>I recently replaced the alternator in my 78MGB with a rebuilt unit.. The
>charge indicator light is not working (with the old alternator the light
>wouldn't go out, so I know it works). Rather than take the unit back to the
>supplier, I would rather test it myself to see if it is charging. The
>method I devised was as follows: Using a voltmeter I testing the voltage at
>the cigar lighter and the trunk light, with the car turned off, the voltage
>was 12 volts at each device. With the car running I repeated the test, the
>results were then 13-14 volts. Can I assume these results indicate that the
>alternator is charging properly? Any advice will be appreciated.
>From the voltage readings I'd say your alternator is working properly.
As I recall, the indicator light is part of the circuit which feeds the
"field" windings on the alternator. When the alternator is running
properly, it produces enough voltage on the field terminal to keep
current from flowing through the light. If the alternator can't keep up
with the load, then the voltage at this terminal starts dropping, and
current begins to flow through the lamp.
On some systems (I think the MG applies here), if the alternator warning
lamp burns out, the alternator quits because the field current stops. I
think later systems use a resistor in parallel with the lamp to keep this
from happening. The point is that if you turn on the key, and the lamp
does not light, but the alternator begins charging when the engine
starts, then my guess is that your mechanic bypassed the lamp with
another wire.
As I recall, your alternator has three terminals. The "top" one is
smaller than the bottom two. The bottom terminals have 12V from the
battery. The top terminal is the feed from your lamp, and only gets 12
volts when the key is on. The main concern I would have is that *if* the
top terminal is getting 12V from the battery at all times, it would
eventually drain the battery.
Of course I could be wrong... it's happened once or twice before ;)
John Van Valkenburgh KE4QBR
Raleigh, NC
BTW, the alternator is currently (no pun intended) the only thing working
on my '70 MGB at the moment...
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