Alternator Replacement Discussion

Faced with the probable replacement of his alternator, Dave experienced some confusion. The Bentley Manual says that VW changed from a generator to an alternator in 1973. Dave's Bug is a 1973 model, as evidenced by the chassis number, 133-3001-550. The engine number (AE320891) says that the engine is 1971, yet it is equipped with an alternator. It is obvious that the PO who replaced the engine removed the various appurtenances from his 1973 engine (such as the alternator) and used them on the rebuilt 1971 engine that he installed in the car.

Dave talked to Speedy Jim about the situation. Both Jim and Rob indicated that an alternator with an internal voltage regulator can be installed on a 1971 engine, though the actual job of replacing the alternator is a fairly difficult task. (See discussion below.)

Dave also had a question about grounding of the alternator, It's actually grounded through the pedestal and into the engine block. This is why there must be a good grounding strap from the transaxle nosecone under the car, to the chassis, and then via the battery grounding strap between the body and the battery. The ground path has to carry 50 amps from the alternator body through the car body to the battery, and over 100 amps from the battery ground post to starter motor body. For more information about alternator grounding, see Alternator Grounding.

While he had it all apart, Dave took the opportunity to clean everything thoroughly, inside and out. He stripped the fan covers and the fan housing down to the bare metal and repainted them (Rob calls him "keen." His wife calls him "obsessive!" :-).

When he installed the alternator in the car, Dave was a bit surprised to find that with the vent correctly oriented on the bottom, the connections on the top of the alternator are cocked to the right about 20 degrees. John Connolly (Aircooled.Net) indicated that 20 degrees to the right (clockwise) is correct.

Per Speedy Jim: The wiring is simple for the internally regulated alternator: The large red wire connects the battery to the B+ terminal on the alternator. The blue wire in the main harness goes to the D+ terminal. This wire connects to the alternator warning light in the instrument cluster -- it must be connected to the warning light for the alternator to work.

Dave replaced an alternator that had the external voltage regulator with one that has an internal voltage regulator. He found that he had more wires (on the old alternator) than he had seen in the schematics. But the internally regulated alternator wiring is very easy - you just ignore the old external regulator wires and connect the heavy red wire from the alternator to the battery positive post - either via the original wire running across under the seat from the battery to the position of the old ectenal regulator) or via a new wire directly to the large red wire connector on the starter motor - either method will work just fine.

Regarding the "Alt" Warning Light: Per John Connolly -- If the bulb is burned out, your alternator won't work! And from Speedy Jim -- If there is an intermittent problem with the "Alt" light, it may well affect the alternator. The alternator depends on the "Alt" light for its initial excitation.

Speedy Jim also told Dave that the symbol with the cross is a terminal for the diagnostic plug. Speedy Jim's Web Page has details (though it really doesn't matter).

To check to make sure the alternator is working properly -- With the VOM dial set to "Volts," put the meter terminals across the battery with the engine off. A fully charged battery should read 12.6V. Start the engine; the voltage reading should jump up to around 14V at high idle if the alternator is working as it should.