In a message dated 2/12/98 12:15:29 AM Eastern Standard Time, boballen@sky.net
writes:
> If you wish to correct me when I'm wrong, get in line.
Bob,
Far be it for me to correct you! However............(you knew there was a
however, didn't you?).
Everything you said was correct, but, you left one thing out. You said "On
the other hand, if something becomes amiss in the voltage regulator, the
Brown/Yellow wire does not see a ground but finds voltage." This is true, but
the Brown/Yellow wire also finds voltage when everything is working as it
should.
The alternator doesn't know that you've shut off the engine. It has no way of
sensing when the key is turned off if it is turning at an rpm above its cutout
speed. If you were to turn of the ignition at the top of a long hill and
leave the car in gear, coasting to the bottom, the alternator would continue
producing voltage as if nothing had changed until the engine speed drops to a
point where the alternator output drops off. On the other hand, if the
alternator is at rest when the key is turned on, it gets a very clear signal
that the key has been turned on.
How can this be? How can the alternator know when the key is turned on, but
not know when it is turned off? OK, I'll 'splain. With the alternator at
rest, it is producing no voltage. Turning the key on applies about 12 volts
to the internal regulator in the alternator. This sends a wake-up call to the
alternator, by energizing the regulator. The regulator is set to adjust the
output of the alternator to about 14 volts. Since the sensing input to the
regulator sees only battery voltage at this time, which is still about 12
volts, the regulator cranks up the field current in an attempt to get the
alternator to provide the requisite 14 volts. Of course, since the alternator
is not spinning, nothing happens.
As soon as the alternator comes up to speed, it begins to produce the normal
14 volts. This 14 volts is applied internally to the same terminal as the
lead from the warning light, which applies it to the regulator. With 14 volts
on both sides, the light goes out. Now, when the key is turned off, the
alternator continues to produce the 14 volts, at least until the engine and
the alternator speed drops off. This 14 volts is applied to the regulator,
just as the 12 volts was when the key was first turned on, and the alternator
keeps on producing. As long as the speed is sufficient, the alternator will
continue to crank out the voltage.
There is a fairly decent description of the workings of an alternator at
http://www.vtr.org/maintain/alternator-overview.html.
Since the alternator continues to provide voltage to the warning lamp after
the key is turned off when every thing is functioning properly, no failure in
the alternator that causes it to produce voltage when it shouldn't could cause
the car to run-on. The warning lamp isolates the alternator voltage from the
remainder of the car's wiring, regardless of the condition of the alternator.
The failure that is causing Bob's car to run on has to be on the ignition side
of the warning lamp, and not on the alternator side.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
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