Originally Posted by Mxfrank

Wrong on theory, maybe right on conclusion. The alternator diodes in these cars (actually, most cars) are neither zeners nor are they avalanche diodes. They are simple half wave rectifiers. Although they are mounted to a mechanical fixture called a bridge, they are, confusingly, not arranged as a full-wave bridge in the electrical sense. The voltage regulator is a separate piece of electronics, which controls output voltage by regulating the duty cycle of the field. Let me explain what's happening here.

First of, the voltage regulator is a completely separate set of electronics from the diode assembly: it's the little can you mount on the back of the alternator. The diodes are internal to the alternator. There are nine, see the photo, nine diodes inside the alternator. This is because there are three A/C windings, creating a three phase output. Each phase has a negative and a positive diode, which accounts for six diodes. These diodes are probably fine in your case (you wouldn't be able to make these high voltages if even one diode was bad).

Which leaves us with three small diodes, which are appropriately called a trio. These three extra output (+) diodes feed the voltage regulator's internal sense connection (see photo). When the alternator is running, the voltage regulator uses these diodes for both power and sense. By power, I mean that the VR is directly powered by the trio. By sense, I mean that it uses the trio voltage to test the output voltage of the alternator. Based on the sense, it will control the duty cycle of the field coil (rotor) keeping the voltage in at spec.

When the car is first starting, the alternator has to be "bootstrapped" before it will charge. What does this mean? If you understand the previous paragraph, you understand that the voltage regulator is powered by the alternator itself when everything is running. But at start up, the alternator isn't generating power. To get things started, a bootstrap current has to be fed to the VR from outside. This is provided, and I know it's confusing, by the dash lamp. One side of the dash lamp is connected to the battery via the ignition switch. The other side is in series with the VR, actually feeding power to it via the same connection as the trio.

Since the trio isn't providing power at rest, the dash lamp finds ground through the VR. Once the engine starts and the alternator starts generating current, the dash lamp will have 12V+ on both leads, and so won't light.

What's wrong here? Assuming the dash lamp is good, you have 16V at the B+ and at the trio output, otherwise the lamp would light. The most likely explanation is that there's something not right about the VR.

There's one other possibility, and it depends on how the dash lamp is wired (I don't have specific info for your car). In many cases, the engineers will add a resistor in parallel with the lamp. If this is the case, the lamp can be blown, but the alternator will still bootstrap. The trio and B+ could be out of sync due to a bad trio diode, and there's no lamp to light. If the lamp never ever lights, even when the key is in position 2, check your bulb.

Your AC readings are garbage. Because these diodes are half wave rectifiers, the output of the alternator isn't flat DC. It's a ripple wave form, which can't be read correctly by many AC meters. Just worry about DC.

What I would do is take the VR out, and reach inside the alternator with some abrasive paper to clean the slip rings on the rotor. They should be bright copper, but I bet yours are fouled with carbon. If that doesn't fix it, then replace the alternator.