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Car not shutting off

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engineerfred

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Car not shutting off

New to this site and to MGs. Bought a used 78 mgb for wife needs lots of work, but go it running, new top. When we got the car the ingition key assembly was torn up. Had to play with the key to get the car to shut off. Replaced the whole key assembly and now the car will not shut off at all with the key. The ignition lite comes on and the fuel pump shuts off but not the car. Any help would be appretiated, thanks
 
Re: Car not shutting off

Hiya Fred! Welcome to the forum!

IIRC there's a diode in the engine compartment, somewhere in the iggy system circuit which can allow "feedback" and create this condition. Go to:

https://www.advanceautowire.com/

download the MG schematics file and hunt your car's diagram. Good luck! And report back? There will likely be a forum member who can more accurately describe a fix.
 
Re: Car not shutting off

or you can put one in yourself in the circuit from the ignition warning light to the alternator. My '76 had the same problem and there didn't appear to be a diode from the factory. Maybe they fixed it by '78.
 
Re: Car not shutting off

Check the vacuum lines for the anti-runon valve. There isn't a diode in the stock wiring that causes this. Some have "fixed the problem" by installing a diode on the brown/yellow wire going to the alternator.

How do you know if the fuel pump is shutting off?
Is it running rough after you turn off the key?
Is the motor really not shutting off or are you have runon?
 
Re: Car not shutting off

Run-on (diesling) is a distinct possibility in a B series engine.

You replaced the ignition switch, was any related wiring messed with that was a consequence of the switched being hacked?

Colin
 
Re: Car not shutting off

The late MGBs (if you look at the Late US Market diagram in the Advanced Auto Wire diagram) in a circuit tied in with the hand brake. I don't know if this will cause the problem that Fred describes. Go to Paul Hunt's web site, Pages of Bee and Vee at: https://www.mgb-stuff.org.uk/ click on Spanners, Electrical, then on Brake Balance and Handbrake Warning. Also click on Fuel, then on Running-on. These two areas should help n finding the cause of the problem.
Cheers,
 
Re: Car not shutting off

David_DuBois said:
The late MGBs (if you look at the Late US Market diagram in the Advanced Auto Wire diagram) in a circuit tied in with the hand brake. I don't know if this will cause the problem that Fred describes. Go to Paul Hunt's web site, Pages of Bee and Vee at: https://www.mgb-stuff.org.uk/ click on Spanners, Electrical, then on Brake Balance and Handbrake Warning. Also click on Fuel, then on Running-on. These two areas should help n finding the cause of the problem.
Cheers,
Click to expand...

The diodes in the braking and service counter circuits would cause the starter to engage if they were to fail.

Paul Hunt wrote the following on another site:

Paul Hunt, Solihull, UK, https://www.mgb-stuff.org.uk paul.hunt1@blueyonder.co.uk
This is due to an original wiring fault from the factory, in conjunction with removed or faulty emissions plumbing or anti-runon valve. Nothing to do with the diode behind the dash, failure of which causes continual cranking while the handbrake is up. Nothing to do with a faulty diode in the alternator either.

Adding a diode in the brown/yellow does work, but is a bit of a frig and removes one of its diagnostic functions. Much better to do what the factory did on UK cars, and move the power supply to the ignition from the white/brown on the output of the ignition relay to the white on its input. FWIW.

Paul Hunt, Solihull UK, paul.hunt1@virgin.net https://www.mgb-stuff.org.uk
Depending on year it could be a problem in the anti-runon system (yes I know it's not dieselling but running normally, be patient).

When the factory added the ignition relay to North American cars they also added the anti-runon system. This was so effective that they didn't realise that a design fault in the wiring meant that the ignition relay stayed operated for a while when the ignition was turned off, so continuing to supply sparks to the plugs. No one knew at the time because the anti-runon system causes the fuel to be sucked out of the carb jets, so stopping the engine instantly as if the ignition had been switched off. It was only when the same relay was added to UK cars *without* the anti-runon system that the problem was discovered and corrected - but only on UK cars!

If there is a blockage or leak in the emissions/anti-runon plumbing, or the valve is failing to operate, it is quite likely to cause this problem. There is an open-ended pipe coming from the valve, you should be able to block the end of this pipe and the engine should stop shortly. If it doesn't the plumbing has a problem. If it does then probably the valve has a problem. This is powered by 12v from the ignition switch OFF and a ground from an oil-pressure switch while the engine is turning. Can't remember the colours of the wires but pink comes into it somewhere, there is also an in-line fuse between ignition switch and valve. I *think* the valve is fitted somewhere at the aft-end of the RH inner wing.

PaulH.
 
Re: Car not shutting off

Yea,

I had this problem 20 years ago on my first 'B' ( a '77').

My fix involved securing/replacing the vacuum lines going to the vacuum canister. If these lines have failed the engine can still pull enough air to 'diesel'.

The previous posts pretty much have everything covered.

Keep on.
 
Re: Car not shutting off

Welcome to the BCF forum Fred! This is one the most active forums on the net for British cars.
happy0034-1.gif
PJ
 
Re: Car not shutting off

Hey guys thanks for the replys, The car is'nt running on or dieseling, and just keeps on running like normal, I have to put in gear and let out the clutch to get it to shut off. the ign lite comes on but its flickering untill I get the engine to stop. Its the first car Ive ever seen that has ever done this. But this the first british made car Ive ever own. I really like the car its fun to drive and really interesting where some of the switches and different things are placed in the car.
 
Re: Car not shutting off

Welcome Fred and Christine :wink:
 
Re: Car not shutting off

engineerfred said:
Hey guys thanks for the replys, The car is'nt running on or dieseling, and just keeps on running like normal, I have to put in gear and let out the clutch to get it to shut off. the ign lite comes on but its flickering untill I get the engine to stop. Its the first car Ive ever seen that has ever done this. But this the first british made car Ive ever own. I really like the car its fun to drive and really interesting where some of the switches and different things are placed in the car.
Click to expand...

Did you read my complete post above? Paul Hunt covers this. It doesn't take much power to keep the contacts closed in the ignition relay. If the contacts are closed in the ignition relay the motor will not turn off.
 
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