P1108

Code

Other P — Powertrain

Dual Alternator Battery Lamp Circuit Malfunction

Brand: Other

Type: P — Powertrain

Views: UK: 41 EN: 65 RU: 41

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Page language: EN

Causes

Blown fuse or fusible link for lamp/ignition circuit
Faulty battery/charge warning lamp (bulb/LED)
Open, shorted or corroded wiring/connectors in lamp or alternator 'L' (lamp) circuits
Faulty alternator internal regulator or 'L' terminal(s) on one or both alternators
Poor ground or battery negative connection
Faulty body control module / charging system control module or incorrect module programming

Symptoms

Battery/charge warning lamp illuminated continuously or not illuminating at key-on
Incorrect charging behavior (battery not charging or alternators cycling)
Intermittent or no charging from one alternator
Possible battery drain or poor starting
Related charging system DTCs or reduced electrical performance

What to check

Retrieve freeze-frame and live data with a scan tool; note status of charge lamp and alternator/runtime flags
Check battery voltage (key off, key on, engine running) and record values
Visually inspect fuses, fusible links and wiring for damage, corrosion, or loose connectors
Check instrument cluster lamp bulb/LED function (bench test if removable)
Inspect battery negative and engine/chassis grounds for corrosion and tightness
Perform wiggle/interaction test of wiring with engine running to reproduce fault

Signal parameters

Battery voltage: ~12.4–12.8 V (engine off); ~13.5–14.8 V (engine running, charging)
Lamp 'L' terminal (key ON, engine OFF): low (~0–1 V) to illuminate lamp
Lamp 'L' terminal (engine running, alternator charging): near battery voltage (~12–14.5 V) — lamp should go out
Open or short: L terminal stuck at battery voltage with lamp not illuminating at key-on, or stuck low with lamp always on
Depending on vehicle wiring, two alternator lamp inputs may be tied together or monitored by a control module — signals should track together or per design

Diagnostic algorithm

Safely secure vehicle. Connect a scan tool and read P1108 details, freeze frame and any related DTCs (charging system codes).
Confirm battery health and resting voltage. If battery is weak, charge or replace before further testing.
With ignition ON (engine OFF) verify instrument cluster battery/charge lamp operation. If lamp does not light, check lamp/LED and cluster fuse. Replace or bench-test cluster lamp as required.
Locate fuse(s)/fusible link(s) for lamp/ignition circuits and verify power is present with key ON. Repair any blown fuses and retest.
Backprobe the alternator 'L' terminal(s) and the instrument cluster/lamp circuit. Measure voltage at key ON (engine OFF) and while cranking/running. Expected: L low (~0 V) with key ON to light lamp, then ~battery voltage when alternator charging. Note deviations.
If L terminal is open (no change) or shorted to ground/battery, follow harness continuity checks: inspect connectors, repair damaged wires, and measure resistance to module/cluster. Repair as needed.
If one alternator L terminal behaves abnormally, isolate alternators one at a time (disconnect the suspect alternator warning lamp lead per service manual) and retest to identify the faulty alternator.
If wiring and lamp OK but a terminal on an alternator remains abnormal, bench-test or replace the alternator/regulator. If both alternators test OK, check control module inputs and grounds; update module software if manufacturer service info indicates.
After repairs, clear DTCs and perform a road/run test while monitoring lamp circuit voltages and charging system performance. Re-scan to verify code does not return.