Engine Run-On Mystery
Bob Mason
Fairhope, AL
A friend had a problem with his 1977 MGB - it would often continue to run (quite smoothly) after the key was turned off, especially when warm.
After racking our collective brains and trying all sorts of things (or as the English say, attempting correction by substitution), I finally said that we'd have to call John Twist. I called, and even before I had finished trying to describe the symptoms, he said he had an answer for me.
In some cases (obviously not all), the alternator will put out just enough of a trickle charge, even at idle, that the contacts of the electric radiator fan relay will stay closed, allowing power to the coil and keeping the engine running. The trick is to pull the one-way diode out of the wiring harness just behind the glove box, reconnect that wire and install this diode in the brown/yellow (small) wire going to the alternator plug, with the arrow pointing to the alternator.
Voila, no sooner said that done...the engine now immediately quits when the key is turned off, no matter the engine temperature.
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