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MG MGB Technical - Overdrive not working

To all,
My overdrive has stopped working, first guess was low transmission oil, this was changed with no success.
I am now checking the switch.
Bearing in mind I am mechanical, if I can see it I can work it out were as electrical is just voo doo, failed Electro technology on my first attempt but always blamed the fact that the subject was on a Friday night (Pub night).
Please refer to attachment which shows my meter which I only use to check battery life.
With simple step by step instructions please explain how to check the switch.
Thank you.



David Levy

Dave. What OD we talking about. From the switch I would say a D type?

Turn the dial on your meter to the bottom to the simbol that looks like a speaker. In this position, if you touch the two probes together, you should get a tone out of your meter.

Then just touch the two probes to the two switch terminals. It should give a tone in the on position and no tone in the off position.

If that switch shows good, you will need to move to either the vacuum switch on the firewall in the engine compartment or to the solenoid.

Neither is difficult to do, but you will need to get under the vehicle to get to the solenoid.
Bruce Cunha

David,
from a recent thread on MGB overdrive, a link to the John Twist page on this site, it refers to USA cars but the basic principles remain the same. -
https://www.mg-cars.org.uk/news/news363.html

And a link to Paul’s excellent site (mgb-stuff.org.uk) on overdrive electrics, but if you’re like me it may appear far too much information to take in but taken in small chunks will help should get you there (but not me). -
http://www.mgb-stuff.org.uk/gearstext.htm#diagnosis

Nigel Atkins

To all,
Thank you for the helpful advice.
It appears my first task will be getting my MG high enough of the ground so that I can get under it.
A combination of ramps and jack stands may suffice. It appears getting access to the 3/4 switch will be an exciting challenge, from the access description it sounds like the switch was hung on a piece of string and the car was built around it on the assembly line.
David Levy

That's the switch on my 73 LH-type, originally the stalk was straight, but changed in April 65 so could be either OD.

If it is an LH-type the electrics can be checked simply by putting an ammeter in series with that switch. If you see about 1 amp with the ignition on, manual switch on, and gearbox in an overdrive gear then electrically the circuit is working. The LH-type may or may not click when powered - neither of mine did until I changed the O-rings on one of them, then that would make a slight click.

After that it's hydraulic or mechanical.

D-type is tricker as not only was there a vacuum switch and relay originally, but it takes an initial 17 amps pull-in current which is beyond most home-use multi-meters, and 2 amps hold-in current. However the D-type should always make a definite click when it operates, so if you don't get that it's either electrical or the plunger is jammed. The relay also clicks though, so you have to check for both. If it's the plunger that's jammed then you should get a noticeable spark when tapping a wire on the switch to complete the circuit.
paulh4

Accessing the gearbox switch again depends on whether it is a D- or LH-Type. D-Types on the 3-synch box with the narrow Mk1 tunnel have a large removable panel on top of the tunnel. LH-Tpe i.e. Mk2 tunnel only have a small panel in front of the gear-lever, but even then if you drop the gearbox cross-member you can pull the gearbox across and access the switch, both electrically and - depending on the size of your hands - remove and replace the switch. If not then some have cut a hole in the side of the tunnel and fitted an access panel.
paulh4

I see from the other thread that David has a 1970 which should be the later LH-OD.
paulh4

Paul you beat me to that as that was what I was going to post - but I got the impression the o/d might be an earlier one because of the drain plug but I don't know change over points.
Nigel Atkins

I'll put up my usual links that I was going to before I got confused, wet end of stick I'm afraid.

Gives the idea of cleaning the things out and possible problems whilst doing so.

104 MG Overdrive Repair Part 1 -
https://www.youtube.com/watch?v=XPSgUeHAy8E

105 MG Overdrive Repair Part 2 -
https://www.youtube.com/watch?v=LCqPu77EWEE

106 MG Overdrive Repair Part 3 -
https://www.youtube.com/watch?v=FHw_o5y0HWE

Nigel Atkins

Originally the change was with the 3-synch to 4-synch at the same time as the change from Mk1 to Mk2, but later gearboxes can be found in earlier cars.
paulh4

Thanks Paul. I remember early o/d with early box and later with later but can't hang on to when MK1 became Mk2.
Nigel Atkins

Check with ignition and OD switched on and car in 3rd or 4th gear on stands that you are getting 12 volts at the solenoid. It may be the 3rd/4th switch that is not engaging (came unscrewed or went defective.) If there is no voltage and there is a relay in the circuit, it may be suspect as well.
Glenn Mallory

Just put an ammeter in series with the manual switch, and that will check the continuity of the whole circuit. You should get 800mA to 1 amp. As a 1970 it should be an LH-type so no relay.
paulh4

Follow up,
My overdrive is the LH model, switch on dash appears O/K so went for what I thought would be the easiest next step ie clean the two filters which have not been touched since I have owned the car (12 yrs).
Attempted to get the car up on drive on stands in my garage, no success floor to slippery, then tried to jack the car up and place the stands under the wheels the car nearly rolled out of the garage despite useless handbrake and wheel chocks.
Eventually got under the car but not enough room to do anything.
This morning the car was dropped of at the local mg specialist.
Ordinary looking house from the front but impressive set up at the rear, hobby that has gotten out of hand and turned into business.
Stay tuned for the outcome.
David Levy

Did you test the current flow with ignition on, gearbox in 3rd or 4th, and manual switch on? Could be something very simple not needing access under the car.
paulh4

Paul,
Thanks for the advice.
On the way to the repairers I tried the overdrive and low and behold it worked once but not a second time.
Thoughts are intermittent electrical problem or hopefully just a dirty filter, I am waiting for a phone call next week from the mechanic which will reveal all.
David Levy

This thread was discussed between 01/09/2019 and 06/10/2019

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