1991 Plymouth Acclaim power

The first thing to do is connect a test light to the battery negative post, then, with the engine running, touch the probe to the metal housing of the alternator. If the test light lights up, repair the corroded ground cable between the alternator and engine block. The alternator is mounted on rubber bushings on that engine to reduce vibration. Those bushings insulate it so it needs that extra ground strap.

If the light doesn't light up, measure the voltages on the two smaller terminals on the back of the alternator. This has to be done with the engine running. One will have full battery voltage, (around 12 volts). The other one must have less, but not 0 volts. Typically you'll find between 4 - 11 volts. That voltage will tell us where to go next.

It's built into the Engine Computer by the battery and has an extremely low failure rate. If the circuit between that pin and the alternator is open, as in a corroded connector terminal, you'll get a diagnostic fault code "Field not switching properly".

I drive an '88 Grand Caravan and let my newer stuff sit. Your Engine Computer sits right next to your battery, just like on my van.

Did you do any of the tests I listed? This is a really easy system to diagnose and repair. Three voltage tests will tell you everything you need to know about where to look for the cause of the problem.

The voltage regulator is built into the Engine Computer and is not serviceable separately.

Did you do the tests I listed? If not, this diagram shows the four test points. The first two are the two small terminals bolted to the rear of the housing. You may not be able to tell them apart because they go through a black plastic block.

For a '91 model, one must have full battery voltage when the ignition switch is on. The other will typically have 4 to 11 volts when the engine is running.

The fat bolted-on output wire must have full battery voltage all the time.

This is one rare time when there can be a ground problem. This alternator is mounted on rubber bushings to isolate it from engine vibration. There's a fat ground wire or a braided ground strap that often gets overlooked when working with the front engine mount's bolt. The strap has also been known to corrode apart.

To test for a good ground, your voltmeter's ground probe or your test light's ground wire must be attached to the battery's negative cable or to any paint-free point on the engine or body other than the alternator's housing. Touch the positive post to the housing. If you find any voltage other than "0", repair that ground cable.

These tests must be done with the engine running and while the problem is occurring. Let me know what you find.

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter