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MG MGB Technical - Running on after switch off

It's not too bad a problem, it just the car won't turn off, it doesn't want to quit. I have a 1979 MGB LE. The problem is that when I turn the key off, the car keeps running (not just for a short time). I have checked the ignition switch, which looked like the problem at first, as inside one of the contact bars had been worn a little and push out of position. I cleaned it up put in the missing ball bearings (that may had cause that problem) and push the contact bar into place. I put the switch in place and the car started ,but still did not stop. I then got a ignition switch from another car that was working fine , but it did not shut off the car. I check the relay, and switch it from another car and still the car won't shut off. Could it be a grounding problem? The problem did start after another problem happened. I not sure how it maybe related. The first problem was I was driving along when my gas line came off at the fuel pump, I had pulled off the road, and went to shut the ignition off and it wouldn't(for the 1st time). So then the engine had stop (with no fuel), but the fan and the fuel pump where still going. I stopped the car by disconnecting the battery. I later read never stop the car by disconnecting the battery, but I hoped that the engine being off will make it ok. Any help would be appreciated.Thanks,
Gordon Thompson

This is due to the electrical trickery in the later cars. I can't remember the solution but it has been documented here before. Try the BBS archive or Paul Hunt's website mgb-stuff.org.uk and click on "Spanners" then follow links. I think he knows about this one. Regarding disconnecting the battery, if the motor is running and you disconnect the battery, you can fry the alternator diodes as it shouldn't operate without the battery load across it. As your motor had stopped, you won't have done any damage.

Mike
Mike Howlett

I had this problem on my 1977 MGB and thought is was unique to this model year, prehaps I'm mistaken. In a 1977 it has to do with an electrical design flaw that is compensated for by the emission system, particularly the anti run-on value. If the system has a problem, and the anti run-on valve isn't/can't doing it's job, the car keeps running. I got around this by moving one of the white ignition wires from the fuse block to the adjacent ignition relay. The specifics on the change is in the archives.
Brian Denis

This thread was discussed on 23/09/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.