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MG MGB Technical - Overdrive overcooking

My primrose beauty ('77 B roadster) became a primrose beast today, when it tried to set fire to itself!

The old OD switch (gearknob-mounted) had worn out after 33 years (not a bad lifespan!), and was no longer engaging reliably.
I bought a replacement on ebay and installed it - however after engaging OD with the new switch it jumped back into 4th and wouldn't re-engage. Found that the switch had arced and mildly melted.

As I thought this may have been a problem with the replacement switch, the seller (Barrie Bishop) kindly sent me a replacement at no cost.

Today that went in, and after first engaging the OD again dropped out, but this time the engine light went on and smoke began to rise from the gearstick!

I hastily killed the power and pulled over - see the result in the photos!

The Switch is only making contact between two points - it shouldn't matter which way round the pins go / whether the car is +ve or -ve earth, surely?!
Why would so much current be flowing through the OD remote to cause this, especially as the old switch had worked without problem and the OD itsle has not been recently touched?

Any advice / next steps greatly appreciated!!

Judah


J Eastwell

Judah

The only thing that could cause the switch to do this is excessive current. From other threads (I've never measured it myself) the current through the switch should be less than 1 amp. This is considerably less than it would take to fry one of these switches. We therefore have to assume that something is drawing excessive current. I assume you don't have a fuse in the OD circuit, if not I'd install one as next time it could be the car that goes up in smoke.

The only things that can cause this excessive current are a) a short in the wiring from switch to OD, most likely place is at the bottom of the gearstick or b) a short in the solenoid that operates the OD.

If you have a meter I'd check the 2 wires that go to the switch. One should have 12V on it (with ignition on), the other goes to the OD via the 3rd/4th gear lock out switch. Measure the resistance between this wire and chassis, I'd expect something over 12 ohms on a working system. My guess is yours will be much less than this. If so disconnect the OD solenoid, if it's still the same it's the wiring.

Good luck

Bob
Bob Davis

Thanks, will do.
J Eastwell

Classic case of a short in the switch wiring, which the gear-lever switches are well known for. Fit an an-line fuse between the white in the main harness and the white in the gearbox harness, in the mass of connectors by the fusebox, as well as locating and fixing the source of the short.
PaulH Solihull

Dont these short where the wire flexes as it leaves the gear lever. I would be inclined to replace it with a fixed switch somewhere else
Stan Best

I would guess that your original switch was OK and that your intermittent overdrive was caused by a broken wire. This wire has presumably now shorted to ground - as mentioned by Paul and Stan - causing the excess current through the switch/wiring.

When I do finally get around to fitting my overdrive 'box, I think I will go for a dash mounted switch and fused supply!
Dave O'Neill 2

I've added fuses to both mine even though neither are the gear-lever switch. Also the fuel pump, which is also unfused from the factory. Both my cars, plus two I have worked on, have all had damage to the fuel pump white.
PaulH Solihull

All v helpful feedback, much appreciated from a newbie like me (this is my first 'proper' car).

Should've checked the wiring before fitting the new switch - there was indeed a short just to the side of the gearbox, with the result that the live wire was melted along its entire 1.5m length! Fortnuately nothing else was involved and the OD has not been damaged.

On with fresh wiring and an in-line fuse! Until I can afford a 5-speed gearbox conversion :-)
J Eastwell

Don't even think about a five speed conversion or you will ruin the whole characher of the car.
Iain MacKintosh

"Don't even think about a five speed conversion"

Quite agree. Why would one change a six-speed gearbox, with two electric ratios (anticipating paddle-changes), for an all-manual five-speed where you have to keep prodding the clutch? As well as the benefits of OD 4th for cruising OD 3rd is an extremely useful gear for twisty bits or long hills. Would someone with a six-speed paddle-change on a modern car swap it for a five-speed? I think not.
PaulH Solihull

I have found the "paddle change" overdrive switch to be the best. Thats the one that uses the wiper stalk to operate the overdrive. If you have not tried one it is like switching between dip and main beam but with the wiper stalk.
David Witham

Agree, it's only minor drawback is that it's less easy to check whether it's in or out. A PO of my V8 has overcome that by fitting an 'OVERDRIVE' tell-tale to the centre console, like the 'BRAKE' one on later cars. Presumably from some other BL marque of the era.
PaulH Solihull

Maybe from a Triumph Spitfire? I can't think of much else that had overdrive at that time - late '70s.
Dave O'Neill 2

Paul a tell-tale might be better than the current method of if the engine boggs down on change up from second I know its in! Did the PO use a relay in the switching circuit like the early cars.
David Witham

Big Rovers and Triumphs had OD as well as the sports cars.

Only the D-type OD has the relay, to ensure it can't be switched out on a trailing throttle. A simple tell-tale doesn't need a relay, it's just a lamp wired from the OD wire off the manual switch to earth same as the OD, although the lamp stays on when in a non-OD of course. Whilst you *could* wire the lamp from the solenoid side of the lockout switch it wouldn't be a good idea for two reasons - there is not much point in having the tell-tale off when the manual switch is on, but more importantly any connections to the solenoid side of the lockout switch should be avoided in case a fault supplies an alternative operate path for the solenoid in a non-OD gear, like reverse!

I use a more sophisticated system involving a relay on the roadster which locks OD out altogether when changing down to 2nd, even when going back to 3rd or 4th, until I turn the manual switch off and back on again. After 15 years I got fed up with forgetting to turn it off when changing down, then getting the jolt and bog-down when going back to 3rd, followed by another jolt as I switched it off.
PaulH Solihull

Paul
That sounds like a good idea. Do you have a circuit diagram for it? I might incorporate that mod when I finally fit my overdrive 'box.
Dave O'Neill 2

Interesting idea Paul. Did you use the lockout switch and a relay wired thorough its contacts?
Really posted to 100% agree that the O/D is so much more fun and characterful than the 5 speed that you should keep it. The early OD with its unintended semi auto mode is perfect for winding country lanes.
Stan Best

For those of us who are not too hot with elecronics, the small (box) ads in the back of a "Well known publication"-EMG p 87 show a "Tiptro". Simply wired across the existing O/D switch. I have it-works well.
Michael Beswick

Details can be found at http://www.mgb-stuff.org.uk/gearstext.htm#sequencer

The lockout switch is left alone, the circuit is inserted between the manual switch and the gearbox switch. It senses the current flowing through the solenoid, if that current ceases but it is still receiving 12v from the manual switch, i.e. the gearbox is no longer in an OD gear, then the relay operates and its contact opens the circuit between the manual and gearbox switches. The relay then stays operated, locking OD out in *every* gear, until the manual switch is turned off and back on again. The module is best placed in the engine compartment so it can be inserted between the main and gearbox harness without cutting any wires. The standard manual switch is used throughout so no need to find somewhere to mount alternative switches (same as the Smartscreen intermittent wiper module, which is why I like that over other versions). An option is an LED mounted somewhere in the cabin to show when OD is locked out.
PaulH Solihull

This thread was discussed between 24/09/2010 and 30/09/2010

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