Short Circuit

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Richard G
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Joined: Fri Sep 28, 2018 4:35 pm
Forename: Richard
Surname: Gormley

Short Circuit

Post by Richard G »

Hello

I have something of an electrical mystery on my 1969 MGB Roadster and would appreciate any suggestions. Symptoms are:

Starts and runs fine however the following don't work at all and as soon as the switches are operated the revs drop dramatically and the car struggles to run:

Indicators - only one faint click on the flasher unit then nothing
Brakes
Reversing Lights
Heater
Wipers
Rev Counter
Fuel Gauge

It seems that the ignition switch related circuits are somehow shorting. Have checked fuses, battery earth and general connections but it is a bit of a confusing set of symptoms that I am not sure what to look for next

Any help greatly appreciated

Richard
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Charles Farran
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Location: Warwickshire

Re: Short Circuit

Post by Charles Farran »

Hi Richard,

Not sure if your car has a 2 or 4 fuse box as the change over was i believe circa 1969. Be that as it may, as i assume from your post the fuses haven't blown, then you possibly have a poor earth somewhere. Have you checked the condition & connections of the earth strap from the engine block to the chassis (and if there is one from the gear box to the chassis)? There is a possibility that when you complete another circuit the route to earth is via the accessory you have turned on . If you haven't already done so check the condition of the earth strap to the battery(ies) with regards to corrosion.
Cheers, Charles
1980 Roadster
David Ramsbotham
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Location: Taunton, Somerset

Re: Short Circuit

Post by David Ramsbotham »

Sorry Richard don’t want to hijack your post but Charles, I have heard a lot of issues can be caused by poor earthing especially block to chassis strap. My 1975 car doesn’t appear to have such a strap — should it have?
Thanks David.
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Paul Scott
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Location: Milton Keynes

Re: Short Circuit

Post by Paul Scott »

David your 75 should have 2 as per Charles comment.

Block to chassis & on the gearbox mount to chassis. Mine on the gearbox is on the drivers side RHD tucked up above.

I ran extra earthing from Battery chassis connection, onto gearbox support plate (bolted to chassis) then from here to block. I metered all conections to ensure they were doing the job.

Regards
Paul
1975 MGB Roadster
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Richard G
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Re: Short Circuit

Post by Richard G »

Thanks Charles,

That's very helpful. I'm getting on to it tomorrow so that gives me somewhere to start

I'll let you know how I get on

Thanks again

Richard
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Charles Farran
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Re: Short Circuit

Post by Charles Farran »

It has been known for the earthing from the block to go via the accelerator or clutch cable when the earth strap is defrctive / missing. My engine earth strap goes from the top nut on the back plate to the mounting bracket on the oil pressure union on the flat face of the driver's footwell. (My 1980 only ever had one earth strap as indicated above , although i believe earlier models had them on the gear box as well. An additional one on the gear box won't do any harm , especially given that the overdrive solenoid route to earth is via the gear box! The latter will also help given an ageing chassis / resistance in getting the earth return back to the battery).
Cheers, Charles
1980 Roadster
Richard G
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Forename: Richard
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Re: Short Circuit

Post by Richard G »

Excellent Charles, thanks so much for your help

Richard
Richard G
Posts: 7
Joined: Fri Sep 28, 2018 4:35 pm
Forename: Richard
Surname: Gormley

Re: Short Circuit

Post by Richard G »

Charles,

Great advice, problem solved. Loose earth seems to have done it. Managed to dislodge bullet connector for the fuel pump lead in the process so spent a good couple of hours wondering why the car wouldn't start! - Still, got there in the end

Anyway, thanks for your help

Richard
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Charles Farran
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Forename: Charles
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Location: Warwickshire

Re: Short Circuit

Post by Charles Farran »

Hi Richard,
That's great news.
The connections between the body harness & the main harness on the inside wing are prone to corrosion & even the youngest cars are 38 + years .If yours haven't been claened / replaced for some time it may pay you to have a renovation exercise with them this autumn / winter if you are putting the csr away for a few months. The connections behind the front grill are even more exposed to the elements.
Cheers, Charles
1980 Roadster
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