* Faulty Voltage Regulator: While the alternator is new, the *internal voltage regulator* within it could be defective. This component regulates the output voltage. A faulty regulator might intermittently undercharge the battery, causing the gauge to plummet.
* Bad Battery: A new alternator doesn't guarantee a good battery. Your battery might have internal damage (shorted cells, etc.) that causes it to lose voltage rapidly under load (like while driving). A load test at an auto parts store can diagnose this.
* Wiring Issues (despite checks): Even if initial checks seemed fine, a loose connection, corroded connector, or a broken wire somewhere in the charging system could be intermittently interrupting the circuit. These are harder to find and often only show up under certain driving conditions. Look carefully at all connections, especially those under stress (vibration).
* Gauge Problem: The gauge itself could be malfunctioning. It's less likely, but a faulty gauge could give inaccurate readings.
* Excessive Electrical Load: Is there any new or unusual electrical load on your system? A faulty component somewhere (like a constantly running accessory or a short circuit) could overwhelm even a new alternator, causing voltage to drop.
* Ground Issues: A poor ground connection somewhere in the charging system can cause intermittent voltage drops. Trace the ground wires from the battery and alternator to ensure they are clean, tight and corrosion free.
Troubleshooting Steps:
1. Battery Load Test: Get your battery tested at an auto parts store. This will reveal its true health.
2. Alternator Output Test: Even with a new alternator, have its output voltage tested under load. This needs a multimeter and ideally a load tester.
3. Voltage Drop Test: Use a multimeter to check the voltage at the battery terminals while the engine is running, and then again at the alternator output. A significant voltage drop between these points indicates a wiring problem.
4. Careful Inspection of Wiring: Visually inspect all wiring related to the charging system, paying close attention to connections under the hood and in the cab that might be subject to vibration.
5. Gauge Check (if other tests are inconclusive): If all other tests check out, then a faulty gauge might be the culprit.
Don't assume a new alternator solves all charging problems. It's only one part of the system. A systematic approach to testing is needed to pinpoint the exact cause.
