Battery charge monitoring device (VW Golf 1)

The control lamp in the instrument panel has two positive connections: on one side, from the D+ terminal of the generator (blue cable) and, on the other hand, from terminal 1.5 (via a multi-pin plug behind the instrument cluster) from the ignition switch.

When the ignition is turned on, terminal 15 is energized. But the generator is not yet running, so contact D+, which has no voltage, is the negative contact. The control lamp flashes, since there is a voltage difference between the on-board network, powered by the battery, and the generator, which is not yet running.

When the engine starts and the alternator reaches its charging speed, the voltage regulator connects the alternator to the on-board electrical equipment. Now the positive current comes from terminal 15 and additionally from terminal D+. There is no longer a voltage difference, the battery charge indicator light goes out.

When the ignition is turned on, the battery charge indicator light should first excite the generator.

Note: In most cases, the battery charge indicator light remains on when you start the engine and it is running at low idle speed. Then the generator pre-excitation is too weak, it does not yet produce electric current. As soon as you lightly touch the accelerator pedal, the light goes out. This is normal, it does not indicate a malfunction.