CASE 006
Battery Warning Light and Weak Charging
intermediate
20 min
Diesel
Vehicle
- ModelTraining Van 2.0
- Engine2.0 dCi
- Mileage210,000 km
Customer complaint
The battery warning light glows faintly and the lights dim at idle. It improves when revved.
History
New auxiliary belt fitted 1,000 km ago.
Live data
- Charging voltage, idle12.6 V
- Charging voltage, 2,000 rpm13.4 V
- Charging voltage, loaded12.4 V
Measurements
- Voltage drop, alternator B+ to battery +0.9 V at load
- Voltage drop, alternator case to battery −0.05 V
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Diagnostic case
Next step — Reasoning.
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Confirmed cause
High resistance in the alternator B+ charging cable or its terminals.
Reasoning
Charging improves with rpm but the loaded voltage collapses, and a 0.9 V drop on the B+ path is excessive. The alternator is producing output; the cable is not delivering it. The ground side is healthy, which localises the fault to the positive cable.
Recommended repair
- Repair or replace the B+ cable and clean/tighten its terminals.
- Re-test voltage drop — it should be small.
- Confirm charging voltage under load returns to the normal band.
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.