MG Car Repair: Alternator Replacement, brown wires, loop wire

Question
QUESTION: Hi Barrie, this is more informational than a question since I couldn't find any thing on the web on this.  I hope you don't mind.

I recently bought a replacement alternator for my 72 MGB.  This alternator has a 3 terminal connector and came with instructions on how to convert from the original 5 terminals.  The instructions said to DISCARD the 'Link Wire' and connect the IND (indicator) wire to the small terminal, and two the brown wires to the larger terminals.

After a heck of a time, scratching my head messing around with the alternator, I realized the small terminal leads into the voltage regulator and thought to myself, how the heck does this know the current voltage needs when there is no current going into the regulator, to check.

When I energized the small terminal (with a battery charger), BAM!  I got the alternator to produce 14+ volts (and resistance trying to spin the pulley).  It was then, I realized what the Link Wire function was for.

The 3 Terminal Alternator configuration should be, Brown wires (Main and B+) on the 2 larger terminals, IND wire to the small terminal, and the LINK wire on the B+ and IND.

I haven't installed this yet in my car, but the numbers work on my bench.  Any thoughts?

Thanks
Joe

ANSWER: Joe, you should never energise the small terminal of the alternator with a battery charger.  You stand a good chance of burning out the voltage regulator inside the new alternator.  

The new alternator does not measure current.  It only measures voltage.

If you inspect the new alternator you will see that the two large terminals are joined together inside the alternator, so in effect there are only 2 terminals now.

The advice you were given is correct.  All the Brown wires go to the large terminal(s) and the Brown/Yellow wire connects the small terminal to the ignition warning lamp.  

If you look at the diagram I sent you earlier you will see a loop wire which is also coloured Brown/Yellow.  This should be removed from the wiring loom and discarded.



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

QUESTION: Then I am confused. You are correct with how both larger terminals are connected. But in the orginal configuration, the loop wire connects the IND to the B+.

If I don't have that connection (with my new alternator), I get the normal .25 no load votage.

Alternatively, I have heard about the need to 'polarize' ones alternator.  Do you think that is essentially what I did?

ANSWER: The original wiring used the following terminals:

Thin Brown to B+ (power supply to the car)

A Brown/Yellow loop between two terminals (both marked IND), and from one of them (either one) directly to the charging INDicator lamp.

Thick Brown to + (power supply from the battery via the starter)

Nothing to -

Your replacement alternator has two large terminals for the two brown wires
and a smaller terminal for IND.

Your alternator does not require polarizing and cannot be polarized.  It only works when connected via a negative earth.   
 

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

QUESTION: I really apologize here, I don't want to beat a dead horse.. but...

I can only tell you what is happening with my config.  The Alternator will put out power, ONLY after I energize the Ind terminal.  I don't need to keep the Ind terminal energized the entire time, I found once I to it,  it charges fine until I shut off my engine...

...this totally brings me back to my old question on the starter.  You got two poles on the solenoid, one goes to the battery, and one goes to the starter itself.  That second pole (the one to the starter) USED to have one of the brown wires which lead to the original 5 terminal alternator.  Remember, that second pole is only energized while the starter is activated.  

So, since I need something to be hot just once, I believe this is my answer...

I appreciate your patience, and reviews.

Thanks
Joe

Answer
No need to apologize.  I can see exactly what you are suggesting, and it sounds good.    

I found the following in a Lucas manual:  

`At first, when the ignition is switched on ... a small pilot current from the battery passes through the ignition warning light ... and the rotor windings.  This current ensures that the alternator voltage builds up rapidly.'

Remember, however, this is only applicable to your old alternator.  Your new one does not require any excitation.