Testing and Repairing Regulator- early alternator equipped Type 1's (1973-1974)

Early alternator-equipped Beetles use a Bosch alternator with an external regulator (BOS-0-190-600-017) screwed to the left wall under the rear seat. The regulator controls the voltage/current fed to the rotor's electromagnet coil windings through terminals D- and DF to provide an output voltage at the D+ or B+ terminals of about 14.5V. When the regulator is working normally D- is connected to ground and around 6V is applied to terminal DF. For the past year I've had problems with the charge indicator warning light coming on intermittently. I eventually found that giving the regulator a sharp whack would make the light go out- for a while. It seems the regulator was developing an intermittent internal open circuit. Although it's possible to replace the entire alternator with a new internally-regulated unit for about US$120, this involves a fair amount of work. Fortunately Hella and Beru offer a replacement regulator module (much smaller than the Bosch original) for about US$40.

Aside from measuring the voltage at D+ and DF, another quick test for the alternator (engine off!) is to disconnect the 3-pin connector and measure the resistance between DF and D- with an ohmeter. A zero reading indicates a short in the coil windings, while an open circuit would indicate that the brushes aren't making contact to the slip rings. It's quite easy to clean the brushes- www.vw-resource.com gives the procedure.

It's worth noting that the coil excitation current flows through the alternator light, so if the lightbulb burns out the battery won't charge. Always check for the alternator and oil pressure warning lights to come on when the ignition switch is first turned on but the engine has not yet started.

I don't think the generator test recommended by "SpeedyJim" in which the full battery voltage is applied to the DF terminal is a good idea for an alternator. This test would raise the alternator output far above the normal regulated 14.5V, which might destroy the rectifying diodes in the alternator.