MG Car Repair: Ignition, coil wire, battery voltage

Question
QUESTION: The engine won't turn off when the ignition key is turned off.
Just replaced my alternator in my 1977 MGB. The pins on the alternator are the same. One is pointed, center is wide and the other is small square. I questioned the plug as it was plugged in upside down in the old alternator. The new one only will let me plug it in one way. Color code on plug. Brown/yellow stripe - center is heavy brown - other is small brown.
With engine off... When I turn the key, the ignition light lights on the dash. The car will then start normaly. The alternator seems to be charging the battery. When I turn the ignition key to the off position, the engine keeps running. I had to pull the coil wire off to stop the engine. With the key off, no power is present at coil.
  No problem with ignition key before I changed the alternator.

ANSWER: Hi Bob,
It could be that the wiring is backwards or even the problem could be in the alternator itself. I believe the charge light circuit may be the problem so do this, Remove the ign bulb socket from the back of the instrument. It has two wires, one it from the ignition switch that supplies power to excite the regulator /alternator into charging. Now you said it "seems to be charging". Meaning what? you didn't check it? Does the "ign" light go out when the engine starts? Put a volt meter across the battery posts and read battery voltage (12v +) then start the engine and read battery voltage. (13.8v to 14.5v normal at 1500 RPM) Now let it idle and turn the key off and when it still runs see if it will run normally and rev up normally. If so let it idle and with the key still off unscrew the bulb out of it's socket or if it is a push and turn type do that and remove the bulb and see if the engine shuts down and let me know.
Howard

---------- FOLLOW-UP ----------

QUESTION: I reversed the alterator plug as it was before I changed it. The IGNITION light does not come on now. Checked battery voltage off 12.5 volts. Got 14.5 volts at battery while running. Will the reversed plug hurt the alternator? Revs up and down OK . All lights bright. Holds charging voltage with lights on.
Key turns off engine now. Could not get my hand up behind the dash to remove the IGNITION light. any suggestions on how to get to it?  Thanks for your help!

ANSWER: Hi Bob,
Great, you fixed it. No need to remove the bulb now, that was just a test to see if the exciter wire was back feeding into the ignition to keep it running. The voltage tests you ran tell us that it is working correctly and now that the engine shuts down with the key tells us that the reverse of the plug was the problem.

The two wires that you reversed were, one was the "Ign" light exciter wire and a (+) power wire that the alternator used to sense battery voltage.

I doubt you did any harm as both receive power when running anyway but in a reversed scenario, the one that was suppose to loose power when the key was turned of still maintained power.

The regulator is working correctly and there is one more test you can do to be sure it is ok and that is a simple sets with a 12v test light.

With the engine OFF and everything on the car turned OFF, remove alternator wires and connect the clip of your test light on the large spade connector on the alternator and touch the test light probe to the large brown wire end. The light should not light. This is a test of the diode pack in the alternator.

If that is ok I don't see anything wrong now. Most of the MGs that I worked on had a formed plastic plug which could not be reversed and I don't remember any of the different alternators having the two outside pins reversed but that don't mean that there weren't any, as I am working with memories of 30 years ago.

All's well that ends well.
Howard

---------- FOLLOW-UP ----------

QUESTION: I was able to remove the ignition lamp fron the dash. I pluged the alternator cable in the way it was when the engine would not shut off. While running and key off, I removed the bulb. The enging did shut off. I know it seems to work fine the other way, but I was wondering about the Ignition lamp. It does not light at all when alternator wires are reversed. Should this be of any concern?

Answer
However, you had said that the charge rate was at 14.5 v which is correct. Something is wrong if it charges ok but there is no Ign light on when the key is on but not started. It sounds like you either have a wiring problem (which I told you how to check) or there is something wrong with that alternator or regulator.
Howard