A typical symptom is sluggish cranking and low battery voltage after sitting (below 12.7 V), even though the alternator itself produces the correct voltage.

You can test this with an easy measurement: Start the engine and turn on all electrical devices (lights, rear window heater, fan). Measure the voltage directly from the back of the alternator (B+ terminal and alternator body). If it reads about 14.4 V, the alternator is good. Measure the voltage at the battery terminals. If the voltage has dropped below 14 V at this point (or even below 13 V), the fault is in the wiring harness or connections in between.

Set your multimeter to the DC voltage range (2V or 20V) and measure the following gaps with the engine running and load on: Ground (minus side): Measure between the alternator body and intake manifold. Battery ground: Measure between the battery minus terminal and intake manifold. Charging current (plus side): Measure between the alternator's B+ terminal and the battery's plus terminal. Rule of thumb: If voltage drop on any gap exceeds 0.1 V, the wiring harness or its connection is faulty (oxidized or "rotten").

The most likely culprit is the alternator's ground strap, which runs from the back of the alternator to the intake manifold (often blue in color). Over time, the wire oxidizes internally and its current-carrying capacity is reduced. Another common location is the thick wire from the alternator to the starter, whose connection at the starter end can be loose or oxidized.

If you find voltage drops, the faulty wire must be replaced or a new cable must be run in parallel: Ground: Replace the old ground wire with at least 6 mm² (or preferably thicker) stranded wire. Connectors: Use proper ring terminals (e.g., 8 mm hole) and crimp them with quality crimpers. Plus side: If the drop is on the plus side, check and clean the connections at the starter motor terminal, where the alternator and battery wires meet.

An oxidized wire may pass a small current without much drop (idle without lights), but when current consumption increases (lights, heaters), the poor connection begins to act as a resistor, causing voltage to "fall off" and not reach the battery at full strength.