MG Car Repair: alternator/electrical, starter solenoid, fuse box

Question
QUESTION: Hi,
Just bought a 1972 MGB and realized the alternator was failing.  All other electronics except for the engine temp guage were functional.  Replaced the ALT with a rebuilt LUCAS and now have the following problems:

Engine continues to run with key out and so does the gas pump...

All guages now failing...only wipers working.

Car will run normal with alt plug out drawing from the batt only.

I was told to try and replace the fuse box, does this sound right?

Thks,

Ken

ANSWER: Hi Ken,

Your symptoms don't pin-point the cause of your problems. If your car came to me in a shop I would do the following. Don't replace any part until you find what is at fault.

First run voltage drop tests on the power cables (battery to starter solenoid# and engine block to battery negative post #always test to the post not the cable ends)

If you are not familiar with "Voltage Drop Tests" of cables, let me know and I will go over it.

Check battery voltage (12+) and start the engine and set the idle at about 1500 RPM and check battery voltage (at the posts not the cable ends) (13.8v to 14.5v)

If I were to not find a problem up to this point I would run the engine and turn the key off and only remove the "IGN" light bulb (charge  light) to see if it shut the engine down.

If that didn't shut the engine off then I would take the wiring diagram and check all of the wires on the ignition switch to see that they are connected to ONLY the circuits that they are suppose to be connected to.

You are obviously getting a feed back from the alternator that is powering the ignition and fuel pump. The "IGN" (charge light) is powered by the ignition switch and grounded by the alternator when it is not turning. As soon as the alternator is turning it is excited by that current from the light and that triggers the alternator to start charging and when it does the connection then is (+) and since both sides of the charge light are now (+) the light goes out. Some MGs experienced enough current from the alternator to feed back through the light and into the ignition system (especially the electronic ignition systems) to keep the engine running. In these cases a diode needed to be installed in the charge light wire on the alternator side of the circuit to prevent back feed.
This is the test of removing the bulb when the key is turned off and the engine still running.

If none of this nets you the cause, you need to take you wiring diagram and start tracing all circuits especially the ones connected to the ignition switch.

If you don't have a diagram let me know and I will put one up on my web site for you to copy.
Howard

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

QUESTION: Hi,
Thks for your quick response. I have never done a voltage drop test(seems to be rather simple), but more info would be appreciated.  When you speak of theIGN light bulb, are you referring to the red engine/alt light on the dash board?  I read about installing a diode the other day, so things are starting to make some sense.

Kind regards,

Ken

Answer
The "Voltage Drop" test is simple. You are testing how much voltage loss is in the cable itself under load.

Put your voltmeter power lead (red) on the battery (+) post (not the cable end) and the (-) test lead (black) on the stud of the starter solenoid that the battery cable is connected to. If your volt meter has a range selection set it on a low voltage range. At this time you should read zero volts. Now have someone spin the starter. Now you will see a very low voltage. It should read below a half of a volt. If it reads higher than you have a poor connection somewhere in that circuit.

Now do the same with the ground circuit. Put the (+) test lead on the engine metal and (-) lead on the (-) post of the battery and hit the starter again and again you should only see a very low voltage.

Yes, on the red light that is the charge light. The charge light is the exciter circuit that starts the alternator working.

Let me know,
Howard