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MG MGB Technical - Very strange indeed

Howdy All,
Well I just got done autocrossing today and my 73 B started quite possibly the strangest behavior I ever seen in a car. After I shut the ignition off and remove the key I open the door to get out...then...the fuel pump starts ticking away and the alternator light comes on. When I close the door it stops. Open it again and it starts back again. Anybody have any ideas what might be going on. I'll be contacting a priest friend tomorrow to try an excorcism, after that I'm lost.

Thanks,
Scott
S J Kelly

Ignition switch is defective???
Frank Baker

Scott,
seems that changes had been done to the electrical system.
You shold check the purple colored wires and the white/black ones. If you use both of these to power a modern radio and have a car with a door alarm system fitted, the ignition might get power from one of the door switches when you get out of the car.

Ralph
Ralph

Scott; You should have two switches inside the driver door jam. With the door open push the switches in to find out which switch is is turning on the fuel pump.
The problem is probably in the ignition key audible warning switch circuit. The circuit ties into the ignition switch, sequential seat belt module (if you have one on your car) and associated switches. I would disconnect the PK (purple/pink) wire between the ignition switch and the door switch to see what happens. If that clears the problem it should be safe to tape any wire ends and leave it disconnected.

Clifton
Clifton Gordon

Could be a bad diode in the alternator.
On my '77, I had to install a switch to shut the engine off as welll.
Turns out the alternator diode. Now I have the Bosch conversion, no more problem.
I am really glad to hear you autox your 'B'
Me too.
but I'm down recovering from hip replacemnt, and my 'B' is down recovering from losing the oil filter.
Yes, I said lost!
Never again will I buy a cheap part!
SF
Dwight

DCM McCullough

Probably the diode in the seat-belt circuit gone short circuit. This has brown/red wires one side and pink/green the other. The diode is supposed to allow current to flow from the brown/red to the pink/green to allow the seat belt switches to sound the buzzer, but block current flow the other way. If the diode goes short-circuit current flows from the purple circuit (always on), through the door switch (closed when door is open) onto the pink/green, backwards through the diode onto the brown/red, through the seat-belt switch (closed when not fastened) onto the light-green/white, and through the gearbox switch (closed in neutral) onto the green. From there it is a mere hop, skip and a jump through the green circuit fuse onto the white to power the ignition and fuel pump.

Not to be confused with the other diode which when *it* goes short-circuit keeps the starter cranking until you drop the handbrake!

The alternator diode situation will leave the ignition powered irrespective of whether the door, seat-belt or gearbox switches are open or closed.
Paul Hunt 2

Dear Paul,
please say you at least consulted a diagram for this excellent information! Otherwise you make others, who try to solve a problem or two on this BBS, feel utterly useless. Priceless!
Willem van der Veer

I have looked at the wiring diagrams for 73-74 MGB's in the shop manual, Haynes and Advance Wiring diagrams and I cannot find a diode in any of these wiring diagrams. I do find a handbrake warning circuit diode and a service interval warning lamp diode in the wiring diagrams for later USA market MGB (75 or later). The car with the question is a 73.

Clifton
Clifton Gordon

I am glad I am stripping all that stuff out of my wiring when I redo it! Handbrake warnings and seatbelt warnings and service interval timers and buzzers. Far fewer things to go wrong then.
Simon Jansen

Howdy All,
Well, I guess the excorcism worked. I've been driving it since I posted the question and I haven't been able to get the problem to reappear. I guess now if it does I'll know where to look. Thank you every one for your advice.

Scott
S J Kelly

Your car is just whining 'cause you worked it so hard. Kidding...:-)
Tom

This thread was discussed between 12/06/2006 and 18/06/2006

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