Charging System Under Load: Alternator or Wiring?
A 2019 truck with a 12V system and a 160A alternator has a complaint of dim lights and a battery that goes dead after a few days. The alternator was replaced 3 months ago. At idle with all accessories on, battery voltage is 13.2V. The alternator 'B+' terminal voltage is 14.8V. The voltage drop from the alternator 'B+' to the battery positive is 1.6V. The voltage drop from the battery negative to the engine block is 0.1V. The alternator is a remote-sense type with the sense wire connected to the battery positive. The battery passes a conductance test but is 4 years old.
1. What is the most likely cause of the low charging voltage at the battery, and how should it be diagnosed?
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The voltage at the alternator 'B+' is 14.8V, but at the battery it is only 13.2V, indicating a 1.6V drop in the charging circuit. This excessive voltage drop is likely due to high resistance in the cable or connections between the alternator and battery. The sense wire is connected to the battery, so the alternator is regulating based on battery voltage; if the sense wire is good, the alternator should increase output to compensate. However, the drop suggests a problem in the main charging cable. Perform a voltage drop test from alternator 'B+' to battery positive while the system is under load; it should be less than 0.5V. Inspect the cable, terminals, and any fusible links for corrosion or loose connections.
2. If the voltage drop is found at a corroded terminal, what is the proper repair?
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Clean or replace the corroded terminal and cable as necessary. Use a suitable terminal cleaner and apply anti-corrosion compound. After repair, re-test the voltage drop; it should be within specification (typically <0.5V). Also, verify the alternator output voltage at the battery is now within the normal range (13.8-14.8V).