I had a perfectly functioning charging system. Red light on when key turned to first position, then off when engine running. Had the battery tested because it didn't seem to hold charge very well. OReilly said battery was bad and replaced it. New battery tested to confirm good.

After installing new battery I have the following conditions:
No light when key turned to first position
No light when engine at idle
Charge light comes on when rpms go up

I'm in a bit of a panic since I need to leave on Wednesday for Red Rocks Classic. I've checked George's Tech Tips book and even my trusty VW Idiots Guide. I can't find anything that matches this behavior. Replacing voltage regulator does not change things.

Any advice is greatly appreciated!

Very interesting. Do you still have the old battery? Do you have a multimeter?

Is the belt slipping at high rpm? I can't imagine that's it but I guess it's possible.

Since the charge light comes on (just not at the right time!) you know the bulb is not the problem… but verify that the wiring harness to the light socket is good.

If the wiring is ok, no (G) light at ignition can be a sign that the alternator diodes have gone bad?

Good luck!

I’ve had a similar condition either on my old Volkswagen Karmann Ghia vert or my 914, I cannot remember which.
It’s possible that one of the diode‘s is going out on the alternator, so at higher rpm charging starting to fail that’s why the lights partially dim under revs. I’m sure one of the other guys on this site can you give you an idea how to test for these conditions?

the light should come on when the key is in the on position. Obviously if the light flickers at higher rpm, the wire has power, red and white, but you may check anyway when the key is in the on position, could have a loose fuse.


If you know there is power to the red and white, check the voltage regulator for a good connection. Cannot think of anything that would have to do with the battery, but check all of the small red wires attaching to the positive post. Finally check the alternator lead to the starter to make sure it is tight

If still no red light when the key is on, I would think that there was something wrong with the alternator

belt shipping at high rpm would flicker the light but the light would still come on when the key is first turned to the on position

Why had old battery gone bad? Are you sure generator light was working properly to start with prior to the battery swap? With engine running just off idle - what is the voltage measured at the battery. Should be higher than 13 volts.

What year is the vehicle? Early cars around 71’ had a diode associated with the generator light located behind the gauge that could cause some flakiness if it has gone bad. This diode is shown in the Haynes schematics but isn’t going to be shown on the simplified schematic I’m posting below.

Start with ensuring all battery connections to new battery are correct and all the red wires are solidly connected. Likewise, ensure your negative cable is solidly connected to body - chassis ground.

From there this gets a lot more complicated and you’ll need a multimeter to troubleshoot.

Here is Spoke’s simplified schematic. Note on some early cars I think it’s fuse 8 whereas later cars are fuse 9 as shown.

Make sure fuse 8 and 9 are seated and not blown - current flows through that fuse at key on to light the generator light.

As far as the light lighting at high rpms - that is much tougher to diagnose.

For now - start with figuring out why the generator light isn’t on at initial key on. That will likely lead to the other 1/2 of the problem once we verify the fuse is there, the bulb we know is working, and the other portion is in the regulator itself that feeds battery voltage to the alternator field windings at start up to bootstrap the alternator to life.

Click to view attachment

This car (a 73 2.0l) had been at my house in Phoenix for the last 4 or 5 weeks. The generator light was functioning correctly during that time but it was cranking slow when started. It even cranked slow after the battery had been on a 2a charger over night. Ground strap, battery cables and battery connections were checked before it left my house. The battery was in question dew to the slow crank. Measured battery voltage off idle at that time was 13.0v to 13.5v.

Great news! Blown #9 fuze was the problem. Not sure why changing the battery would cause a blown fuze. Never saw any wires ground out, but the fuze was definitely blown. Everything now works as it should.

Thank you all for the help!

Dr. 914 said (emphasis added): "If you know there is power to the red and white, check the voltage regulator for a good connection. Cannot think of anything that would have to do with the battery, but check all of the small red wires attaching to the positive post. Finally check the alternator lead to the starter to make sure it is tight"


Wondering why a voltage regulator would not have a good connection with the relay board? Would it be mis-alignment of regulator prongs with the regulator board plug? Or possibly a dodgy connection within the relay board or the voltage regulator? I seem to have that issue with my car, where it will stop charging (charge light on, 12.2 volts across the battery), and by unplugging the voltage regulator and plugging it back in, it starts charging again (charge light off, 13.6 volts at battery).

These cars use ancient spade type terminals. Although they have been proven to be highly reliable over the years they do become loose over the years from vibration and repeated removal - replacement of components.

Very easy to tighten them back up with a pair of needle nosed pliers.

Same thing occurs with the various relays and socket connectors on the relay board. Need to be tightened up occasionally.

The simple act of removing and replacing a relay or the voltage regulator can scrape a fresh new connection that changes something from non-functional to functional . . . For a while. Cleaning and tightening connectors is the long term solution.

good job!

OK, that sounds like the right place to focus next. I might go ahead and get another voltage regulator also to see how that does. What are folks buying for VRs now? I like the digital adjustable ones but of course reliability trumps all!