Repairing a Bosch, VW 120 A
alternator. Date Oct. 25, 2004
MY LEGAL STATEMENT.
This worked for me, but that doesn't
mean it will work for you.
Do this at your own risk! I will not
be held liable for your own actions. If you hook-up a wrong wire, and
burn your home or car down, thats your fault! If you feel you don't
have the skills to do this, ask someone who does.
VW part #: 028 903 803D (replaces many Bosch #s).
Transpo #: IB388
WAI #: 35-9108
Heres a step by step to for the Bosch
To replace a voltage regulator, remove the black cover (3 phillips
screws, and disengage the 3 pry tabs on the side of the cover).
Carefully lift and remove the cover, set it aside.
You'll see that the voltage regulator is secured by 2 phillips screws.
Remove them and set aside, and carefully lift up the old regulator and
place it someplace secure (if you will reuse it).
Now would be a good time to replace or repack your bearings
(details below). Also I would suggest you clean up the slip rings and
remove any oxidation of the voltage regulator contacts (on the
alternator case, and the rectifier assembley tab).
Use 600 fine grit wet or dry Silicon Carbide sandpaper on the slip
rings, then wipe clean with a rag when finished. Use the same sandpaper
to clean up any oxidation as noted above.
Carefully align the new voltage regulator, be carefull not to damage
Set the regulator inplace and reinstall the 2 phillips screws. Give the
alternator pulley a clock wise spin, check that the brushes are making
proper contact with the slip rings.
If your going to repack or replace the rear bearing, remove the 4
phillips bolt head screws located around the outer perimeter of the
rear case. They are 8mm, you will need to use a socket and 1 closed end
wrench (the wrench is needed due to the limited space as its under the
W and D+ plug).
Remove the voltage regulator before attempting the following.
Using a flat head screwdriver, carefully pry the case at all four sides
as shown in the pic. The two halves will come apart, some may be a bit
The rotor will stay with the front pulley half, the stator will come
off with the rear rectifier assembly.
Now depending on the age of your alternator, the rear bearing may not
be worth saving. It may have a hardened seal and prying it off may
damage it, rendering it useless. The only solution would be to replace
it with a new one.
Please take the time to repack the new bearing with synthetic grease.
Failing to do so will result in premature failure of the bearing.
There just isn't enough grease in it from the factory.
Don't loose this bearing housing AKA the tolerance ring
If you suspect the front bearing needs attention, the pulley needs to
be removed. The use of an impact gun makes quick work of this.
Not having one handy, your in for a lot of knuckle busting. I would
advise to find a friend with an air compressor or take your alternator
to friendly mechanic.
With the pulley off, you'll see 4 phillips screws, these secure the
front bearing to the front case. You may need to use an impact
screwdriver to remove these stubborn screws.
Pulley and screws removed.
Here is the thrust washer. Note that it is tapered on the inside,
facing the bearing.
Before the bearing can be removed, the thrust washer must be removed
The following were removed:
Thrust washer, bearing and bearing retaining plate.
This picture shows the orientation of the parts.
Make sure the bearing retaining plate and the thrust washer are
The brearing plate threaded holes will face the rotor assembly.
Rotor with slip rings.
Rear-small: 6203 RS or 6203 2RS (rubber seal) 17mm ID x 40mm OD x 12mm
Front-large: 6303 RS or 6303 2RS (rubber seal) 17mm ID x 47mm OD x 14mm
Seals: Labyrinth closure system that keeps the dirt out and the grease
Do Not use 6203 ZZ or 6303 ZZ bearings, these are metal shielded
bearings, but are not sealed.
Grease will quickly fly out of it, and you'll have a noisy alternator
in no time.
Notice the rear bearings inner lip seals are damaged, this is due to
Thank you Bosch for using cheap Chinese made bearings.
According to the Bosch web site, they use 100% new high quality
Bosch web site.
Bearings come in many different varieties. The thing to keep in mind is
cost and performance. The low end bearings are usually ABEC 1 rated.
ABEC stands for Annular Bearing Engineers' Committee. This committee
works to determine the standards for bearings for the Anti-Friction
Bearing Manufacturers Association (AFBMA).
The ABEC scale classifies different accuracy and tolerance ranges for
The higher the ABEC number the more precise the bearing specs are.
Just pick a name brand such as the following, and you can't go wrong.
Most will have a Hi-temp, Hi-speed bearing.
BCA, FAG, KOYO, NTN, SKF, TIMKEN
Following is just some info on replacing your rectifier assembly
Remove the 3 phillips screws and set aside. Assuming you already
removed your voltage regulator, desolder the 6 terminals where the
stator wires go. This is easier said than done, but having a 140 watt
soldering gun and a solder sucker helps. You may need to use a tiny
pick or jewelers screwdrivers to pry apart the rectifier assemblies
Thats pretty much the basics of the Bosch alternator.
If I get additional info I'll add it to this page.