MG Car Repair: Alternator works on bench, not in MG, bench tests, lucas alternator

Question
Dear Barrie:

The battery in my 1980 MGB (US market) has a sufficient charge, and my Lucas alternator has recently been rebuilt.  Two bench tests show that it's generating about 13.2v.   
However, when I recently installed in my MG, it wasn't charging.

I discovered that the exciter wire (brown/yellow) going into the alternator was not carrying a charge so I jumped it with a lead from the battery, but the alternator still did not
generate a charge. My B was running off the battery, so I've parked it in the garage.  

This leaves me with a couple of questions.  First, despite the fact that the alternator checked out OK, could there still be a problem with it? (I've ordered a Bosch, just in case.)
And second, if I wanted to trace the fault in the exciter wire, where is a logical place to start?

I appreciate any attention you may give to this.
Thanks,  John

Answer
Hi John.  Alternators do not require any exciting.  The Brown/Yellow wire is an output from the alternator, not an input.  It is only intended to go to the ignition warning lamp.  Connecting a battery to this wire may have done permanent damage the alternator.
  
Just because an alternator is producing 13.2 volts doesn't mean that there is nothing wrong with it.  Your alternator is a 3-phase current generator.  If one of the phases is blown, it will still be able to produce 13.2 volts, but it will not be able to produce much current.  A good alternator can generate 45 amps.  With just one phase blown, this drops to less than 10 amps.  You need to do a load test, not a voltage test.

There is a diode pack inside the alternator.  My guess is that one or more diodes inside the diode pack have blown, so the entire unit requires replacing.  There is also a voltage regulator unit inside the alternator.  This is a sealed unit.  Either one or both of these components may have failed.  Unless you are a competent electrician with all the right test equipment, you would be better off exchanging the alternator for a replacement unit.  

Fitting a Bosch alternator would be fine, but be careful which one you buy, because the wiring may be different or the drive end may not fit correctly into your pulley.  It depends which Bosch unit you purchase.  Try to buy one with slightly higher output, say 55 amps.