P1879

What Does Code P1879 Mean?

DTC P1879 indicates an “open circuit” condition detected within the electrical circuit of the Transmission Transfer Case Disengaged Solenoid. In layman’s terms, the Powertrain Control Module (PCM), or in some vehicle architectures, the Transfer Case Control Module (TCCM), has detected a break in the electrical continuity to the solenoid responsible for shifting the transfer case out of an engaged 4-wheel drive mode or into 2-wheel drive (or neutral, depending on system design). The module monitors the expected electrical resistance and current flow through this solenoid when it commands it ON or OFF. If the measured resistance is infinite or the current flow is zero when the solenoid should be active, it signifies an open circuit, meaning electricity cannot flow through the component as intended. This specific subsystem affects the vehicle’s ability to engage or disengage the four-wheel-drive system, potentially leaving it stuck in a particular drive mode or preventing mode changes.

Common Symptoms

• Service 4WD or 4×4 warning light illuminated on the instrument cluster.
• Inability to shift the transfer case into or out of 4-wheel drive modes (e.g., stuck in 2WD, stuck in 4HI).
• Difficulty engaging or disengaging the transfer case, often accompanied by grinding noises during attempted shifts.
• Reduced fuel economy if the transfer case is stuck in a 4WD mode on dry pavement.
• Unusual noises emanating from the transfer case area.
• Vehicle operating solely in 2WD mode despite driver input for 4WD.

What Causes the Code P1879?

• Faulty Transfer Case Disengaged Solenoid: The most common cause, where the internal coil of the solenoid has an open circuit, preventing electrical current flow.
• Open Circuit in Wiring Harness: A break, cut, or severely corroded wire in the circuit leading to the transfer case disengaged solenoid. This could be due to physical damage, rodent damage, or chafing.
• Poor Electrical Connection: A loose, corroded, or damaged connector at either the solenoid itself or at the Transfer Case Control Module (TCCM) / Powertrain Control Module (PCM).
• Faulty Transfer Case Control Module (TCCM) / Powertrain Control Module (PCM): While less common, an internal fault within the control module, specifically the driver circuit responsible for activating the solenoid, could set this code.

How to Diagnose and Troubleshoot

Diagnosis of P1879 requires systematic electrical testing. Always begin by reviewing relevant Technical Service Bulletins (TSBs) for your specific vehicle make and model, as manufacturers often have known issues or diagnostic procedures for this type of fault.

1. Retrieve Freeze Frame Data: Connect an OBD-II scanner and record P1879 along with any associated freeze frame data. This data provides conditions (engine speed, vehicle speed, selected 4WD mode) under which the code was set, which can be valuable for replication or understanding the context of the failure.
2. Perform a Thorough Visual Inspection:
   • Inspect the entire wiring harness leading to the transfer case disengaged solenoid. Look for signs of chafing, cuts, pinch points, corrosion, or rodent damage. Pay close attention to areas where the harness passes through metalwork or near exhaust components.
   • Examine the electrical connector at the transfer case disengaged solenoid. Check for bent, pushed-back, or corroded pins. Ensure the connector is securely seated and the locking tab is engaged.
   • Locate the transfer case disengaged solenoid (typically mounted on the transfer case housing or an external actuator assembly). Inspect for any visible physical damage or loose mounting.
3. Electrical Testing of the Solenoid:
   • Disconnect the electrical connector from the transfer case disengaged solenoid.
   • Using a Digital Multimeter (DMM) set to the ohms (Ω) scale, measure the resistance directly across the two terminals of the solenoid itself.
   • Compare the reading to the vehicle manufacturer’s specifications (found in the factory service manual). Typical solenoid resistance values range from 20 to 40 ohms.
   • An “OL” (Open Loop) or infinite resistance reading on the DMM confirms an internal open circuit within the solenoid, indicating it is faulty. A reading significantly outside the specified range also points to an issue.
4. Electrical Testing of the Wiring Harness:
   • With the solenoid still disconnected and the ignition ON (engine OFF), use the DMM to test for power supply at the solenoid harness connector. One pin should typically have battery voltage (e.g., 12V) relative to a known good chassis ground. If no voltage is present, trace the power circuit back to its fuse or relay.
   • Test for continuity on the control wire(s) from the solenoid harness connector back to the TCCM/PCM connector (with both ends disconnected). Use the vehicle’s wiring diagram to identify the correct pins. An “OL” reading indicates an open circuit in the wiring.
   • While performing continuity checks, also check for shorts to ground and shorts to voltage on both the power and control wires by measuring resistance between each wire and chassis ground, and between each wire and battery positive.
5. TCCM/PCM Diagnosis: Only if the solenoid and all associated wiring test perfectly within specifications should the TCCM/PCM be suspected. This usually requires advanced diagnostic equipment capable of bi-directional control and monitoring the solenoid driver circuit output.

Recommended Repairs and Solutions

Addressing a P1879 code typically involves repairing or replacing the component identified as faulty during the diagnostic process. Given the nature of an “open circuit,” the solution is often straightforward once the exact location of the break is found.

• Replace the Transfer Case Disengaged Solenoid: If the solenoid’s internal resistance tests as an open circuit (OL), replacement is necessary.
  • Procedure: Disconnect the vehicle’s battery. Depending on the solenoid’s location, you may need to drain a small amount of transfer case fluid. Disconnect the electrical connector, remove any mounting bolts, and carefully extract the old solenoid. Install the new solenoid, ensuring any O-rings or gaskets are properly seated. Reconnect the electrical connector and mounting hardware. If fluid was drained, refill the transfer case to the specified level with the correct fluid type. Reconnect the battery.
  • Mechanic’s Tip: Always use an OEM or reputable aftermarket solenoid to ensure proper function and longevity.
• Repair or Replace Wiring Harness: If an open circuit is located in the wiring harness, the damaged section must be repaired.
  • Procedure: For minor damage, carefully splice in a new section of wire using crimp connectors and heat-shrink tubing to ensure a weather-tight and robust connection. For extensive damage or multiple breaks, it may be more efficient to replace the entire segment of the harness or the full harness if available.
  • Mechanic’s Tip: When repairing wiring, ensure the gauge of the replacement wire matches or exceeds the original, and use solder connections for maximum reliability, protected by marine-grade heat shrink. Secure the repaired harness away from potential chafing points.
• Clean or Replace Connectors: If corrosion or damage is found at an electrical connector, thoroughly clean the terminals using electrical contact cleaner and a small brush. If pins are bent or severely corroded, the connector body or individual pins may need to be replaced. Apply dielectric grease upon reassembly to prevent future corrosion.
• TCCM/PCM Replacement: This is a last resort. If all other components (solenoid, wiring, and connectors) are confirmed to be in perfect working order, and the control module’s driver circuit for the solenoid is proven faulty, then the TCCM/PCM may require replacement. This often necessitates programming or flashing the new module to the vehicle’s specific VIN, a task typically performed by a dealership or a specialist with factory diagnostic tools.

After any repair, clear the DTCs with an OBD-II scanner and perform a thorough test drive, attempting to activate and deactivate the 4WD system multiple times to confirm the repair is successful and the code does not return.