Alternator Replacement Discussion

These discussions, together with Dave's alternator replacement experience, culminated in our Alternator Removal/Installation Procedure. Details of the alternator replacement are given there.


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. Jim 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, which he posed to Speedy Jim. Jim responded to say that the alternator is grounded through the pedestal and into the engine block. This is why there must be a good grounding strap from the transaxle to the chassis and from the chassis to battery. The ground path has to carry 50 amps from the alternator body to battery, and about 50 amps from the battery ground to starter 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!" :-).


Dave's New Alternator
and Refurbished Fan


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.



Alternator Wiring

For more information, see our article on Alternator Wiring.

Per Speedy Jim: The wiring is simple: The large red wire connects the battery to the B+ terminal on the alternator (just as before). 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 -- there was a large red one and a black one attached to the B+ terminal on the old alternator, and then small red, brown and green wires going to the D-, D+ and DF terminals (these were combined in one connector -- it was hard to tell which wire goes to which terminal).

The new alternator only has a large red wire to the B+ terminal on the alternator (going to the splice with the two other red wires that were on the B+ terminal on the voltage regulator), and a green wire on the D+ terminal that splices with the blue wire that was on the 61 terminal on the voltage regulator and goes forward to the "Alt" light in the instrument cluster.

John Connolly advised Dave to "just ignore the old regulator wiring. B+ is the big red one to the battery. The other slip-on spade connector needs to go to the dash warning light. That's it."

Dave further explained the old alternator and voltage regulator wiring: There are four wires coming out of the old voltage regulator to a push-together connector -- a red one (D+), a green one (DF), brown one (D-), and a blue one (61 -- "Alt" light). A schematic that Speedy Jim sent to Dave also shows the heavy red wire from the B+ terminal on the alternator going to the B+ terminal on the voltage regulator, with another heavy red wire from the B+ terminal on the alternator to the battery. On mine the heavy red wire from the B+ terminal on the alternator doesn't go to the VR at all, but proceeds right across to the right on the the rear seat to the battery. And presumably there is a connection to the solenoid on the starter motor, too.

So Dave wasn't finding the three heavy red wires that he needed to splice together. Obviously the red wire (smaller gauge -- 16?) that went from the D+ on the alternator to D+ on the voltage regulator is removed from the car entirely.

Speedy Jim advised: Since you're replacing an existing alternator with an external voltage regulator with a new one having an internal regulator, ignore the diagrams! The heavy red wire from the alternator B+ terminal to the battery is all you need for the output; no splicing at the regulator. But you must extend the thin blue (#61) wire back to the alternator D+ terminal. Use the thinner red wire. Connect one end to D+ and splice the other to the thin blue wire. Then you can throw away the regulator.

Dave clarified: So essentially all that is needed is to connect the existing heavy red wire to the B+ terminal on the alternator and connect either the thinner red or green wire to the D+ terminal and splice it to the blue wire that runs up front to the "Alt" light.

The bottom line: The wiring consists simply of a large red wire from the battery, through the firewall into the engine compartment to the B+ terminal on the alternator. The green wire attaches to the D+ terminal on the alternator, then runs to the area under the rear seat where it splices to the blue wire that goes forward to the "Alt" light in the instrument cluster. Speedy Jim confirmed that this is correct.

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 13.5-14V at high idle if the alternator is working as it should.


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