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MG MGB Technical - MGB O/D jerks on release

Just fitted a LH type o/d which I got from e-bay to my 69 roadster.Before installing I partialy stripped it and all seemed fine.on the road test the o/d engages straight away and works as it should but when I switch the toggle off on the dash the o/d takes about 2-3 seconds to release and the car gives a jolt like when you changs down say from 4th to 2nd.I can live with this but not sure if it will do any damage,alternatively I can move the stick out of gear and then switch off. any info on this would be appreciated.
John Parker

They usually work the other way round, that is there is often a slight delay when engaging the OD, but an instant release when disengaging. At what speed in what gear are you disengaging? Obviously, the faster you are travelling the bigger the shock.
Mike Howlett

I agree with Mike. Also how are you using it? The OD is designed to be engaged and disengaged with the power on (the Type D had a vacuum switch and relay to ensure this was the case on disengagement) to minimise jolting, mine do give more of a jolt on the overrun, one slightly more than the other.

The speed of engagement (and hence the jolt) is set by adding or subtracting shims from between the low pressure valve plunger and its spring (behind the hex plug) - adding a shim makes it sharper/faster, removing one makes it softer/slower. Try removing one and see what happens, oil will probably drain out when you remove the hex plug from the base of the OD.
P Hunt

I mostly when slowing down give a quick dab on the Clutch with the throttle closed will usually allow a smooth change. If dropping down to get more power keeping the throttle open , but not foot flat, and a very quick dab on the clutch will sync the box. With the delay the timing will be difficult.
When I bought my 1980 B it had hypoid in the box and not Castrol GTX. The box appeared to be a lot smoother once the hypoid was replaced and the filter changed etc.
R. E Bester

Ive tried all speeds even allowing the revs to go as low as 1500rpm but no difference.I agree with paul the o/d should be operated as designed by the switch.I also suspect it could be either the pressure relief not operating correctly whether releasing to late or early,also possibly the ball in the solenoid not releasing the oil.I filled the box with castrol 20w50. i noticed the 3 shims on the plunger of which one is slightly thicker, which one should I remove.i also might add i have not replaced any o rings in the o/d.
John Parker

What weight oil are you using in your gearbox/OD? I have hard of some people using 90W and having a problem with hard shifting in and out of OD. The shop manual says to use motor oil - I use Castrol 20W-50 and the OD shifts perfectly smooth. Cheers - Dave
David DuBois

How about just disengaging when you come off the highway and to a stop? you could just do it like that. Have you tried to disengage at a standstill?
Mike
MK Mike K

90wt and 20w/50 make no difference, the V8 uses 90wt as standard.

You defintely shouldn't use the clutch when switching the OD in and out, the drivers handbook specifically says not to, and it rather nullifies the point of having the convenience of electric overdrive. Neither should you need to feather the throttle.

Come to a standstill everytime you want to disengage OD? That's even worse!
P Hunt

I see on page 30 of the handbook "DO NOT depress the clutch when selecting o/d or normal"I removed the thick shim from the PR valve which took a couple of minuets with only a couple of drops of oil (20W50)in the tray.took it for a run and still the same,jerks you in the seat on disengagement.!!. Maybe I should try the solenoid valve with an extra gasket or ATF oil.? John.
John Parker

ATF might help, I've seen claims that it can flush out problems. I've also seen claims of people leaving ATF in when replacing it with 20W/50 (or whatever grade of oil is correct for local climatic conditions) brought the problem back. Personally I wouldn't be happy with that as it is obviously just covering a problem up, but I suppose if it avoids removing and rebuilding with no guarantee of success ...

I don't think shimming the solenoid will do anything, when energised the plunger moves up until it is stopped by the ball on its seat, and when de-energised its just pushed down a fraction of an inch by oil pressure. Shimming it will just cause it to operate within a slightly different area of the coil. It operates pretty-well instantaneously in either direction.

Is there any tendency to free-wheel when disengaging?

Useful to see undoing the hex plug didn't drain the oil.
P Hunt

Is swithcing off OD at a standstill worse for the unit than having it slam out of OD while the car is moving in gear? or is it more of a inconvienence to have to wait until you stop?
MK Mike K

Im not sure on the freeweeling but it takes a couple of seconds before coming out of o/d with a shudder in your seat.Im also curious why hardly any oil drippped into the tray (large teaspoon)
should you switch off o/d before declutching and changing to 2nd as changing down will automatically deenergise the inhibitor switch.Thanks John.
John Parker

There is no need to switch off the OD as you change down to second if the inhibitor is working. It will drop out as you move the lever across to the 1-2 plane. On my car you couldn't notice this happening, but it did happen. Then as you change back up from 2 to 3 it comes back in by itself. On mine there was always a very small delay, so you got the rev drop due to taking third, and then another drop as the OD kicked in. This isn't generally desirable as it kills the acceleration because its similar to changing from second straight into fourth.
Mike Howlett

John,
my 73 BGT has had a jolt when disengaging the o/d since I bought the car in 1986 uptil today & I use it more or less on a daily basis. So far there have been no other negative side-effects but I am interested to hear what the experts think the cause may be.
rgds & a happy Christmas to all.
BrianR
B Reeves

Thanks ,I can live with it but it would be peace of mind knowing what the reason is. Thanks to all for the advice and have a merry christmas and smooth gearchanges for 2010.
John Parker

Switching off OD when the car is at a standstill has no effect at all as OD is no longer engaged anyway. It's only hydraulic pressure from the pump on the OD input shaft, together with the solenoid being energised, that causes OD to engage. Slowing to a stop disengages OD much the same as switching off OD does, except that the loss of hydraulic pressure is usually more gradual than switching off the solenoid.

Slam it out of OD? No such concept, it's nothing like banging a gear lever in and out of gears where softer or more gradual is generally better for the synchro cones and drive dogs. An electrical switch is either on or off and changes relatively instantaneously from one to the other. It's how OD is supposed to be engaged and disengaged.

OD should drop out the instant you move the gear lever from the 3/4 plane towards the 1/2, and should never be engaged by the time 2 (or 1) has been engaged. However going up again there is usually a short delay in OD engaging which is enough time for you to select 3rd and let out the clutch, only then getting the jolt of OD re-engaging. After about 15 years I still couldn't remember to switch OD out when changing down to 2nd, and hated that jolt, plus the other one when switching OD out manually for reasonable acceleration, so built a circuit that locks OD out when changing from an OD gear, even when going back in to an OD gear, until the manual switch is flipped off and on again.
P Hunt

John, I had problems with my LH O/D that came out of a 78 MGB. It was very slow to engage when cold, and I mean up to 30 seconds, but came out instantly when dis-engaged. Someone on this site recommended using ATF for a short while to see if it would cure the problem. It did. In fact it worked so nicely, I've only just changed back to Catrol GTX after 2 years. And now with the 20/50 it still works great. It could be some crud or dirt somewhere that the thinner ATF cleaned out. BTW when running with the ATF I had no more gear noise than normal, it ran quietly.

Andy 74 MGB GT
Andy Preston

This thread was discussed between 22/12/2009 and 24/12/2009

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