Post by Bob Noble
Maybe it wouldn't stop. That car was before my time and likely had an old
generator that I may have wrongly suspected needed the battery to charge
Bob, I'm beginning to suspect that we're on the same team here. Please read
this whole thing before you make up your mind. You've seen what static
discharge does to components. Read on and see what over-voltage does to other
systems in the car.
The old saying was that you have to have power to make power. You had to have a
battery to begin with but after you got it going, you could run without it. That
was true with later DC generators and was partially true with the AC generators.
I was a general line automotive technician during the time when America changed
from DC to AC generators. I received professional training from GM, Ford, and
Chrysler on their charging systems. They all strongly cautioned against
removing the cable while it was running, for the following reasons:
The older DC generators were the only ones that you could safely take the cable
off, for three reasons. They didn't have anything solid state to risk getting
cooked from a voltage spike, they didn't have enough capacity to make a lot of
excess voltage or amperage due to their limitation from a permanent magnet
setup, and it was only 6 volts to start with. The car with the blown light
bulbs almost certainly involved the secondary of the ignition coil.
The amount of electricity generated is limited by the strength of the magnetic
field and speed of rotation (actually the speed of the field vs the conductor
but let's not split hairs here). In DC generators the magnets were fixed to the
shell of the unit and the armature rotated inside the permanent magnetic field.
With these permanent magnet setups, it may have been possible to bump-start one
without a battery in place, I never tried. The only thing that could increase
the output was to rotate it faster which changed the amount of times that a
conductor was passed through a magnetic field. This, of course, had limits. If
you rotate it fast enough, the armature would sling apart.
Later DC generators used coils instead of magnets which increased the output
because the magnetic field was stronger than a permanent magnet. When these hit
the market, you could no longer generate power without a using battery to get
the system going. Some smaller industrial engines used the generator as the
starter, the starter drive being the fan belt. The permanent magnets were
replaced by very heavy magnetic coils that when energized by battery power,
caused the armature to rotate. There was a limited amount of space for the
magnetic coils and they wouldn't crank the heavier engines due to their limited
cranking power. They never got fast enough to sling it apart. Gear drive
starters used a much higher gear reduction in relation to engine speed and if a
starter drive hung in the flywheel when the engine caught and sped up, it would
sling the starter armature apart if it wasn't shut off pretty quickly. Most
later starters have overrunning clutch setups to try to protect them but if that
fails, you're looking at a new starter.
The AC generators, or alternators (GM used to call them Delcotrons) don't have
permanent magnets. The rotor is a couple of, for lack of a better term, cups,
that partially surround a coil of wire. They're actually multi-pole magnets
when in operation. The more poles you have in your magnetic field, the more
times it passes over the conductor to generate electricity. When current is
applied to this coil it magnetizes the cups which rotate inside the stator, a
mass of loops of wire. The more loops you have, the more power is generated
when the field passes over them. In the older GM alternators it was possible to
increase the output just by putting in a stator with more wire loops. Why they
made different output alternators was always a mystery to me, but they did.
The output of the alternator is varied by changing the amount of power supplied
to the rotor. If voltage goes up, the magnet get stronger, output goes up which
increases system voltage which increases rotor voltage which increases power
output until something releases its internal smoke*.
*electronic devices run on Internal Smoke. If you see something release its
Internal Smoke, catch the smoke and take it with the device to a qualified
technician. He will reinsert the smoke so it will work again.
<foghorn> That's a joke, son. Don't pop a diode. </foghorn>
It was a vicious cycle that kept increasing until something popped. Some
systems popped just as soon as the cable was pulled off. Some built heat for a
while and then something failed. Some degraded and decreased output until the
maximum voltage was below what it took to smoke something. Pulling a cable off
on an AC system is very harmful to the system. Eventually it will kill or
degrade the system until it will no longer maintain the battery.
Sure the main diodes are heavy duty but they still fail. That's why they're
replaceable. Now they come in a preset tree that must be replaced as a unit but
in the old days they were pressed into a couple of horseshoe shaped heat sinks
and could be individually changed on an arbor press. In some models they're
built in to the regulator and it gets changed all in one lump, along with the
Nobody rebuilds alternators in the shop anymore. You can buy a rebuilt one
cheaper than you can buy the parts and do it yourself. But I used to rebuild a
lot of them. I'd just replace whatever was bad. Now when you get a rebuilt
one, you get a new diode tree, new brushes on the slip rings of the rotor, new
bearings, and a new voltage regulator. You still have the same old rotor and
stator unless they're bad. I've seen many stators overheat and short together or
short to ground. I've seen a few burn open. I've seen one or two rotors fail
in either short or open condition but a bad rotor coil is quite rare.
The arguments about JD bulldozer systems don't apply here. We're talking about
Chevy car alternators which were designed to keep the service department busy.
The JD stuff was designed to keep working under harsh conditions and so was
heavy enough to run a long time in overload condition. That's why industrial
parts are so massive. They don't have any greater output than a car alternator,
but they're built to take a lot more abuse.
Due to cost cutting and planned obsolescence, car stuff is designed to be as
light duty as possible and so has a higher failure rate when abused. If they
built the stuff as heavy as tractor systems, the overall weight and bulk would
be a lot more, the cost would be a lot more, and the service department would
look like it was staffed with Maytag Repairmen. It's a sad fact that Detroit
makes almost as much money fixing their cars than they do selling them and we
all pay for it. That's why they try to keep repairs so mysterious or
complicated that you can't do it at home.
When some new system comes out, the public won't be able to buy a service manual
or test equipment for it until at least two years or until it is replaced by a
newer system requiring new manuals and test equipment. Part of the reason I got
out of mechanics was the ever-changing systems that were pushed on us in the
80s, complete re-designs every year, more schools to go to, more stuff to
remember, more test equipment to buy. We had to get our test equipment from the
manufacturer and that crap wasn't cheap but it was useless the next year.
Add to that some goober pulling off his battery cable and frying an engine and
transmission* control computer buried deep in the bowels of the dashboard
requiring a day of taking stuff apart just to reach...no wonder I have high
blood pressure. ;-)
*Since the early 90s or earlier, automatic transmissions have been computer
controlled. Gone are the vacuum modulator valves, the governor valve, most of
the valve body. Replaced by throttle position sensors, speed sensors, and other
electronic stuff that is very sensitive to over-voltage.
I'll say one more thing and then I'm done with the subject. Go ahead and pull
the cable off when it's running. Somewhere there's a mechanic who needs the
money. He can figure out why your fuel injection system started acting funny or
your transmission started shifting differently or not at all, or your timing
advance doesn't advance anymore, or your air conditioner stopped working, not
long after you "tested" your charging system.
But it ain't me, not any more.