Why does my 1992 Passat charge erratically?

improper or damaged wiring when reinstalling need a multi meter to test the wires

Had this problem in a Lincoln. Only car that showed 45mph at a stoplight with brakes on. Odometer kept churning up miles as car stood still. Turned out that Alternator was shorted and although battery kept charge, the alternator case was losing 16 amps into body of car.

I even installed another digital dash. Take off the alternator and have it bench checked.

- Switch on the head lamps for 5 minutes.
- Keeping the head lamps ON, start the engine.
If the lights die,
- Check the battery terminals.
- Check battery state of charge.
- Charging system.
If the lights still glow sharply,
- Check fuses and relays in under the hood fuse and relay box junction.
- Battery to starter motor cable.
- Battery to engine ground and chassis ground cable.
- Solenoid on starter motor.
- Starter motor.
- Wiring.
-Ignition switch.

There is a controller called a BCM of GEM that is a small computer in the dash that controls all those things. They are expensive.

The first thing to do is have the battery tested, just because you charged it doesn't mean it will stay charged if the cells are sulfated up or shorted.

try turning the headlight knob left to right several times, and jiggle the switch at night. Also check your fuse panel should be a 10 amp or less fuse that says "Insr" or something like it.

I would get a multimeter and while it is running check to see what the charge is at the battery, it should be above 13amps 14 is good. If not then your alternator is bad and isn't charging the battery while it is running.

What happenes when you try to jump start the vehicle? If it starts - then it is most likely the battery or alternator. If it doesn't start then the solenoid or the starter or the engine. To get it down to the starter or engine - disconnect the serpentine belt and turn the crankshaft with a wrench. If it turns - then it is your starter/solenoid.

Check the dash light fuse. Maybe a problem with the headlight switch.

Battery is weak.