following is mostly based on Dave's experience
with the replacement
of the alternator in his '73 Super
Beetle, with input from Rob and others,
questions and answers added as appropriate.
material is subdivided as follows -
-- General Discussion --
A technical point
to begin with to clear any confusion -- generators and dynamos
are the same thing - direct current generators. An alternator
is different -- it produces alternating current, which must
be 'rectified' to direct current before it's used to charge
the battery etc. The word 'dynamo' is often used in the UK,
but not so much elsewhere -- most other countries call it a
generator has the same diameter down its entire length, between
the pulley and fan housing. The alternator is the same diameter
to the generator in the area where it sits on the support stand,
but it is larger in diameter at the pulley end, where it overhangs
the support stand.
common problem -- the battery goes dead. It has no charge, but
when the battery is 'jumped' the engine will start and continue
problem almost everyone has experienced -
I left my parking lights on all day (it was raining on the way
to work) and the battery was 90% flat when I got in the car
(dim dash lights - no cranking). Luckily the car park is on
a slight slope so I was able to push it, jump in and pop the
clutch, and away I went. I don't think the battery really had
enough volts for the ignition, but with generators, you only
need a couple of volts for it to self-energise and so if the
clutch popping gives you enough rpm the generator immediately
runs up to it's 14.5 volts and the coil says "great - sparks
are on the way". The alternators won't do that - they need almost
the full 12 reference volts (via the blue wire and the dash
light) before they can start generating power. So you are more
likely to need a jump start to get 12 volts from a good battery.
In other words, generators are more tolerant of the sick/flat
new battery may solve the problem of the battery going dead
for a while, but then a few months later the car won't start
again. If jumped it works fine, but the battery doesn't seem
to be getting a charge.
faced with this situation, take an electrical meter (see our
discussion of VOM operation) and first
measure the voltage across the battery. With the engine off,
set the meter to "DC volts" (15 or 20 volt range), then simply apply
the two leads from the meter to the two battery terminals. The
reading for a fully-charged battery is about 12.6 volts. A dead
battery will read ten volts or less.
check the output of the generator/alternator (that is, to make
sure a charging current is going to the battery), start the
engine and read the voltage across the battery terminals as
before. As the engine speeds up, the voltage should increase
to about 14.2-14.5 volts. If it does, then the generator/alterator
is working properly and charging the battery as it should.
you don't have a meter you can try the following (less effective)
test: remove the positive terminal from of the battery with
the engine running. If the engine continues to run, the generator/alternator
is okay. If the engine dies, there is a problem in the electrical
Some have suggested disconnecting the wires from the (D+) and
(DF) terminals on the alternator/generator, then connecting
an ammeter across the terminals to check the operation of the
generator/alternator. To do this you would need to put a high-amp
ampmeter in series with the main red wire (D+) to read the amperage
going into the battery. The simple tests above are the easiest.
the alternator/generator test shows an output of less than 14
volts or so, check the voltage regulator before writing off
the generator/alternator. If you have an external voltage regulator
it with be under the rear seat on the left.
Up until 1967 the voltage regulator was located on top of the
generator. From 1967 until 1974 it was located underneath the
rear seat on the left. From 1974 on the voltage regulator is
internal to the alternator. If the 6-volt Bug has been converted
to 12 volts, the 12-volt regulator is usually screwed to the
fan shroud just above the generator so the existing wiring can be used.
function of a voltage regulator is to maintain a precise voltage
regardless of the current drawn by the load. With meter switched
to DC volts and probes on positive and negative terminals of
the battery, read the voltage as before (again, should be a
bit more than 12 volts). Then start the engine but don't rev
it up. Slowly increase the engine speed to maybe 2000 rpm. The
voltage should climb up to 14.2-14.5 volts and remain level
there. If so, the alternator/generator and voltage regulator
the voltage does not go up as the engine speed is increased,
check to see that the generator brushes are not worn excessivly.
If you can, apply a little pressure on the brushes while the
engine is running and see if the generator lights goes out or
the voltmeter reading increases. If it does, the brushes may
be worn to the point that they don't exert enough pressure on
Jim gives a good method for testing
a generator on his Web site.
if the alternator/generator and voltage regulator are both okay,
it is likely that there is a short somewhere in the system that
is draining the battery. Test for this as follows -
the positive cable off the battery.
the VOM meter between the positive battery cable and the positive
post on the battery.
set to DC Volts, the meter will respond to the slightest drain.
If it shows a drain, switch to DC milliamps. Any reading over,
say, 5 milliamps is suspect.
that point you must disconnect things to find what is causing
the electrical drain. Start by removing the fuses one by one
until you find the culprit.
the circuits associated with the offending fuse to find the
the car has been left out and not run for a period of time,
electrical problems could result. VW used fairly simple connectors
in the electrical system, and quite often just shining up the
contacts can fix problems. Connections to consider include the following -
the case of a generator or alternator, clean the contacts
on the generator/alternator itself, and then the regulator
contacts under the back seat, left-hand side.
is a heavy red wire from the regulator to the battery on the
other side. Brighten up the connectors on both ends of this
wire. If your car has had an alternator installed, you might find that a previous
owner has taken a heavy red wire from the alternator direct to the starter motor,
(connected it to the battery cable from the + post on the battery)
rather than through the wiring harness to under the rear seat.
This usually works fine, but it does mean that the battery charging wire
is subject to the dirt and grime under the car.
your car has no regulator under the seat, it's a later alternator
model which has the regulator inside the alternator itself.
On these models, there should still be a heavy red wire tracking
from the left side across to the battery, and this might have
some connectors on it -- clean them up. With the stock VW wiring, this is the wire which
charges the battery, and also supplies the car with power
from the battery.
look at the heavy red wires in the luggage compartment. The
red wire from the wiring loom on the left side leading to
the headlight switch is the main power lead, then two red
wires come from the headlight switch -- one to the ignition
switch, and one straight to the fuse block (this one works
those electrical things which operate even with the ignition
off, such as interior light, radio etc). Make sure all these
connectors are clean so the power can get where it needs to.
is one additional wire (a thin one) from the alternator to
the "Gen/Alt" indicator light in the instrument cluster. It
is essential that this wire be in place -- otherwise the alternator
cannot charge. On alternator cars, even if the indicator light
burns out, the alternator can't charge -- it uses a tiny signal
current through this bulb to activate the alternator.
It's not the same situation with the older generator
models -- if the indicator light blows it can still charge.
are often asked regarding the need to polarize a generator.
If the generator is charging (the battery stays charged) then
you don't have to polarize it. When a generator has been left
unused for a long time (months or years), the metal core looses
it's magnetism, and all generators NEED a little residual magnetism to
start the charging process. Polarizing just gives it enough
"boost" in magnetism to start it charging properly. When it's
being used, it keeps its core polarized so there is no need to do
Why do generators need to be "polarized"? Automobile generators
need some magnetism to get started. This "residual" magnetism
remains in the field pole pieces even after the engine has stopped.
The next time the generator starts up, the residual magnetism
creates a small voltage in the armature windings. Not enough
to charge the battery, but enough to allow the field windings
to draw current. As the field current increases, the pole pieces
create even more magnetism. That makes even more voltage in
the armature, and the cycle continues until the generator is
capable of producing maximum output.
happens though to a generator which has been stored a long time
or is freshly rebuilt? The residual magnetism may have decreased
to the point where it can no longer get the generator started
producing voltage. In the case of a new generator or one which
has been mis-treated, the residual may even be of the wrong
direction (North and South poles reversed). Polarization is
a simple process used to restore the field pole residual magnetism
and ensure the magnetic direction is correct.
polarize a generator, connect a jumper wire from the (DF) terminal
on the generator to the generator frame. Remove the fan belt,
then connect a wire from the positive terminal on the battery
to the (D+) terminal on the generator. The generator shaft should
start to spin.
Don't run the generator this way for more than a few seconds
to avoid overheating.
generator will now be properly polarized. If the generator did
not spin during this process, the generator is most likely
the fan belt back on and re-test the generator voltage with
the (DF) terminal grounded. If the output voltage is still low,
the generator is defective.
Voltage regulators do not need to be polarized -- they are not
polarity sensitive. Even if voltage regulator came with instructions
to polarize it -- these instructions actually polarize the generator,
not the regulator. The regulator manufacturer simply wants to
make sure that your generator will work properly so you don't
blame the voltage regulator.
alternators never need to be polarized. Alternators use an electrical trigger
current to start the process of charging - electromagnetism,
so don't need any internal magnetism to begin with, the way a generator
trigger current is supplied via the alternator dash light (it's
a tiny current and won't make the light glow when the engine
is running) so if that bulb blows - the battery won't get charged
and it will go flat. Curious circuitry VW used, but it works
just fine. So that means that with an alternator equipped VW,
you should always glance at the dash as you turn the key and
make sure the Alt light does glow with just the key on, just
to make sure that that circuit is intact.
Miscellaneous Questions and Answers
Our Generator Troubleshooting
article may be useful.
- I have a 1972 Beetle with a red generator light permanently
on. I have changed the brushes and regulator, and still a red
light. A problem with further fault finding is the wiring isn't
all original, lots of do-it-yourself stuff. I have checked continuity
though and all seems fine. But could you please help deciphering
which contacts are which -- in my manual (D+), (D-) and (DF)
are all close together, yet well labelled. But unfortunately
the connections on my generator are positioned differently.
Two towards the pulley and one towards the front (fan). Do you
know which are which?
responded - Does the battery go flat, or is it keeping
its charge? If it's going flat, then the red light is telling
the true story; but if the battery is being charged, then the
light is telling lies and IT'S at fault.
my '71 engine, there are only (D+) and (DF) connections - there
is a black plastic connection block with two connectors for
wires. Some models do have a separate (D-) connection -- this
is the one nearest the fan, and should be a brown wire.
(D+) connection has the thick red (or red/black) wire attached
to it -- it should be a large spade connector on the left side
of the connector block (in earlier years is has a brass bolt/nut
type connection). This is the main charging wire.
(DF) connection (usually a thinner green wire with a smaller
spade connector) is on the right side of the connector block.
The (DF) connection supplies power to the field windings to
produce the magnetic field inside the generator.
(D+) red (or red/black) wire goes to the upper regulator (under
the back seat) connection on the engine (rearmost) side of the
regulator. The (DF) green wire goes to the lower connector on
the same side of the regulator.
the front end of the regulator, the upper connection (#61) should
have a thin blue wire attached. This blue wire goes to the "Gen"
warning light in the instrument cluster. The lower connector,
(B+), has two thick red wires -- one to the battery and one
to the headlight switch up front (which is used as a junction
box for the main power supply up front). There may also be a
thinner red wire connected there too -- I think that one goes
to the headlight dipping relay up front.
- I want so much to have the generator of my Volkswagen reconstructed
or have the rewinding done.
responded - The VW generator can be rewound by any
competent automobile electrician -- it works the same as any
older style generator, and most auto electricians can still
- I've heard that generators needed a higher rpm to charge the
battery, like cruising speed, but that alternators need only
the idling speed to charge the battery.
responded - That's partly true. Alternators can charge
the battery at idle rpm because they usually have a smaller
pulley, so in fact they spin faster than a generator for the
same engine rpm. That means they will charge the battery at
lower ENGINE rpm but the alternator itself is spinning faster
than a generator does.
VW generator is called an "early cut-in" generator, so it starts
to charge the battery at speeds just above idle - from about
1000-1200 engine rpm. It will provide more current at higher
rpm of course, up to it's rated maximum of 30 amps (for the
- In the case of a failing generator, would it be wise to convert
to an alternator instead of having the generator repaired?
responded - You can do that if you want to, but it
requires a little alteration to the car's wiring. Speedy
Jim has the necessary changes to the circuits on his web
- Does a few Amperes make a difference?
responded - The higher amperage of the alternators
makes no difference at all if you have stock VW electrics --
it's only useful if you have spot lights, a big stereo and stuff
like that added.
- The alternator that by Beetle has is rated at 45 amperes,
for Toyota cars. There's one available, specifically made for
the Beetle, rated at 51 amperes. Physically, the 51 amp alternator
looks about a third bigger and much heavier than the 45 amp
alternator. Maybe it gives more power? Will it put less strain
on the battery?
responded - The beetle's wiring will handle the VW
50-amp alternator OK, but if you use a higher power alternator
than that ('75 VW alternators are available) then you might
need to increase the wire size for the charging circuit to the
bigger size alternator won't reduce the load on the battery.
It's main load comes from using the starter motor, and that
won't change. A bigger alternator will recharge a flat battery
faster of course, but the 30 amp generator does work just fine
for most old VWs. The 50-amp alternator was added in 1973 when
they added fresh air fans and other extra electrical items to
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