[Vwdiesel] Alternator wires

[Sandy Cameron]

well, lets think this through.

When you switch on the ignition switch it connects 12v from the battery to
the instrument cluster. In all A2's (not A3's) the indicator is an LED, so
can only light when the alternator side is lower in voltage than the
"ignition" side. They are a one-way device and will not light if the
unballance is in the other direction.

There is a current limiting resistor of a fairly high value (500 ohms?) in
series with it that protects it from high currents in either direction.The
current through the LED is not enough to start the alternator charging by
itself. ([ Alternators, unlike generators/Dynamos are not able to self
excite from residual magnetisim, and require current from the battery to get
"excited"])

In the A2 VW, there are 2 other resistors, in parallel, (just for a size
reason, one big one could do it too) from the 12v ign bus, located in the
cluster beside the LED resistor, that connect to the blue alternator wire.
They permit sufficient battery current to flow to the exciter terminal on
the alternator, to get it charging once started. They are there for 2
reasons. First, so the voltage on the alternator side of the resistors will
be low enough that the LED will light before starting,  but high enough to
get the alternator charging. second, they limit the current in the circuit
when the key is on, but the engine is not running.

At the alternator, where the blue wire arrives, It connects directly to the
"upper" (hot) end of the field winding. also, internally, there is a set of
small diodes that take current from the stator windings (the charging
windings) so the alternator can excite itself as long as it is running.
These are connected to the inner brush on the regulator.
There is a contact point on the regulator that presses against a spring tab
inside the back of the alternator. This is where all the above come together.
There is one more connection at this point, the sense line for the regulator
circuit.

The other end of  the field (rotor) winding comes out on the outer brush of
the regulator, and thence to ground through the transistor regulator.

The alternator and its regulator are a nation alone, that produces about 14
volts when running, regardless of what conditions are like outside itself.

If the wiring to the battery, including the ground return, have any
significant losses in them or their connections, the battery will not get
all the voltage/current it needs to remain healthy.

The elves in the black forest did a reasonable job of getting the + side to
the battery, but the -- side is a travesty of dissimilar metals, totally
exposed to the elements. Brass terminal rings secured to aluminum castings
with ferrous fasteners. And a ground cable several times too thin for
starting a diesel engine.

My A2's get a treatment of #2 (6 volt) ground cable direct from neg battery
to one of the starter mounting bolts.

Also a #6 from one of the case bolts on the alternator, direct to the neg
battery clamp bolt. Although there are 2 brown wires attached to the neg
terminal clamp bolt intended to ground body circuits, its wise to use the
original ground cable too, to ensure a GOOD ground.

I also replace the short + cable to the starter with a new #2 as well
Amazing winter starts.

Now. the sometimes dim glow of the alternator indicator while running.

All it is saying, is that the voltage at the "field" terminal on the
alternator  is slightly lower than that coming to the ign switch from the
battery. This is most likely inside the alternator, although a poorly
grounded alternator (via the bracket, engine block, tranny housing, rusty
bolt skimpy ground cable, etc. etc. can result in this.

The important thing. Measure the voltage across the (clean) battery
terminals. With the engine running, it should be about 14 volts, with the
heater blower and headlights on.

If not there are wiring and connection issues.

Usually the alternator either works, or quits, they seldom limp, except when
the brushes are about to give up. Sometimes the dim glow just means the
alternator is working hard charging the battery and running all your toys.
If you have your 14 volts at the battery, it means nothing
An after-market voltmeter is a comforting additio I recommend on any car
that is made without it. It can mean finding a problem long befor the
battery goes dead far from home.

Another little thing related to dissimilar metal idiocy.

On the back of the regulator, there is a groundingstrap that encircles one
of the regulator mounting screws. Invariably, the aluminum of the alternator
case that it mates with, will be severely corroded and make a poor contact.
Polish up both surfaces, and smear on some contact grease (sold for
sparkplug connectors) to keep the grunge out.

If you take out the regulator once a year, and grease the screws when you
replace it, they will come out easily instead of snapping off, which happens
if you Don't do it annualy. You will also see how the brushes are wearing,
and avoid being left in the dark some dark and stormy night.

Confidence is born of due dilligence.

Sandy