P1108

What Does Code P1108 Mean?

The OBD-II diagnostic trouble code P1108 signifies a “Dual Alternator Battery Lamp Circuit Malfunction.” This code is specific to vehicles equipped with two alternators, often found in heavy-duty trucks, commercial vehicles, or vehicles with high auxiliary electrical demands. The Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), monitors the operational status of both the primary and secondary charging systems. The P1108 code indicates that the ECM has detected an electrical fault within the control circuit responsible for illuminating the battery warning lamp specifically associated with one of the alternators, typically the secondary unit, or a specific circuit shared by both. This fault could manifest as an open circuit, a short to voltage, a short to ground, or an implausible voltage signal within the monitored circuit, preventing the accurate indication of a charging system issue via the instrument cluster warning lamp. The ECM sets this code when it detects that the actual voltage or continuity on this dedicated lamp control circuit does not correlate with its expected state based on system operation, thus indicating a potential failure in the ability to alert the driver of a charging system problem.

Common Symptoms

• Battery Warning Light Illuminated: The most direct symptom, indicating a problem with the charging system.
• Intermittent or No Battery Warning Light: If the circuit is open, the light may not illuminate during a bulb check or when an actual charging issue occurs.
• Reduced Electrical Accessory Performance: In some cases, if the underlying fault affects the alternator’s ability to charge, accessories may perform poorly.
• Difficulty Starting or Repeated Battery Drain: If the charging system is compromised, the battery may not be adequately charged, leading to starting issues.
• Other Charging System Related Codes: P1108 may be accompanied by other codes such as P0560 (System Voltage Malfunction) or P0622 (Generator Field F Circuit Malfunction) if the lamp circuit issue is symptomatic of a larger charging system problem.

What Causes the Code P1108?

• Faulty Secondary Alternator: An internal defect within the secondary alternator’s voltage regulator or its integrated lamp control circuit is a common cause.
• Wiring Harness Issues: An open circuit, short to voltage, or short to ground within the wiring connecting the secondary alternator’s lamp control output to the ECM or instrument cluster. This includes damaged insulation, chafed wires, or corroded connectors.
• Corroded or Loose Electrical Connections: Poor electrical contact at the alternator, ECM, or instrument cluster connectors can disrupt the lamp circuit signal.
• Blown Fuse: A blown fuse in a circuit supplying power to the alternator’s regulator or the battery lamp circuit itself (less common as direct lamp circuits are often low-current).
• Faulty Instrument Cluster: A rare but possible failure within the instrument cluster that affects the functionality or monitoring of the battery warning lamp.
• ECM/PCM Failure: While uncommon, an internal fault within the ECM/PCM preventing it from correctly monitoring or controlling the battery lamp circuit can trigger this code.

How to Diagnose and Troubleshoot

Diagnosing P1108 requires systematic electrical testing and adherence to manufacturer-specific wiring diagrams.

1. Verify the Complaint: Connect an OBD-II scanner to confirm the presence of P1108. Note any accompanying diagnostic trouble codes. Clear the code and road test the vehicle under various electrical loads to see if it immediately returns.
2. Visual Inspection: Begin with a thorough visual inspection of the secondary alternator, its wiring harness, and connectors. Look for signs of chafing, corrosion, melted insulation, or loose connections. Pay close attention to the small gauge wires leading to the alternator’s regulator or lamp output terminal. Inspect battery terminals and ground straps for cleanliness and tightness.
3. Battery and Charging System Health Check:
   • Measure the battery voltage with the engine off. It should be approximately 12.6V.
   • Start the engine and measure the charging voltage at the battery terminals. With both alternators operational, it should typically be between 13.5V and 14.7V, depending on the vehicle and state of charge. Note if the battery light is on.
   • Perform a voltage drop test on both the positive and negative charging circuits to ensure there’s no excessive resistance.
4. Isolate and Test the Lamp Circuit:
   • Locate the specific battery lamp control wire for the secondary alternator using the vehicle’s wiring diagram. This wire typically runs from the alternator’s regulator terminal (often labeled “L” or “Lamp”) to the instrument cluster or directly to the ECM.
   • With the ignition ON (engine OFF), disconnect the electrical connector from the secondary alternator. Using a Digital Multimeter (DMM), measure the voltage on the lamp control wire terminal at the harness connector. This circuit often receives an initial “exciter” voltage from the ECM or ignition circuit. An open circuit (0V) or a short to ground (0V when expecting voltage) indicates a wiring fault between the ECM/instrument cluster and the alternator.
   • Test the continuity of this wire from the alternator harness connector back to the ECM connector or instrument cluster connector. Check for shorts to power or ground within the harness.
5. Alternator Bench Test (if suspect): If wiring tests pass, the secondary alternator itself is highly suspect. Many automotive parts stores or repair shops can perform a bench test to verify the alternator’s output and regulator function, including its lamp circuit control. This is particularly useful for dual alternator systems where one unit might be failing silently in terms of its indicator function.
6. ECM/PCM Verification: As a last resort, if all wiring and alternator tests confirm proper function, and the code persists, the ECM/PCM may be at fault. However, this is rare for a P1108 code and should only be considered after exhaustive testing of all other components.

Recommended Repairs and Solutions

Addressing P1108 primarily involves identifying and correcting the electrical fault within the dual alternator’s battery lamp circuit.

• Repair or Replace Wiring Harness: If visual inspection and DMM tests reveal damaged, chafed, or corroded wiring, the affected sections must be professionally repaired or the entire sub-harness replaced. Ensure all connections are clean, tight, and free of corrosion. Use heat-shrink connectors for durable repairs.
• Replace Faulty Alternator: If the secondary alternator’s internal voltage regulator or lamp control circuit is determined to be the cause through bench testing or elimination, the alternator must be replaced. Always opt for a high-quality OEM or reputable aftermarket replacement to ensure proper system integration and longevity.
• Clean or Replace Connectors: Corroded or damaged electrical connectors at the alternator, ECM, or instrument cluster should be thoroughly cleaned using electrical contact cleaner, or replaced if damage is extensive. Ensure a tight, secure fit.
• Replace Blown Fuses: Although less common, if a fuse directly supplying the lamp circuit or the alternator’s excitation circuit is blown, replace it with a fuse of the correct amperage. Investigate why the fuse blew to prevent recurrence.
• Instrument Cluster Repair/Replacement: In the rare event of an internal instrument cluster fault affecting the lamp circuit, specialized repair or replacement of the cluster may be necessary. This often requires programming to the vehicle.

Mechanics’ Tips:

• Always disconnect the battery before working on charging system components to prevent electrical shorts.
• When replacing an alternator, ensure all mounting bolts are tightened to specification and that the drive belt tension is correctly adjusted.
• After any repair, clear the DTCs and perform a charging system test and a road test to confirm the fault is resolved and the battery lamp functions correctly.
• Be aware that some dual alternator systems may have different control strategies (e.g., one primary, one secondary; or both actively controlled by the ECM). Consult the specific vehicle service manual for accurate wiring diagrams and diagnostic procedures.
• Ensure the battery is fully charged and in good health before starting diagnostics, as a weak battery can sometimes mask or complicate charging system issues.