P1106

What Does Code P1106 Mean?

The diagnostic trouble code (DTC) P1106 is a manufacturer-specific code, meaning its precise definition can vary significantly depending on the vehicle’s make and model. Historically, and most commonly, P1106 refers to a Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent High Voltage condition. In this context, the Powertrain Control Module (PCM), also known as the Engine Control Module (ECM), detects an intermittent signal voltage from the MAP sensor that is outside the expected operating range, specifically registering higher than the calibrated threshold for given engine operating conditions. This typically implies the MAP sensor is reporting manifold pressure closer to atmospheric pressure (low vacuum) when the engine should be operating under a higher vacuum state, or there is an electrical short to voltage within the sensor circuit itself.

Alternatively, for certain General Motors (GM) vehicles, particularly those equipped with dual alternator systems (e.g., some trucks or heavy-duty applications), P1106 signifies a Dual Alternator Lower Fault. In this application, the PCM monitors the output and operational status of both alternators. A P1106 indicates that the lower, or secondary, alternator is not functioning correctly, not contributing its expected share of the electrical load, or has an internal fault such that its generated field duty cycle or voltage output is below specification or intermittent.

Common Symptoms

• For MAP Sensor Circuit Intermittent High Voltage:
  • Rough or unstable engine idle
  • Reduced engine performance and acceleration
  • Poor fuel economy or excessive fuel consumption (black smoke from exhaust)
  • Engine hesitation or misfires
  • Difficulty starting the engine
  • Engine stalling, especially when coming to a stop
  • Illuminated Check Engine Light (CEL)
• For Dual Alternator Lower Fault (GM specific):
  • Battery discharge or low battery voltage warning light
  • Dimming of headlights, interior lights, or dashboard lights, particularly under electrical load
  • Intermittent electrical system malfunctions
  • Difficulty starting the engine due to insufficient battery charge
  • Illuminated Check Engine Light (CEL)
  • Visible wear or damage to the lower alternator drive belt

What Causes the Code P1106?

• For MAP Sensor Circuit Intermittent High Voltage:
  • Faulty MAP Sensor: An internal electrical fault within the sensor causing an intermittent high voltage output.
  • Vacuum Leaks: Leaks in the intake manifold gasket, vacuum hoses, PCV system, brake booster, or throttle body gasket, which can cause erratic or low manifold vacuum, making the MAP sensor report higher pressure.
  • Wiring Harness Issues: Damaged, chafed, corroded, or intermittently shorted wiring in the MAP sensor circuit (specifically a short to voltage).
  • Poor Electrical Connection: Loose or corroded connector terminals at the MAP sensor or PCM.
  • Blocked or Damaged MAP Sensor Vacuum Port: Restriction preventing accurate manifold pressure readings.
  • Faulty PCM: While less common, an internal PCM fault in processing the MAP sensor signal could trigger this code.
• For Dual Alternator Lower Fault (GM specific):
  • Faulty Lower Alternator: Internal mechanical failure (e.g., bearings) or electrical component failure (e.g., rectifier, voltage regulator).
  • Damaged or Loose Alternator Drive Belt: A slipping or broken belt preventing the lower alternator from being driven effectively.
  • Wiring or Circuit Issues: Open circuit, short to ground, or high resistance in the field control circuit, battery positive circuit, or sense wire for the lower alternator.
  • Corroded Battery Cables/Terminals: Can impede proper current flow and charging system integrity.
  • Faulty Voltage Regulator: An issue with the voltage regulator, often integrated within the alternator, preventing proper output.

How to Diagnose and Troubleshoot

Given the dual nature of P1106, the initial diagnostic step is to determine the vehicle’s specific interpretation of the code. Consult a manufacturer-specific service manual for the exact definition for the year, make, and model in question.

General Diagnostic Steps (Applicable to Both Interpretations):

1. OBD-II Scan Tool Data Review:
   • Connect an OBD-II scan tool and record freeze frame data. This provides crucial information on engine operating conditions (RPM, engine load, coolant temperature, fuel trim, etc.) when the DTC was set, which can help replicate the fault.
   • Clear the DTC and perform a road test under conditions similar to those in the freeze frame data to confirm if the code returns.
2. Thorough Visual Inspection:
   • Inspect all visible wiring harnesses for signs of damage, chafing, corrosion, or loose connections, paying close attention to the MAP sensor circuit or alternator control circuits and main battery cables.
   • Check for obvious vacuum leaks (hoses, manifold gaskets) if applicable to the MAP sensor.
   • Inspect the serpentine belt condition and tension for the alternator system.

Specific Diagnostic Steps for MAP Sensor Circuit Intermittent High Voltage:

1. MAP Sensor Circuit Voltage Check (using DMM):
   • With the ignition ON (engine OFF), backprobe the MAP sensor connector to verify the 5-volt reference voltage and ground circuit integrity.
   • With the engine running at idle, measure the signal voltage from the MAP sensor. Typically, a healthy MAP sensor at idle will output low voltage (e.g., 1.0-1.5V) due to high manifold vacuum. A high voltage (e.g., >4.0V) at idle indicates a problem.
   • Observe the signal voltage fluctuation during throttle application. The voltage should rise with increasing throttle (decreasing vacuum) and drop with decreasing throttle (increasing vacuum).
   • Perform a “wiggle test” on the wiring harness and connector while monitoring the MAP sensor signal voltage with a DMM or oscilloscope to identify intermittent opens or shorts.
2. Vacuum System Inspection:
   • Connect a handheld vacuum gauge directly to the intake manifold (or a known good vacuum source) and compare its reading to the live data reading from the MAP sensor. They should correlate closely.
   • Perform an intake manifold smoke test to precisely identify any vacuum leaks that could cause erroneous high pressure readings at the MAP sensor.
3. Continuity and Resistance Test:
   • With the battery disconnected and PCM connector removed, check for continuity and resistance between the MAP sensor connector and the PCM connector for each wire (reference, ground, signal). Look for opens, shorts to ground, or shorts to voltage.

Specific Diagnostic Steps for Dual Alternator Lower Fault (GM specific):

1. Battery and Charging System Test (using DMM and Load Tester):
   • Measure battery voltage with the engine off (should be approx. 12.6V).
   • With the engine running, measure charging system voltage across the battery terminals. It should typically be between 13.5V and 14.7V. If significantly lower, investigate further.
   • Perform a battery load test to ensure battery integrity.
2. Alternator Output Test:
   • Using an appropriate automotive diagnostic scan tool capable of monitoring GM specific PIDs, observe the Generator Field Duty Cycle, Generator Status, and system voltage for both alternators. The PCM commands the alternators, and their response (duty cycle) indicates their contribution.
   • If possible, perform an amperage output test on both the primary and secondary (lower) alternators using an inductive amp clamp. Compare outputs to manufacturer specifications.
3. Wiring and Circuit Integrity Check:
   • Inspect the main battery cable connections to both alternators, ensuring they are clean and tight.
   • Check the integrity of the field control wire and sense wire between the lower alternator and the PCM. Look for opens, shorts to ground, or shorts to voltage using a DMM.
4. Alternator Diode Ripple Test:
   • Using an oscilloscope, check the AC ripple voltage at the alternator B+ terminal. Excessive AC ripple can indicate a failing diode rectifier bridge within the alternator.

Recommended Repairs and Solutions

For MAP Sensor Circuit Intermittent High Voltage:

1. Address Vacuum Leaks: Systematically repair any identified vacuum leaks by replacing cracked or worn vacuum hoses, intake manifold gaskets, throttle body gaskets, or a faulty PCV valve.
2. Replace MAP Sensor: If diagnostic testing confirms the MAP sensor itself is faulty and producing an intermittent high voltage signal, replace it with an OEM quality sensor.
3. Repair Wiring Harness/Connectors: Repair any damaged, corroded, or shorted wiring in the MAP sensor circuit. Clean corroded connector terminals and ensure all connections are secure. If a connector is severely damaged, replace it.

For Dual Alternator Lower Fault (GM specific):

1. Replace Lower Alternator: If diagnostics confirm the lower alternator is not producing adequate output, has an internal fault (e.g., regulator, rectifier, or bearings), or is unresponsive to PCM commands, replace it.
2. Replace or Adjust Drive Belt: If the alternator drive belt is worn, stretched, or broken, replace it. Ensure proper belt tension as per manufacturer specifications.
3. Repair Wiring: Fix any open circuits, shorts, or high resistance points in the wiring connecting the lower alternator to the PCM or the main battery cables. This includes cleaning corroded battery terminals and ensuring secure connections.

Mechanic’s Tips:

• Always clear DTCs after repairs and perform a comprehensive road test to confirm the fix and ensure the code does not return. Pay attention to freeze frame conditions for intermittent faults.
• For intermittent issues, recreating the exact conditions under which the code set is paramount for diagnosis. This might involve driving cycles, specific engine loads, or temperature ranges.
• When replacing electrical sensors or alternators, always opt for OEM or high-quality aftermarket components to ensure compatibility and longevity.
• After replacing an alternator, it’s good practice to fully charge the battery using an external charger to minimize the initial load on the new alternator.
• For MAP sensor issues, always rule out vacuum system integrity before condemning the sensor itself.