P1107

What Does Code P1107 Mean?

DTC P1107 is a manufacturer-specific diagnostic trouble code that typically indicates an electrical malfunction within one of two distinct automotive subsystems, depending on the vehicle manufacturer and model. It most commonly refers to either a Dual Alternator Lower Circuit Malfunction or a Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent Low Voltage.

When interpreted as a Dual Alternator Lower Circuit Malfunction, the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an issue with the voltage or current output, or the control circuit, of the secondary or “lower” alternator in a dual-alternator system. These systems are often found in heavy-duty trucks, fleet vehicles, or those with significant electrical loads. The ECM monitors the charging system’s integrity, including the individual performance of each alternator. If the voltage or current from the designated lower alternator falls below a calibrated threshold or exhibits unexpected fluctuations, indicating a problem within its dedicated circuit, the PCM will register P1107. This implies the secondary alternator is either not contributing sufficiently to the system’s electrical demand or its control circuit is compromised, potentially leading to undercharging and electrical instability.

Alternatively, in many passenger vehicles, P1107 signifies a Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent Low Voltage. In this context, the ECM/PCM monitors the voltage signal output by the MAP sensor, which provides crucial data regarding the absolute pressure within the intake manifold. This data is essential for calculating engine load, fuel delivery, and ignition timing. An “intermittent low voltage” condition means the ECM has detected that the MAP sensor’s signal voltage sporadically or consistently drops below a pre-defined minimum operational threshold set by the manufacturer, without necessarily indicating a complete circuit open or short. This suggests the sensor itself, or its associated wiring and connections, is failing to provide a reliable and accurate pressure reading, directly impacting the PCM’s ability to precisely control engine operation.

Common Symptoms

• For Dual Alternator Lower Circuit Malfunction:
  • Illumination of the battery warning light or “Check Charging System” message.
  • Erratic voltage gauge readings on the dashboard.
  • Dimming of headlights, interior lights, or flickering dashboard illumination.
  • Slow or inoperative electrical accessories (e.g., power windows, HVAC fan).
  • Difficulty starting the vehicle due to an undercharged battery.
  • Premature battery wear or failure.
• For MAP Sensor Circuit Intermittent Low Voltage:
  • Illumination of the Malfunction Indicator Lamp (MIL – Check Engine Light).
  • Reduced engine performance, including sluggish acceleration or loss of power.
  • Rough idle, stalling, or hesitation during acceleration.
  • Increased fuel consumption due to incorrect fuel mixture.
  • Difficulty starting the engine.
  • Possible black smoke from the exhaust (due to a rich fuel condition if the PCM defaults to a higher pressure reading).

What Causes the Code P1107?

• For Dual Alternator Lower Circuit Malfunction:
  • Faulty “lower” or secondary alternator (e.g., internal regulator failure, rectifier diode bridge malfunction).
  • Damaged, corroded, or loose wiring in the main power output circuit or the control circuit of the secondary alternator.
  • Corroded or loose battery cable terminals or main power distribution connections.
  • Defective external voltage regulator (if applicable to the system design).
  • Blown fuse or fusible link protecting the secondary alternator circuit.
  • Internal ECM/PCM fault (less common, but possible).
• For MAP Sensor Circuit Intermittent Low Voltage:
  • Defective MAP sensor itself (internal failure causing intermittent low voltage output).
  • Damaged, frayed, or corroded wiring in the MAP sensor signal, reference voltage, or ground circuits.
  • Loose or corroded electrical connector at the MAP sensor.
  • Intermittent short to ground in the MAP sensor signal circuit.
  • Vacuum leaks in the intake manifold or the hose connecting the MAP sensor to the manifold (less likely to cause a low voltage signal directly, but can affect the sensor’s reading).
  • Internal ECM/PCM fault (less common, but possible).

How to Diagnose and Troubleshoot

Diagnosis of P1107 requires a methodical approach, starting with identification of the vehicle’s specific system (dual alternator or MAP sensor related) and then performing targeted electrical tests.

1. Verify Code and Context: Connect an OBD-II scanner to confirm P1107. Note any other related or pending codes, as they can provide critical diagnostic context. Clear the code and road test the vehicle under conditions where the fault typically occurs to see if it immediately returns.
2. Visual Inspection:
   • Inspect the battery terminals for corrosion or looseness.
   • For dual alternator systems: Visually inspect both alternators, their wiring harnesses, and all associated connections for signs of damage, corrosion, or looseness. Check the alternator drive belts for proper tension and condition.
   • For MAP sensor systems: Locate the MAP sensor and inspect its electrical connector and wiring for any signs of damage, fraying, or corrosion. Verify any vacuum lines connected to the MAP sensor are intact, properly connected, and free of cracks or leaks.
3. For Dual Alternator Lower Circuit Malfunction:
   • Battery Voltage Check: With a Digital Multimeter (DMM), measure battery voltage with the engine off (should be approximately 12.6V). Start the engine and measure voltage at idle (should be between 13.5V and 14.7V). If the voltage is low, activate electrical loads (headlights, HVAC fan on high) and observe the voltage drop.
   • Individual Alternator Output Test: Using a DMM, carefully measure the voltage at the main output terminal of each alternator with the engine running and under load. Compare the readings. A significantly lower reading from the secondary alternator indicates a potential issue.
   • Circuit Integrity Test (Secondary Alternator): Refer to the vehicle’s wiring diagram. With the engine off and battery disconnected, use the DMM to check for continuity and resistance in the primary and control circuits of the secondary alternator, paying close attention to power supply lines, ground circuits, and field control wires. Look for open circuits or excessive resistance. Perform a voltage drop test on the main power feed from the secondary alternator to the battery under load.
   • PCM Input Check: If accessible, use an advanced scan tool to monitor the PCM’s live data for individual alternator output commands and actual voltage/current readings from each alternator, if supported by the vehicle.
4. For MAP Sensor Circuit Intermittent Low Voltage:
   • MAP Sensor Connector Voltage Checks: Disconnect the MAP sensor connector. With the ignition ON and engine OFF, use a DMM to test for:
     • Reference voltage (typically 5V) at the appropriate pin.
     • A good ground signal at the ground pin.

     If either is missing or out of spec, trace wiring back to the PCM.

   • MAP Sensor Signal Voltage Test: Reconnect the MAP sensor. Backprobe the signal wire with the DMM. With the engine idling, observe the voltage reading (typically 0.5V-1.5V, varying with vacuum). Increase engine RPMs and observe the voltage changes (it should increase with decreasing vacuum/increasing load). The key here is to look for intermittent drops or fluctuations to abnormally low voltages. An oscilloscope can be very useful for capturing intermittent faults.
   • Wiggle Test: While monitoring the MAP sensor signal voltage on the DMM or oscilloscope, gently wiggle the MAP sensor connector and the wiring harness leading to it. Observe if the voltage reading fluctuates or drops intermittently, indicating a loose connection or damaged wiring.
   • Vacuum Check: Using a handheld vacuum pump or a separate vacuum gauge, apply vacuum to the MAP sensor (if it’s a port-style sensor) or connect a gauge to the intake manifold. Compare the actual manifold pressure/vacuum to the MAP sensor’s voltage output (higher vacuum = lower pressure = lower voltage output from sensor).

Recommended Repairs and Solutions

• For Dual Alternator Lower Circuit Malfunction:
  • Replace Faulty Alternator: If diagnostics confirm the secondary alternator is not producing adequate output or its internal regulator has failed, replace the entire alternator assembly.
  • Repair Wiring and Connections: Address any damaged, corroded, or loose wiring and connectors found during inspection and testing. Clean all battery and alternator terminals and ensure they are securely fastened.
  • Replace Fuse/Fusible Link: If a blown fuse or fusible link is identified in the secondary alternator circuit, replace it after verifying the underlying cause of the overload.
  • Replace External Voltage Regulator: If the vehicle uses an external voltage regulator and it tests faulty, replace it.
• For MAP Sensor Circuit Intermittent Low Voltage:
  • Replace MAP Sensor: If the sensor itself is found to be intermittently outputting low voltage despite proper power and ground, replacement of the MAP sensor is the most common and effective solution.
  • Repair Wiring Harness: Repair or replace any damaged, corroded, or shorted wiring within the MAP sensor’s electrical circuit. Pay close attention to the signal wire.
  • Clean or Replace Connector: If the MAP sensor connector is corroded or shows signs of intermittent contact, clean it thoroughly or replace the connector housing and terminals.
  • Address Vacuum Leaks: Although less directly related to “low voltage,” ensure all vacuum lines to the MAP sensor and the intake manifold are sealed and free of leaks to prevent secondary issues or misdiagnosis.