1991 Mazda B2200 alternator light

The most common cause is worn brushes making intermittent contact. That would cause no output from the generator, but some voltage regulators will turn on the warning light for an overcharge condition too. That can lead to a damaged battery if you let it go too long. What you need to do to start with is measure the battery voltage while the engine is running and the light is on. It must be between 13.75 and 14.75 volts. Holler back with what you find.

I assume that was done on the car. The next step is to determine what is taking place when the light turns on. There has be be a low-charge, no-charge, or over-charge condition. Not many car models turn the light on for an over-charge condition. Measure the battery voltage while the problem is occurring.

Also, what did they test for or what were the results? If they just tested to see if there was some output voltage, that was only part of the full test. Usually they'll also test for output current, but you have to compare that to the rating of your generator. To just have some current is not sufficient. If there's a bad diode, you will only get 30 amps from the common 90 amp generator, and that isn't enough to run the entire electrical system under all conditions.

15.2 volts is too high. That is typical of a bad diode. If the voltage regulator is shorted, the battery voltage will be much higher, especially when you increase engine speed.

AC generators all put out three-phase current. When one diode is defective, you'll be missing one of those phases. It also shorts out half of each of the other phases. That's where the loss of two thirds of the output current capacity comes from. On some designs, the regulator responds to those momentary dips in voltage during the missing phase, and it tries to bump up the output current to try to get battery voltage back up. In effect, it over-reacts and can't respond fast enough to bring the voltage back down to below 14.7 volts.

The generator needs to be tested for maximum output current. Most auto parts store testers don't test for that, and if they do, the motors in them aren't strong enough to run a generator wide-open. That test needs to be done with the generator on the car. The test only takes five seconds once the cables are connected, but it should be done while the warning light is on.

Not until testing proves it has a problem. We never condone replacing random parts even though we sometimes have to do resort to that. Sears Auto Center used to do a charging system test for five bucks. It tested maximum output current, regulator voltage, battery capacity, and ripple voltage. The test was free if you needed and bought a new battery or generator, and the second test to verify the repair was also included.

12.9 volts is too low. You need at least 13.75 volts just to start putting a charge back into the battery. I suspect you have worn brushes inside the generator. Unfortunately, only Chrysler alternators and a few Ford generators, ("alternator" was copyrighted by Chrysler in the early '60s), have external test points for measuring continuity between the brushes. Most imports have the voltage regulator built into the generator which makes taking measurements impossible. The only thing you can do is measure the voltages on the other wires going to the unit. One must have full battery voltage all the time or all the time the ignition switch is on, and that may be the same as the large output wire. There has to be a different wire that tells the voltage regulator when to turn on. That one usually comes from the ignition switch through the dash light. If the dash light turns on at some time or other, that circuit is okay. At the generator, if you can identify that wire, you'll find close to battery voltage when the dash light is off, and typically around 2 volts when it is turning the dash light on.

Also, when you find 12.9 volts at the battery and the engine is running, measure on the generator's large output terminal. You must find the same voltage there. If you find a higher voltage on the output terminal, there's a break in that wire going back to the battery. Look for a corroded splice at a fuse link wire or a blown fuse bolted into the fuse box.

Dandy. Off-car testing is never accurate. All that can do is tell you if the unit is producing some output current, not how much or if there's a bad diode. Happy to hear it's solved.