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rick_ingram

rick_ingram

Yoda
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From Jim Stuart:

A friend of mine has a stock 1978 MGB that often will not shut off when the ignition is turned off. This is not uncommon with late model MG's and is caused by a wiring anomaly allowing the alternator to back feed and provide an ignition source.

I have forgotten the simple solution which involved adding a diode to the alternator wiring. Hopefully, someone remembers what wire to cut, what diode.

Remember, this is a stock Lucas alternator.
 
Not sure about the B's - so, wait for this to be confirmed, but, on Midgets, two red/white wires from ignition, one goes toward coil the other to diodes under the dash above the glove box - cut the diode side.

OR, what I did was disconnect the brake warning switch on the brake fluid splitter.
 
The diode can be fitted in the brown/yellow circuit between the alternator and the ignition warning lamp.Diodes 1n4002 or IN4004 can be used.


Stuart. :cheers:
 
Also received this from Paul Hunt:

Paul Hunt 2010, West Midlands, United Kingdom, paul.hunt1@blueyonder.co.uk_https://www.mgb-stuff.org.uk
Adding a diode is a bit of a frig and removes one of the diagnostic features of the ignition warning light. This problem only occurs if there is something wrong with the emissions plumbing or anti-runon valve wiring. If the emissions system is still fitted then the problem with this should be investigated first, I suggest.

The problem was corrected immediately on UK cars as they didn't have the valve to stop the engine in lieu of switching off the ignition. Originally for 1978 this was done by moving the white warning light wire from the ignition switch i.e. the ignition relay winding (which is the source of the current to keep the relay operated). But for 1979 they made another change, this time because they were having problems with ignition relays sticking on for other reasons, they moved the coil connection from the ignition relay contact to its winding. This would have done the job on its own i.e. without moving the ignition warning light wire, as is the easiest change to make.

I'm pretty sure that the white/brown wire comes from the ignition relay to the fusebox, with a second white/brown wire in the same spade connector teeing off which is feeding the coil. There should be a separate white/brown spade connector beide the first, which again I'm pretty sure feeds the cooling fans via an in-line fuse. If this is the case then you will have to cut one of the white/browns at the spade connector that contains two wires, and see if that is the feed from the relay or the fed to the coil.

If it is the one for the coil you are in luck, simply move that to the white on the ignition relay which should be terminal C1 or 87, and push the other wire with its original spade connector back onto the fusebox. Even though the current through the ignition warning light will still be there, the low resistance of the coil will pull the voltage down below that at which the ignition relay can remain operated.

If the wire you have cut off is the feed from the relay, you will have to fit a new spade to that and put it back on the fusebox, and conenct the other wire to the white at the relay.

If you find the single wire is the one that is feeding the coil, then simply connect that to the white on the relay.
 
I used Paul Hunt's fix on my '78 and it definately solves the problem. FUnny thing is, I never had that problem. Then one day, after doing something totally unrelated (installed a new fuel pump) the engine wouldn't shut off! I remembered seeing Paul Hunt's explanation online...so I searched it out.
 
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