P1859

What Does Code P1859 Mean?

DTC P1859 indicates an Open Circuit in the Transmission Transfer Case Contact Plate ‘D’ circuit. This code is typically set by the Powertrain Control Module (PCM) or the Transfer Case Control Module (TCCM) when it detects a loss of electrical continuity on a specific circuit related to the transfer case’s internal mode selection or position sensing mechanism. The ‘D’ designation refers to a particular contact or circuit within the multi-position switch or sensor array that monitors the transfer case’s operational mode (e.g., 2WD, 4WD High, 4WD Low, Neutral). The PCM/TCCM supplies a reference voltage to this circuit and expects a specific voltage return or resistance value based on the selected transfer case position. An open circuit implies that this expected electrical path is broken, preventing the module from accurately determining or confirming the transfer case’s state. This loss of communication or feedback can prevent proper engagement or disengagement of four-wheel drive modes, as the control module cannot verify the actual position of the transfer case.

Common Symptoms

• Inability to shift into or out of 4WD modes.
• The 4WD indicator light (e.g., 4WD, Service 4WD, or specific mode lights) may be flashing or illuminated erratically, or not at all.
• The vehicle may be stuck in 2WD, 4WD High, or 4WD Low.
• Check Engine Light (MIL) illumination.
• Reduced traction control system performance (if integrated with 4WD system).
• A clunking or grinding noise from the transfer case during attempted mode changes (less common but possible if the control module tries to actuate without proper feedback).

What Causes the Code P1859?

• Damaged or corroded wiring harness: An open circuit can be caused by a broken wire, damaged insulation leading to a short to ground (though less common for “open circuit”), or severe corrosion within the wiring harness connecting the transfer case contact plate switch/sensor to the TCCM/PCM.
• Faulty electrical connectors: Loose, corroded, or damaged terminal pins within the connectors at either the transfer case side or the TCCM/PCM side can interrupt the circuit.
• Internal failure of the transfer case contact plate switch/sensor: The contact plate itself may have worn out, suffered internal damage, or experienced a mechanical or electrical failure, leading to a permanent open circuit within the component.
• Transfer case shift motor assembly failure: If the contact plate is an integral part of the shift motor assembly, a fault within the motor’s internal gearing or electrical connections could cause this specific circuit to open.
• Faulty Transfer Case Control Module (TCCM) or Powertrain Control Module (PCM): While less common, an internal fault within the control module itself, such as a failed driver circuit or monitoring resistor, could falsely report an open circuit or fail to provide the necessary reference voltage.

How to Diagnose and Troubleshoot

Diagnosis of P1859 requires a systematic approach, focusing on electrical circuit integrity:

1. Retrieve & Verify DTCs: Use an OBD-II scanner to confirm P1859 is the active code. Check for any related transfer case or communication codes that might provide additional context. Record any freeze frame data.
2. Visual Inspection:
   • Inspect the entire wiring harness leading to the transfer case, specifically looking for signs of damage, chafing, corrosion, or modifications. Pay close attention to areas near exhaust components, sharp edges, or points of common vibration.
   • Carefully examine all electrical connectors associated with the transfer case, including the transfer case shift motor, position sensors, and the TCCM. Look for bent or pushed-out pins, corrosion, or signs of moisture intrusion. Ensure connectors are fully seated.
3. Electrical Circuit Testing (with DMM):
   • Isolate the Circuit: Disconnect the connector at the transfer case shift motor/sensor and the TCCM/PCM (refer to service information for exact pinouts and module locations).
   • Continuity Test (Wiring Harness): Using a digital multimeter (DMM) set to ohms, test for continuity on the specific ‘D’ circuit wire between the TCCM/PCM connector and the transfer case connector. An open circuit will show infinite resistance (OL). Test for continuity to ground and to battery voltage on this same wire; there should be none, indicating no shorts.
   • Reference Voltage Check: With the TCCM/PCM connector reconnected (but the transfer case side disconnected), verify that the TCCM/PCM is outputting the correct reference voltage (typically 5V or 12V, depending on the system design) and a good ground to the transfer case contact plate ‘D’ circuit pin.
   • Transfer Case Contact Plate Switch/Sensor Test: With the transfer case shift motor/sensor disconnected from the harness, measure the resistance across the pins corresponding to the ‘D’ circuit and its common return. Consult service information for the expected resistance values for each transfer case mode. If the internal component shows infinite resistance on the ‘D’ circuit regardless of mode, the switch/sensor is likely faulty.
   • Wiggle Test: While monitoring the DMM on continuity or resistance tests, gently wiggle the wiring harness and connectors. An intermittent open circuit will show fluctuating readings or momentary loss of continuity.
4. Check TCCM/PCM Operation: If all wiring and component tests pass, and reference voltages are correct, but the code persists, it may indicate an internal fault within the TCCM or PCM. This should be a last resort after ruling out all other possibilities.

Recommended Repairs and Solutions

• Repair or Replace Damaged Wiring: If the visual inspection or continuity test reveals a broken or corroded wire, repair the specific section using appropriate automotive-grade wiring and connectors, or replace the entire harness if damage is extensive. Ensure proper soldering and heat-shrink tubing for environmental protection.
• Clean or Replace Corroded Connectors: If corrosion is found, clean terminals thoroughly with electrical contact cleaner and a small brush. If pins are bent, broken, or heavily corroded, replace the affected connector shell and/or individual terminals. Ensure proper terminal tension for a secure connection.
• Replace Transfer Case Contact Plate Switch/Sensor or Shift Motor Assembly: In most cases, the contact plate ‘D’ is integral to the transfer case shift motor assembly or an internal mode sensor. If electrical testing indicates an internal fault, replacement of this assembly is typically required. Follow manufacturer’s procedures for removal and installation, paying attention to any specific indexing or calibration steps.
• Inspect Transfer Case Fluid: While not a direct cause of an open circuit, ensure the transfer case fluid level and condition are correct. Incorrect fluid can contribute to mechanical issues that might indirectly stress electrical components.
• TCCM/PCM Replacement: Only consider replacing the TCCM or PCM after thoroughly verifying that all other components and wiring are functioning correctly. These modules often require programming or calibration after replacement, which typically requires specialized diagnostic tools.
• Clear Codes and Test Drive: After any repair, clear the DTCs from the PCM/TCCM memory. Perform a test drive, engaging various 4WD modes, to confirm the repair and ensure the code does not return.