelerator to get it to re-engage and it may only stay engaged for only a short time. Has anyone experienced this problem before and if so what did it turn out to be and how was it "cured"? When this problem first occurred, I changed the gear box oil from the 90 wt gear oil that was in it to 30 wt non-detergent. Didn't seem to make any difference. Appreciate any help.

Cheers - Dennis
Dennis Rainey

Two things I can think of. First is that the switch is out of adjustment. Try pushing the shift lever side to side as you drive with overdrive engaged. If it shifts in and out as you do this, then the problem is the switch adjustment.

Another possibility is that the little ball above the solenoid plunger isn't seating properly. Remove the cover plate, solenoid, plunger and ball. Put a dab of grease on a punch and stick the ball to it. Insert the ball into its seat and tap the punch a few times. Reassemble the rest and give it a try. It worked for me a couple times.
Steve Simmons

Oh, one more thing. Make sure the solenoid is getting enough voltage and that it is strong enough to fully engage the plunger.
Steve Simmons

Clean/change the 3rd/4th gear switch . . . ?


Dan Robinson

Those dash switches are getting on so if it hasn't been replaced then maybe that is breaking contact. The same applies to the gear lever inhibitor switch - this can easily be dismantled & cleaned. To get At it remove the carpet and the bolted on oblong cover. The switch can then be loosened using a large screwdriver and levering it. The soenoid cover plate also gets dirty and therefore has a bad contact with the bottom of the coil.
Garth.
Garth Bagnall

The filter on the bottom of the overdrive unit does need cleaning periodically - quite easy to access when on ramps - gear box oil should be engine oil as you have discovered - but more likely to be an electrical fault as posted above - cheers Cam
Cam Cunningham

Last year, I had OD slippage during throttle lift (deceleration),
as well as sluggish engagement and disengaement when the
engine was cold and luke-warm.

Renewing all of the "O" rings and filter cured all.

Except for the OD pump (VERY difficult to re-install!) - all
of the other "O" rings are not too difficult to swap.

My OD has been 100% ever since.
Daniel Wong

It is easier to to lower the gearbox when removing the inhibitor switch - don't forget to remove the speedo cable - support on a jack & just remove the cross member bolts the lower it down onto the body x member.
O/D clutch slip on deceleration is usually caused by the sliding member lining becoming glazed. It is usually accompanied by no reverse when the O/D gets hot. Usually the only cure is to replace it. How-ever it can sometimes be cured temporarily by changing the oil - usually thinner than the old oil. Another problem on the earlier O/D - up to 76? is caused by the bush in the first casing turning and blocking off the oil hole that feeds the mainshaft and bleeds off the oil pressure. The result is very sluggish dis-engagement when cold - sometimes you need to stop.
Garth
Garth Bagnall

Same thing happened to me...I tried to get a wrench on the switch yet no amount of persuasion or lowering of transmission, etc could I get the thing to move.I found out later that the dis-similar metals had fused together.
A friend suggested to remove the carpet, drill or cut an opening -adjust or replace the switch and replace the opening...no one will ever know.Thirty minutes later I had the O.D.back in service :)
jim

Top up the oil.
Dan Neu

Maybe *check* the oil level, then top-up if needed.
Paul Hunt 2

This thread was discussed between 20/01/2007 and 26/01/2007

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