Generator Light On!

by Robert Kuhn

The information listed is my experience with this problem and I will assume that you have a Type-I. If you have a Type-II, III or IV, there will be some variations but that is mostly where the location of the voltage regulator is mounted. For the most part, the information provided should be the same for all Type's. I recommend that you get a decent repair manual to aid you (Clymer, Chilton, Robert Bentley, Haynes and I guess Muir's Idiot's Guide).

Eeek! My generator light is on!

Don't panic, pull over to a safe place and turn the engine off. Most people assume that the generator (or alternator if you have one) is at fault. This is not always the case.

Let's look at some of the common areas/things that can cause your generator light to come on.

Some tools you will need:

  • Flat blade screw drivers
  • A set of open end wrenches
  • Volt meter (digital if possible)
  • Test light
  • Fuses
  • A wire brush
  • Crimp connectors (push on style)
  • Wire strippers
  • Wire clippers
  • Crimpers
  • Hook up wire (aligator clips on both ends)
  • Electrical tape

For the most part, you will probably be using your volt meter.

Fan belt tension/defelction.

Make sure that there's about a half inch of play/deflection.

If there's more than that, see if you can tighten the belt. Do this by removing one (or more) shims that's between the two pulley halves.

If after removing all the shims and there's still too much play in the belt, then the belt is stretched and needs to be replaced.

If the belt is OK, then check the generator/alternator mounting bolts are tight. If they are nice and tight, then we can rule out a bad/loose belt and generator/alternator.

Wire harness and connectors.

Check the generator/alternator wiring harness and the connectors at the regulator.

Depending on the year of your VW, the regulator can be found under the rear seat (left side) or be mounted right on top the generator. If you have an alternator, then regulators are usually built in to the alternator itself.

Typically, on a 6V car (1966 and older) it's mounted on top the generator (a silver box). On a 12V car (1967 and on) it's usually under the rear seat off to the left. In Ghia's, I think the regulator is in the engine compartment and I believe that's the same with the Bus.

Anyway, check and make sure that the wires are on good and tight and not burned or damaged. Replace any wire.connector that looks bad.


Disconnect the battery cables, negative cable first then the positive. Inspect the battery posts, for a few bucks, you can buy a battery post cleaner or use a wire brush and clean the posts and the battery clamps. For about $1.00 (USA), you can buy these little felt rings that you place over each battery posts which helps keep them nice and clean. There's also a gooey gell that you can put on the posts once the cables are re-attached to help them from corrison build up.

If you suspect that the battery has a bad cell or two, you can buy a hydrometer from any FLAPS. This simple device checks the specific gravity of the battery electrolyte in each cell.

A bad cell can cause the generator/alternator to overcharge the other cells which will trigger the generator light in the speedo.

With the key in the OFF position (remove it from the ignition to be safe!) and the negative cable removed from the battery (keeping the positive cable still attached) connect a test light between the negative battery post and the disconnected negative battery cable.

  • If the light does not come on, then reattach the negative battery cable and move on to checking the voltage output check.

  • If the test light comes on, there's a short in the electrical system. You must first find out where that short/drain is coming from before we can check out charging system.

    A suggestion that might help you track this down would be to disconnect the generator/alternator harness, if the light goes out then the generator/alternator is bad, if the light stays on then move to the fuse block and start removing fuses one at a time until the lamp goes out. This will at least help you figure out which components might be causing the short/drain.

Get out your digital volt meter (DVM) and check the battery voltage. A healthy 6V battery should show about +6.3VDC. A healthy 12V battery should show about +12.6VDC.

Start the engine and run it at fast idle (about 2000 RPM) and check the battery voltage again. On a 6V battery, you should be getting around +7.4VDC to +8.1VDC, on a 12V battery, you should be getting around +13.5VDC to +14.5VDC.

Flip your head lights on, the voltage should drop a bit then come back up again if the charging circuit it working properly.

If the voltages are more than what's specified, then the regulator is probably shot and needs to be replaced.

If the voltages are less than specified, then the problem lies in the generator/alternator.


Inspect the brushes first.

On generators, you can can try a simple trick; remove the brush (use a magnatized flat bladed screw driver, it will make it easier when you go to re-install the brush), the copper wire that is attached to one end is usually long enough for you to rotate the brush 180 degrees. If it is, re-install it (do this for the other brush too) and re-check the voltage outputs (the generator light may take several seconds before it goes out). If the generator light goes out and the voltages are within spec, then you have a set of worn brushes ... replace them.

If after doing this brush trick doesn't fix the problem, then you probably have a bad generator.

Remember that when replacing the generator, it's a good idea to replace the regulator too. A generator that's gone wonky can damage or weaken the regulator which may come back to haunt you later down the road.


In the alternator, you can't do this trick as the brushes are built into the internal regulator and they both get replaced as a single unit.

If after replacing the internal regulator/brushes in the alternator does not fix the problem, then the alternator is probably bad and needs replacing. Hang on to the new regulator/brushes and keep them as a spare (most places will not let you return electical parts, if they do, cool deal!).


Well, that's about the basics in what can cause your generator light to glow (aside from there being a short in the generator light itself).

It's also beyond the scope of this page to pin point the exact problem, but what I have outlined here is something that the basic "Do it yourself" type person can do. It is my hope that this the information that I have provided will help/aid in your troubleshooting.

Keep in mind that if you are running a generator, the generator light will glow dimly and is most noticable when it's dark (ie: at night). This is common. Ever notice when sitting at idle and you have your headlight on how *dim* the lights are at times, then when you rev the engine up over 1000 RPM they glow brighter? This is normal for a generator supplied car. Try running the lights and the wiper motor on at the same time in a 6V car. (c=

And last but not least, don't be ashamed if you have to enlist the help of a shoppe. It's best to have it done right. Electrical troubleshooting is time consuming and fustrating (even in a simple car like the VW), if not done properly, you can not only toast the wiring/electical system, but you can toast your VW!

On the 12V generator, the regulator is connected as follows:

  • D+ = Connects to the battery
  • DF = Connects to the generator
  • B+/51 = Connects to the battery +ve and terminal 31
  • 61 = Connects to ignition warning light (aka: The generator idiot light)

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