According to the acknowledged VW electrical guru "Speedy Jim", re-building alternators is quite a chore, and it may be difficult getting the proper bearings. Jim says he would opt for a rebuilt unit if it's bad.
There are two bearings, one at each end, and two small brushes which ride on "slip rings"
- on the spinning part. There should be a brush-block on the side or top which may be
removable so you could see the state of the brushes. The old style generator brushes
are replacable in the car, and so are the alternator brushes. Alternator brushes wear
much slower that generator brushes because they ride on smooth slip rings and carry
only a small current, so I may just have a slow deterioration of these. If they are
worn, also check that the slip rings aren't too worn down into "valleys" where the
brushes ride. If this happens the alternator would definitely need to go to an
overhaul place to have the rings machined or replaced so new brushes will ride properly.
Alternator Brush Refurbishment
First an interesting saga, then Dave's fix procedure -
Dave had a worrisome problem with his "Alt" light. He wrote to Rob - The light was
on all the way home from work (~20 miles) tonight, with the headlights on. I checked
the fan belt a couple of days ago - seems to be all right.
Rob responded - If the fan belt had come loose enough to cause the "Alt" light
problem, then the engine would be overheating, as the cooling fan runs off the belt too.
Check the battery voltage with the engine off - it should be 12.6V or therabouts, and
with the engine running 14.2-14.5V. If the voltage doesn't change and the fan belt is
good, then it might be the rectifiers inside the alternator or something like that.
If so it would be an odd one since alternators are very reliable (as an example, my
old truck has 900,000km and the alternator has never been touched).
Dave wrote - The fan belt is not loose, cracked or frayed. Voltage across the
battery terminals (engine off) is about 12.5V. With the engine running (charging
voltage) it's about 14.5V. The alternator was replaced on 4/29/08 when I had the
This morning when I started the car (it starts instantly) the "Alt" light came on
and immediately went off, just like it should.
Rob responded - If the voltage is still at 14.5 with the engine on, then the
alternator is charging the battery OK. But maybe it's an intermittent problem, like
a dirty brush (yes - there are two small brushes inside all alternators - they
normally wear very slowly as they run on smooth slip rings at low amps, not like the
generator which has large brushes carrying the full amp load on segmented commutators
which spark/wear a lot). Just keep an eye on it and if the Alt light starts coming on
more, then you might need to do some more checks. It might also be an intermittent
fault with one or more of the diodes they have inside (which change the alternating
current into direct current) - maybe getting hot and going open circuit. The rectifier section is
replaceable - on some versions of the alternator it comes out with the brush holder,
on others it's built into the void space inside the alternator and is much more
difficult to replace.
Dave wrote to Speedy Jim - I´m having a strange situation with the "Alt" light in
my instrument cluster. Sometimes (but not always) it will stay on, but will become
dimmer (but not go completely off) as the engine speed increases. Other times it will
come on when the engine is started, then go off normally. I have checked the fan belt
- it is the proper size (912mm x 11.3mm wide), in good condition, 1/2-inch deflection. The alternator was replaced when I had the engine overhauled in April 2008. With the engine off, the battery reads 12.5 volts across the terminals; with the engine running, it reads 14.4 volts, which tells me it's charging.
Jim responded - It could be a flaky regulator. I would connect the multimeter to
the battery semi-permanently, with the meter in the cab where you can see it. See if
the voltage is 14.4V even when the light appears to be acting up. If the voltage
*is* correct when acting up, then the regulator is OK and there is some flaw elsewhere.
Dave lengthened the leads on his VOM and set the meter on the passenger seat so he
could read the voltage across the battery terminals with the engine running. He found
that when the "Alt" light came on the voltage across the battery terminals dropped to
about 12.5V, indicating that the battery was not being charged.
The problem really had Dave baffled. He wrote to Speedy Jim - I just went to see my
doctor in Pasco, a distance of about 20 miles, highway driving. The "Alt" light
didn't come on at all, either going or returning. It did flicker briefly in town...
Then the other morning on the way to work the "Alt" light was on and the voltage
remained at 12.5V. Half way to work (about 10 miles) the "Alt" light went off and the
voltage jumped to 14.5V, indicating that the battery was being charged. Same
situation on the way home, only this time the "Alt" light was off and the voltage read
14.5V on the first leg of the trip. Then I went into a store; when I came out and
started home, the "Alt" light was on and the voltage read 12.5V for the duration of
It seems (at least to me) that there is something wrong with the alternator which
causes it to charge only intermittently.
Jim responded - It may be that the brushes are making poor contact, intermittently.
You can pull the brush holder out (along with the regulator, if internal) and check
them. You can also polish the brass slip rings by spinning the shaft and holding a
pencil eraser against the slip rings. If that's no help, replace the regulator. I
doubt that there is a serious problem with the alternator innards.
Note from Rob: A quick explanation of the way an alternator works. When you
turn the ignition on, the spinning part of an alternator gets a small current from the
battery (initially through the Alt/Gen dash light - to those slip rings and brushes) to produce an electomagnetic field in
the spinning core. When you start the engine,
this field sweeps past the fixed outer wire windings, inducing a large alternating current
through those windings. This alternating current flows through sets of diodes
(which allow only one-way electrical flow) to convert the electrical flow into direct
current, which is then used to power the car and charge the battery. If there is no power
flowing through the brushes and slip rings into the spinning core, then no power is
produced in the outer fixed wiring, and the car and engine will be running on battery
power alone. The Alternator dash light plays a critical part in this process. When the
ignition is turned on (engine off), the alternator core sees a tiny current through
the Alt light to those windings and this produces a tiny electromagetic field in the spinning windings.
This starts the process of the alternator making power when you start the engine.
The process then becomes self-sustaining, and the Alt/Gen dash stops glowing. so if the Alternator light does not glow
when you first turn on the ignition, the alternator will not produce any power. Dont
confuse this with the older generators, which have completely different electrical circuitry
- they can make power with an inoperative dash Alt/Gen light.
Dave approached the job with fear and trepidation, as always with a procedure he
hasn't done before. But this one turned out to be a piece of cake. Here's Dave's
procedure for refurbishing the alternator brushes -
- Take a look at Dave's alternator -
Dave's Motorola Alternator
- Remove the ground strap from the battery.
- Remove the air cleaner to provide access.
- Note the white ceramic box on top of the alternator (voltage regulator?).
There is a Phillips screw on either side of this box. Remove these screws.
- Pull the box straight up (there's a wire that will prevent you from moving it
up too far). Note the two brushes that extend down into the alternator and rub on the
slip rings. (See the picture below.)
- Look down into the alternator and note the two brass slip rings.
- Remove any deposits from the ends of the brushes with the eraser on the end of
a pencil. (Don't use emery paper, it's too absrasive - a pencil eraser is usually enough
to brighten up the slip rings).
- Polish the slip rings by rubbing a pencil eraser on the rings as you turn the
alternator over (19mm box-end wrench (ring spanner) on the alternator pulley nut).
Note: I used 200-grit sandpaper on the slip rings as well as on the
brushes, before both Rob and Speedy Jim recommend using the "gentler" eraser - I hope I didn't
do my slip rings any harm!
- Slip the two ears on either side of the ceramic box down into the slots in the
- Replace the two Phillips screws on either side of the ceramic box; tighten them
- Replace the air cleaner.
- Replace the grounding strap on the battery.
* * * *