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MG MGB Technical - Could not shut down engine

Just finished replacing a cracked exhaust manifold in my 77B with Stromberg Carb. Car had not been run in awhile during repairs and battery needed to be charged. Car was difficult to start but finally turned over ran rough and began to stall. I turned ignition off but car would not shut down. There was no combustion but the alternator and crankshaft kept turning over and could not be stopped with the key. Only way it would turn off was to disconnect battery. I put my hand on the starter and it was very warm. Never had this happen before and not sure how to proceed. Is it a possibility that battery was bad and car was operating off of power from alternator? Haven't had battery tested yet. Any help would be greatly appreciated.
JCH Hibbard

Hi JCH
Could be that the starter is not disengaging the flywheel and therefore constantly powered and turning the crank until you disconnected the battery.
This might explain why the starter was hot, could be starter solenoid, ignition switch connections? something else electrical

Regards Pete
PJ Thompson

Check starter relay. I have had a batch of Lucas labelled relays all of which stuck in the "ON" position with the key turned off, until given a tap with a screwdriver handle to release them. I then used a much cheaper Durite relay to replace, which has operated with complete reliability.
jim soutar

Thanks PJ and JIM was hoping it wasn't going to be starter related as I haven't had to deal with the MG starter at all and will need to gain some knowledge. Where are the relays located? Time to dig out the service manual and start studying.
JCH Hibbard

Yep, sounds like starter relay, Right side inner wing. White and brown is the feed out to the starter solenoid, Disconnecting that should stop the starter, if the screwdriver handle doesn't.
Allan Reeling

Relay is mounted on the inner wing near the fuse box on UK cars. Silver metal box with 4 terminals, some of them double. Get a friend to switch key on and off you should be able to hear and feel the box click.
( Some times the relay is in black plastic box) Wires are Black Brown, White/Red White/Brown.
jim soutar

There's a Diode in the Brake test/warning circuit, that if it goes bad, lets a continuous live feed circulate even with the switch off/key out.

Try cranking the engine with the handbrake/parkbrake/ebrake (your name here) off. If it doesn't keep going, suspect the diode has failed. It's under the dash on the green/orange white/red wires. A small black cylinder.

It could just be a dodgy relay, but this is one more thing to think about.
Roadwarrior

Check the leads on the starter solenoid. you've changed the exhaust manifold, you might have moved the connectors slightly causing a short cct. It can happen, don't ask me how I know.The above mentioned diode would be my next choice. Barrie E
B Egerton

Thanks for the ideas I will check this weekend to see what I can find.
JCH Hibbard

Had my battery tested over the weekend. It was five years old it was a Westco that I bought from Moss motors with 475 cold cranking amps. It was only showing 8 A on the test could be part of the problem.
JCH Hibbard

the battery could be a victim to the problem

as John Twist says batteries are the most over sold part on a car

the vast majority of "battery problems" are not caused by the battery at all, quite rare to have a faulty battery but they can me made to be faulty by faults elsewhere so whilst the battery is off the car being fully charged up check all leads, connections, earths all need to be clean, secure and protected

on a B the main lead from battery to starter is long and exposed, one poster found that under the plastic insulation the main wire had crud hidden by the insulation

recently I've had two mates change batteries, one on the advice of an emergency breakdown service that also sells car batteries when he got the car home went to fit a new battery that he'd bought elsewhere he found the battery clamp that was hidden from view was covered in crud he cleaned that off then fully charged the battery and two weeks later it's still hold that charge

the other mate is more relevant to your case, he has a classic with the battery in the boot, he put a new battery in but after a very short time it had no charge on installing its replacement he noticed a fat spark from behind the metal battery holder, the main live lead had been moved a few inches and had rubbed or nicked the insulation on the live lead so there was a short circuit, not seen before as it's a black cable in a black lined boot with a black painted battery holder but as he was putting the replacement battery back in his head and eyes just happened to be at the right angle to see the spark
Nigel Atkins

I can't see how the battery,good or bad can cause your problem.When you reconnect the battery does the ignition light come on ? If it does,there's is a short between the green & white ccts. If the 2 centre fuses on the fuse block get shorted in any way, that will also cause the problem. I still think the brake light diode is the culprit. Barrie E
B Egerton

Times 2, a weak battery cannot cause an engine to continue to crank, and if it is only delivering 8 amps it wouldn't crank anyway.

"short between the green & white ccts"

The green and white circuits are connected together anyway by the fuse. You may be thinking of the brown/purple and white/green circuits, which will be connected together if the 4-fuse fusebox is fitted upside down. However that will only cause the ignition to be on all the time, not cause it to crank all the time, which is what JCH originally said was happening.

One of the brake test diode short-circuit, a stuck starter relay contact, a stuck ignition switch contact, or a short between a brown wire and either the starter relay or the solenoid operate wires are the probably cause. If it stops when you release the handbrake (and the handbrake warning circuit is working correctly) then it is the diode.
Paul Hunt

This thread was discussed between 05/02/2014 and 12/02/2014

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