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MG MGB Technical - Overdrive woes

My 66 Roadster has developed a fault with the D type overdrive.
When in 3 or 4th gear under load and before I engage the overdrive the overdrive unit tries to engage then immediately disengage making the car jump like the preverbial kangaroo.
Now, if I then engage the overdrive she runs as sweet as a nut until I disengage when slowing down then the same shenanighans happen again.
Has anyone any idea as to where the problem lie? I suspect electrical and have changed the relay and dash switch (mainly because I had new spares on shelf!)
The circuit must have some current going to it even when the overdrive switch is in the off position to make the system try and function?
Many thanks in advance.

Steve.
S Collins

Have you checked the switch on top of the gearbox? When the "hunting" starts try pushing the gearlever towards you, and then the other way away from you across the gate, not backwards and forwards The fault could be a worn or loose switch. Check also for chaffed wires in the harness down to the gear switches especially where the cables go through the metal clips.
jim soutar

If it's trying to come in *before* you manually engage it, or seems to stay engaged after you manually disengage it, then there is current getting through when their shouldn't be. Wired correctly then the manual switch or relay closed when they shouldn't be can be a cause as you have surmised, but the vacuum switch could also be a factor. Disconnect the yellow/red between the overdrive sub-harness and the gearbox harness to eliminate any of those three sources of voltage when there shouldn't be. If it still happens, then either there is an intermittent short between the green wire for the reverse light switch (or some other live wire in the vicinity) and the yellow/red to the gearbox switch or yellow/pink from the gearbox switch to the solenoid, or it is not electrical but mechanical. If it only happens in 3rd or 4th, and never in 1st or 2nd, then it almost certainly *is* electrical, voltage getting to the yellow/red when it shouldn't be. If it happens in 1st and 2nd as well it could either be voltage getting to the yellow/pink, or it is mechanical. I don't have so much info for the D-type as for the LH-type, but in the LH-type if there is a blockage from the pressure relief valve to the sump then there can be enough pressure developed at higher revs to move the pistons and partially if not fully engage OD.
PaulH Solihull

Paul,
The overdrive is indeed trying to engage in third and fourth before I manually switch
I have checked and adjusted the vacuum switch, however to no avail I am now believing that I have a short somewhere!!
I am not really great with wiring so if you have any pointers as to where/ what I should be looking for/ testing I would appreciate it.
S
S Collins

As I say you need to disconnect wires in the circuit until it no longer comes in by itself in 3rd and 4th.

I'd start with the yellow/red on the manual switch. If that stops it, reconnect it and makes sure it starts happening again (in case the fault is intermittent and it has stopped anyway) and if it does the manual switch is faulty - permanently closed.

If that doesn't stop it then disconnect yellow/red that goes between the vacuum switch and relay and the gearbox harness, i.e. disconnect that wire in the gearbox harness. If it still happens then the problem is in the gearbox harness or the gearbox switches.

If that stops it, and reconnecting it starts it happening again, then disconnect the wires from the relay, making sure the white and yellow don't short to anything. If the relay contact has stuck, or the wires are connected incorrectly, it can cause this problem. Black and yellow should go to W1 and W2, doesn't matter which way round but originally it would have been yellow to W1 and black to W2. White and yellow/red should go to C1 and C2, again it doesn't matter which way round, but originally it would have been yellow/red to C1 and white to C2.
PaulH Solihull

Thanks Paul, will give it a go as you suggest, I do know that if I remove both wires from the vacuum the overdrive works as it should, reconnect and the fault reappears!!
I will remove one and see what happens - watch this space.
S.
S Collins

In that case the vacuum switch contact is stuck closed, although in theory that on it's own can't cause the problem, unless the relay is incorrectly wired. The vacuum switch is simply a normally open contact, so removing either wires should have the same effect as removing both. Which *should* be that the overdrive comes out immediately the manual switch is turned off, even on a trailing throttle i.e. high manifold vacuum. The purpose of the vacuum switch is to keep the overdrive engaged when the manual switch is turned off until you press the accelerator, or take it out of 3rd or 4th gear.
PaulH Solihull

Paul, I removed the vacuum switch from the car and checked the contact and cleaned them up (the diaphragm is in remarkably good condition) using a multimeter and gunsons brake bleed kit I adjusted the vacuum(which was working at around 5.5" of mercury) to the recommended 7+" of mercury, the copper contact is working fine.
It really must be a fault with the wiring (having already changed the relay,switch and had the inhibitor switch checked and adjusted)I guess it needs hooking up and checking whilst driving rather than static in garage.
I have looked at the wiring all seems ok but that doesn't of course mean it is when driving.
S
S Collins

Well would you believe it..... Hurrah, I have fixed the fault.
It turned out to be the y/p to the solenoid shorting
Boy am I a happy bunny again :)
Paul/Jim many thanks for your input to my woes it was much appreciated.
S
S Collins

Shorting to the green, I trust! Shorting to earth wouldn't cause this, but would burn your wiring when you tried to engage OD.
PaulH Solihull

This thread was discussed between 14/03/2012 and 10/04/2012

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