1999 Plymouth Voyager Battery Light on

Start by measuring the voltages on the two small wires on the back of the alternator while the engine is running. One must have full battery voltage. The other wire should have less, but not 0 volts. Those voltages will tell us where to go next.

Caradiodoc

I take strong issue with "on its last leg". It is very common for these alternators to be intermittent. A worn tire can be on its last leg. A rusty muffler can be on its last leg. The parts inside an alternator, with the exception of the brush assembly, are good or bad, no in between. Only the brush assembly, when worn, will cause the unit to start working again at times. The test is to measure the voltages on the two small wires while the engine is running. I've done it numerous times, but it is easier from underneath. Clip leads can also be run up to a place that's easy to reach from the top.

If one diode fails in the alternator, you will lose two thirds of its capacity, but that alone will not turn on the battery light and will not cause a battery to die while driving. Some testing equipment will indicate a bad diode, but it won't cause your symptoms.

The issue of access is irrelevent anyhow since the alternator has been replaced. That eliminates one possibility and leaves two suspect circuits. The feed circuit is the least likely to cause a problem because the same circuit feeds the injectors, coil(s), and fuel pump or pump relay. Since the engine runs, you know that circuit is working. Only a short piece of wire or a corroded splice are in the circuit to the alternator.

The other circuit is the dark green wire from the alternator field to the voltage regulator in the Engine Computer. You can measure that voltage in connector 1, pin 4. It's a very thin 16 gauge wire, (dark green). There are three possibilities. 0 volts indicates a break in that wire, typically a corroded pin in a connector. Well, ... Typically the worn brushes in the alternator, but it has been replaced already. Full battery voltage indicates a defective regulator, (Engine Computer), or a loose connection in that connector which isn't very likely. If the system is working properly, there will be typically somewhere between 4 and 11 volts on that wire. In that case, the alternator's output is not getting back to the battery. Here again, measuring on the large wire bolted to the back of the alternator will provide the clue.

Caradiodoc

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