Larry,
You don't say what year your car is, so this may not apply to you. On later Bs there was a wiring flaw that was masked by the anti-run-on setup that was intended to stop Dieseling. It also masked the flaw, which was that the ignition system could remain energised whilst the ignition switch was off. Here's a description of the problem I grabbed from the MGOC bbs:
Quote: "Paul Hunt, Solihull, UK, http://www.mgb-stuff.org.uk paul.hunt1@virgin.net
If the ignition warning light comes on when you turn the ignition off then that *proves* the switch is working as it should. The problem is simply as has been said many times before, leakage back through the warning light from the alternator and nothing to do with Dieseling or faulty ELC plumbing. Because this era of North American car is wired the way it is this leakage holds the ignition relay operated and hence the car continues to run normally. The quick fix has been to put a diode in the brown/yellow from the warning light to the alternator to block this leakage current but this has the side-effect that it removes one of the diagnostics available if your engine should suddenly cut out whilst bowling along. UK cars never suffered from this problem because the warning light (and incidentally the fuel pump and overdrive) are all wired to the ignition relay contact (white/brown) and not the operate winding (white). Note that the wires to the afore-mentioned items are still white but they are wired back to a white/brown. The best fix is to alter the North American wiring to do the same, the schematics for both UK and North American cars are on the above web site ('Spanners', 'Electrics', 'Schematics' and 'Ignition') I am sure you can puzzle out what needs to be done from those. The diode quick-fix is also shown.
There has been a note that the fix is to disconnect the coil from the relay contact (white/brown) and wire it to the winding (white) instead and this will indeed stop it happening, but since the ignition relay was provided to greatly reduce the electrical load on the ignition switch and since the coil is about the greatest consumer of current on the ignition circuit one is just likely to cause premature failure of the ignition switch.
PaulH. "
