P1105

What Does Code P1105 Mean?

DTC P1105 specifically indicates a malfunction within the primary or “upper” alternator circuit in a vehicle equipped with a dual alternator system. The Engine Control Module (ECM) or Powertrain Control Module (PCM) continuously monitors the charging system’s performance, including the voltage output and load characteristics of both alternators. In a dual alternator configuration, one alternator is typically designated as the primary or “upper” unit, managing a specific portion of the vehicle’s electrical load or serving as the main charging source, with the secondary or “lower” unit supplementing as needed for heavy electrical demands or redundancy. When the ECM/PCM detects a voltage output from the “upper” alternator that deviates significantly from the commanded or expected range, or when it identifies an open or short circuit within its control or output pathways, it interprets this as an “upper fault” and triggers P1105. This code signifies that the ECM/PCM has lost proper control or reliable feedback from this specific alternator, directly impacting the overall charging system’s stability and capacity. The subsystem directly affected is the vehicle’s electrical charging system, potentially leading to inadequate battery charging and an unstable electrical supply to all vehicle components.

Common Symptoms

• Battery Warning Light Illumination: The primary indicator, often displaying a battery symbol on the instrument cluster.
• Check Engine Light (CEL) Illumination: The Malfunction Indicator Lamp (MIL) will be activated.
• Dimming Headlights or Interior Lights: Especially noticeable at idle or during high electrical load conditions.
• Electrical Accessory Malfunctions: Erratic operation of power windows, radio, climate control, or other electrically powered components due to insufficient voltage.
• Slow Cranking or No-Start Condition: If the battery is not adequately charged over time.
• Unusual Engine Sounds: Potentially a squealing belt if the alternator’s pulley bearing is failing or the belt is slipping due to seizing.
• Vehicle Entering "Limp Mode": Some vehicles may reduce engine performance or disable non-essential systems to conserve power.

What Causes the Code P1105?

• Faulty "Upper" Alternator: Internal failure of the alternator itself, such as worn brushes, a defective voltage regulator, failed rectifier diodes, or a shorted stator/rotor winding.
• Open or Short Circuit in Alternator Wiring: Damaged wiring or corroded connectors in the main B+ output cable from the "upper" alternator to the battery/distribution block, or in the field control/sense wires.
• Faulty Voltage Regulator: If the voltage regulator for the "upper" alternator is an external unit or has failed internally within the alternator housing.
• Blown Fuse or Fusible Link: A blown fuse or fusible link protecting the "upper" alternator’s charging circuit or its control circuit.
• Loose or Corroded Battery Terminals: High resistance at the battery terminals or ground points can mimic an alternator issue by impeding current flow.
• ECM/PCM Failure: Although less common, a faulty ECM/PCM unable to properly control or monitor the "upper" alternator’s output can trigger this code.
• Slipping or Damaged Drive Belt: An overly loose, worn, or damaged serpentine belt driving the "upper" alternator can prevent it from generating sufficient power.

How to Diagnose and Troubleshoot

Diagnosis of P1105 requires a systematic approach, often involving a Digital Multimeter (DMM) and an OBD-II scan tool.

1. Retrieve & Analyze DTCs: Connect an OBD-II scan tool and retrieve all stored DTCs. Note any accompanying codes, especially those related to charging system voltage (e.g., P0562 Low System Voltage, P0563 High System Voltage). Access live data to observe battery voltage with the engine off and running, and monitor the commanded and actual alternator output voltages for both the primary (upper) and secondary (lower) alternators, if available.
2. Visual Inspection: Begin with a thorough visual inspection of the "upper" alternator and its associated wiring.
   • Check the serpentine belt condition and tension for the "upper" alternator. Ensure it is not cracked, frayed, or excessively loose.
   • Inspect all wiring harnesses and connectors for signs of damage, chafing, corrosion, or loose connections. Pay close attention to the main B+ cable from the alternator, the ground strap, and the field control/sense wires.
   • Examine battery terminals for corrosion and ensure they are clean and tight. Check battery cables for damage.
   • Locate and inspect any fuses or fusible links in the "upper" alternator’s charging circuit as per the vehicle’s service manual.
3. Battery Load Test: Perform a battery load test to ensure the battery itself is capable of holding a charge and delivering sufficient cranking amperage. A weak battery can place excessive strain on the charging system.
4. Voltage Measurement (DMM – Engine Off):
   • Measure battery voltage directly at the terminals with the engine off. A healthy, fully charged battery should read approximately 12.6V.
   • Test for parasitic drain if the battery is consistently low.
5. Voltage Measurement (DMM – Engine Running):
   • With the engine running, measure the voltage at the battery terminals. A healthy charging system should show between 13.5V and 14.7V. If it’s significantly lower (e.g., below 13.0V) or much higher (above 15.0V), a charging issue is present.
   • Measure the voltage directly at the B+ output post of the "upper" alternator. Compare this reading to the battery voltage. A voltage drop greater than 0.5V between the alternator B+ post and the positive battery terminal (while charging) indicates high resistance in the B+ cable or its connections.
   • Identify the field control (L-terminal) or sense (S-terminal) wire for the "upper" alternator according to the vehicle’s wiring diagram. Measure voltage at this terminal with the engine running. The voltage should typically be commanded by the PCM/ECM. Refer to manufacturer specifications for expected values. An absence of voltage or an incorrect voltage indicates a control circuit issue or an internal regulator fault.
6. Continuity and Resistance Checks (DMM – Engine Off, Battery Disconnected):
   • Disconnect the battery. Disconnect the "upper" alternator’s electrical connectors.
   • Check for continuity on the main B+ cable from the alternator to the battery. Test for any short to ground or short to voltage.
   • Check the continuity and resistance of the field control and sense wires back to the ECM/PCM connector. Look for open circuits or high resistance.
   • Verify proper grounding of the alternator housing to the engine block, and the engine block to the chassis.
7. Alternator Bench Test (if facilities allow): If suspicion remains high on the alternator itself, removal and bench testing at an automotive electrical specialist can confirm internal failure.

Recommended Repairs and Solutions

Once the root cause of P1105 has been pinpointed through diagnosis, the following repairs or solutions are typically recommended:

• Replace the Faulty "Upper" Alternator: If internal failure (e.g., worn brushes, failed voltage regulator, rectifiers) of the alternator is confirmed, replace the entire unit. Always use a high-quality OEM or reputable aftermarket replacement.
• Repair or Replace Damaged Wiring/Connectors: If an open circuit, short, or high resistance is found in the alternator’s B+ cable, ground strap, or control wires, repair or replace the affected section of the wiring harness. Ensure all connections are clean, tight, and corrosion-free.
• Replace Blown Fuses or Fusible Links: If a fuse or fusible link in the charging circuit is found to be blown, replace it with a new one of the correct amperage. Investigate why the fuse blew, as it may indicate an underlying short circuit.
• Clean and Secure Battery Terminals/Grounds: If high resistance was noted at the battery terminals or main ground points, clean them thoroughly and ensure they are securely fastened.
• Replace Drive Belt: If the serpentine belt driving the "upper" alternator is worn, stretched, or damaged, replace it and ensure proper tension according to manufacturer specifications.
• ECM/PCM Reprogramming or Replacement: In rare cases where the ECM/PCM is definitively diagnosed as the cause, it may require reprogramming or replacement. This should only be considered after all other possibilities have been exhaustively ruled out.

Mechanic’s Tips:

• Always disconnect the negative battery cable before performing any work on the charging system to prevent accidental short circuits.
• After replacing an alternator, always verify the new unit’s output voltage and current under load using a DMM and an amp clamp.
• Clear all DTCs with a scan tool after the repair and perform a comprehensive test drive to ensure the code does not return and the charging system operates correctly under various load conditions.
• For dual alternator systems, consult the vehicle’s service manual carefully to correctly identify the "upper" alternator and its specific wiring/control schematics, as these systems can vary significantly between manufacturers.
• Ensure proper belt routing and tension when reinstalling the serpentine belt. Incorrect tension can lead to premature bearing failure in the alternator or other accessories.