1968 Volkswagens -- like any vehicles with an alternator or generator and a battery -- have voltage regulators. A regulator prevents an alternator, generator or battery from loading its circuit with too much current. Too much current blows fuses, burns out light bulbs and melts electrical systems. Fortunately, checking, or even replacing, a voltage regulator is simple. If you can locate the battery and you have a multimeter, you can test the regulator on your old VW.
Turn the dial on the multimeter to "volts." Place the positive multimeter probe on the positive battery terminal and the negative probe on the negative terminal. With the VW's engine off, the multimeter should display a reading of slightly higher than 12 volts; between 12.2 to 12.4 volts is acceptable..
Have your assistant turn the car on and allow it to idle. Carefully touch the probes to the battery terminals. If the alternator or generator is working properly, the multimeter display number should increase to read between about 13.6 to 13.8 volts.
Ask your assistant to slowly increase the engine to 2500rpm, then to 35,000. The multimeter display should climb to between 13.8 to 14.5 volts and stop. If the multimeter displays more than 14.8 and continues to climb as the engine revs to greater rpm, the voltage regulator is bad and must be replaced.