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MG MGB Technical - overdrive has stopped working

The lacock overdrive on my '79 MGB GT (rubber bumper) has suddenly stopped working.

I have drained the gearbox, cleaned and re-fitted the solenoid and filters, checked that the solenoid is working OK, and is operated correctly by the gear knob switch, but the overdrive just doesn't engage. There are no nasty noises coming from the gearbox or overdrive.

I know that the overdrive could be engaged in *any* gear on this car, but I'm sure that it was never engaged in the lower gears (or in reverse!).

Any ideas anyone?
Dave Taylor

Either there is a bit of dirt or metal swarf under one of the ball valves - pump valve is prime candidate, or the lining on the cone clutch has come away - these were only bonded on the later OD units - not rivetted.

The pump valve lurks under the main filter - under the plug with two holes. The spring can weaken or get trapped by dirt and the pump will not then produce enough pressure (400 psi) to operate the OD.
Chris Betson

Thanks Chris. I was afraid I might have to take the thing to pieces again!

Any tips on doing this job with the minimum of disruption (i.e. draining the gearbox - again)?
Dave Taylor

I am puzzled by your reference to OD working in any gear

The solenoid shd only be activated in 3rd and 4th -
in 1st 2nd and reverse an inhibitor switch shd prevent current reaching the solenoid

I am wondering whether there is a bizarre fault with the inhibitor which is causing your probs

Chris
chris storey

Not really, the only hassle you are likely to run into is undoing that plug with the two holes.

You can use a couple of drills and a big screwdriver if the plug isn't too tightly done up.

If it is tight then you can sometimes tap it round using a small chisel or old screwdriver alternately in the two holes.

Before tackling the pump - have you checked the low pressure valve under the hexagonal plug by the solenoid - you do not need to drain the box to remove it and if that is the source of the problem then you will save the aggro of taking out the pump.
Chris Betson

Chris,

Is there a way to allow od to work in 3rd without disassembly? Presently, mine is limited to use in 4th gear.

rn
RN Lipow

RN, you may have the much later ('76? up)type of LH Overdrive which only works in 4th gear. I installed one on my '69 B from a '77.
Luis

i do, but i understand that there is some kind of limiter associated with the gearshift.

rn
RN Lipow

Chris,

I'm not sure why the inhibitor isn't working - the previous owner of the car told me that overdrive could be engagged in any gear, and I found this to be the case.

As I understand it, the inhibitor is an eletrical device, which only passes current when 3rd/4th gear is selected. I assume therefore that it's connected to the solenoid, so that the solenoid is only activated when 3rd/4th is selected. If this *is* the case, then I don't think the inhibitor is causing my problem, since I have verified that current is being passed to the solenoid.

So I'll set about checking the low pressure valve, and then the pump. I presume that I'm looking for a piece of crud or a broken spring.

Just one more thing, I'm a bit worried that this might be in vain, if the lining has become detached. I don't think that this has happened, but are there any tell-tale signs or noises?

Dave.
Dave Taylor

RN-
Since you have a Big Bore engine that puts out a lot more low-end torque, you might want to consider the fact that the reason for the later MGB transmissions having their overdrive units restricted to 4th-gear-only operation was that the same unit was used on the V8 models. The whole idea behind restricting the overdrive to 4th-gear-only operation was to protect it from the prodigious torque output of the V8. Unless your overdrive unit has been completely rebuilt, you might want to leave it as it is.
Steve S.

thanks

rn
RN Lipow

RN,

The inhibitor switch is operated by a pin driven by a pad on the side of the selector mechanism. The only way to get a 4th only box to operate in 3rd is to change the mechanism which is a gearbox out operation.

You could bypass the inhibitor switch and use a relay in the circuit linked to the reverse light to block operation in reverse - that will give you OD in all forward gears.

Dave,
If the inhibitor switch is bypassed then OD works in all gears - the switch is way up on the top of the unit and impossible to get at with the gearbox in place ( you can cut an access hatch in the tunnel). So if it has gone bad in the past a DPO may have just bypassed it.

Unfortunately a broken clutch makes no noise at all but the problem is much more likely to be electrical or hydraulic.
Chris Betson

Dave,
I'd take it to a specialist before you decide to pull the OD. They can check the pressure and such to make sure the problem isn't pressure related.
J Arthurs

This thread was discussed between 31/08/2002 and 03/09/2002

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