Jim Adney's electrical system diagnosis procedure

Questions with regard to generators and voltage regulators come up frequently enough that I have written this FAQ in the hope that it can answer most of those questions and allow some of you to determine the root of your problems.

To do these tests you need some kind of voltmeter. It doesn't need to be fancy and it doesn't need to be particularly accurate. Buy something cheap, analog or digital. All voltage readings should be taken WRT (with respect to) ground.

VWs '69 and later have the brown ground wire running from the generator body to the regulator frame. Earlier cars just used the body of the car, which is adequate.

Here's how you can check out your system:

Turn ON the ignition key.

The dashboard generator light should come on and stay on. If it does not stay on, then you have a burned-out bulb, a bad connection, or a bad regulator. To determine which, pull the small blue wire off terminal 61 of the regulator and touch the wire to ground; if the dashboard bulb lights, then the bulb and the wiring are okay. Reinstall that wire when you are finished.

Turn the key OFF.

Connect a voltmeter to any fuse that gives you a reading. This should now read about 12 Volts.

Start the engine.

Increase your engine speed a bit; your voltmeter reading should increase. This is the sign that your charging system is working and there is output from it and into your battery.

Stop the engine.

If you have no output from the generator with the engine running at fast idle this is wrong. You need to determine if the fault is in the generator or the regulator. To do this, disconnect the DF wire from the DF terminal on the generator and fold the DF wire back out of the way so it is not shorted to anything.

With the green wire disconnected from the DF terminal of the generator, clip the voltmeter (-) to ground and (+) to the D+ terminal of the generator.

Start the engine.

Be careful of the fan belt. Now, using any metal tool, short the DF terminal on the generator to ground; the voltmeter should deflect upward. If you give the engine some throttle while doing this, you should see voltages well over 15 V. Do not do this for more than a couple of seconds at a time.

Stop the engine.

If the generator will put out voltage under these conditions it is fine, if not check the brushes. There is also a screw connecting two straps that is visible through the brush end plate of the generator. If this screw has fallen out the generator will not work.

The brushes are rectangular blocks of carbon which make contact with the commutator in the generator. They are piloted within a rectangular steel sleeve and there is a spring which pushes them against the commutator to insure good contact. New brushes stick up above the edges of the steel sleeve; brushes worn enough that they have sunk below the face of the sleeve are worn out. The upper brush can be checked visually, and the lower one can be checked by feel.

Type III generators need to be positioned correctly in the mounting saddle of the fan housing. There is a "dot" punched into the body of these generators, and this dot must be positioned just in front of the similar dot in the retaining strap. This assures that the cooling air hole is pointing straight down and is receiving cooling air from the fan.

If the generator tests out okay above, but your charging system is not working, then the problem is either bad wiring or a bad voltage regulator. Worn out voltage regulators are very common. Bad wiring is uncommon, but you should check yours out visually just to make sure that there are good connections everywhere and that all the wires are intact.

When everything is connected and working properly, the dashboard light should come on when you turn the ignition ON and go off when the engine starts. With the engine idling, the generator voltage (D+) should measure ~13 V and it should rise to about 15 V as the engine is speeded up. The regulating voltage is determined by the regulator and should be 14.1-14.4 V; this is measured across the battery, but will only be achieved at fast idle and above when the battery is well charged, so don't expect to see this immediately after fixing an inoperative charging system.

In general, I find that regulators last 3-10 years and brushes about the same. Generators usually last forever, but can sometimes develop faults that require replacement with a rebuilt unit. For replacement regulators and rebuilt generators I strongly recommend genuine Bosch, although there is nothing to keep someone else from doing a good job on rebuilding a generator.

Jim Adney
1/22/99