1990 Chrysler Le Baron New battery n alternator. But wont h

Could be the computer controlled of the field generation of the alternator

Measure the voltage on the two small terminals on the back of the alternator while the engine is running. 0 volts on both; look for a broken feed wire or corroded pin in a connector. Exactly the same, about 12 volts, on both; look for an open wire going to the voltage regulator in the engine computer, or a defective regulator. If one has a lower voltage, 4-10 volts typically, the alternator is working. In that case, measure the voltage on the fat wire bolted to the back of the alternator. It must be between 13.75 and 14.75 volts and the battery voltage should be the same. If the alternator voltage is higher than the battery voltage, the heavy wire is open, there's a corroded connector near the battery, or there's a high amperage fuse in the fuse box under the hood that's blown.

If you find 12 volts on one terminal and 0 volts on the other, the brushes are worn. Not likely since you have a new alternator.

These voltages come through the Automatic Shutdown relay. The engine must be running for the relay to turn on, or you can bypass the relay and make the measurements without running the engine.

Caradiodoc