P1820

What Does Code P1820 Mean?

DTC P1820 signifies a detected electrical malfunction within the Transmission Transfer Case Clockwise Shift Relay Coil Circuit. In vehicles equipped with an electronically controlled transfer case, the transfer case control module (TCCM) or powertrain control module (PCM) monitors the operational status and electrical integrity of the components responsible for shifting the transfer case between various drive modes (e.g., 2WD, 4WD High, 4WD Low, Auto 4WD). The transfer case shift motor, a bidirectional DC motor, changes the internal gearing by moving a shift fork. This motor’s direction and activation are controlled by a set of relays. The “clockwise shift relay coil” refers to the specific coil within one of these relays responsible for energizing the circuit that drives the transfer case shift motor in the clockwise direction, typically to engage a higher range or specific 4WD mode. When the TCCM/PCM commands this relay to activate and detects an abnormal voltage, current, or resistance value on its control circuit – such as an open circuit (high resistance), a short to ground, or a short to voltage – P1820 is set. This indicates that the module is unable to properly energize or de-energize the relay coil, preventing the shift motor from functioning as commanded and potentially leaving the transfer case stuck in a specific mode or unable to shift.

Common Symptoms

• Inability to Shift Transfer Case Modes: The primary symptom is the inability to select or engage different 2WD/4WD modes (e.g., unable to shift into 4WD High or Low, or stuck in 2WD).
• Service 4WD Light Illumination: The “Service 4WD” or “Service AWD” indicator light will often illuminate on the instrument cluster.
• 4WD Indicator Lights Malfunction: The 2WD/4WD selector switch indicator lights may flash erratically, not illuminate, or indicate the wrong mode.
• Grinding or Clunking Noises (Rare): If the transfer case attempts to shift but fails to fully engage, grinding or clunking noises might occur, though this is less common for a circuit failure than for mechanical binding.
• Lack of Transfer Case Motor Activity: No audible sounds of the transfer case shift motor attempting to operate when a mode change is commanded.

What Causes the Code P1820?

• Faulty Transfer Case Shift Relay: The relay itself may have an internal coil failure (open circuit), burnt contacts, or internal short.
• Damaged or Corroded Wiring: An open circuit, short to ground, or short to voltage in the wiring harness between the TCCM/PCM and the transfer case shift relay coil. This includes corrosion at connectors.
• Poor Electrical Connections: Loose or corroded terminals at the TCCM/PCM connector, the relay socket, or the shift motor connector.
• Faulty Transfer Case Control Module (TCCM/PCM): Although less common, an internal failure within the TCCM or PCM could prevent it from properly controlling the relay circuit or accurately monitoring its feedback.
• Incorrect Relay Installed: Installation of an incorrect relay with different electrical specifications.

How to Diagnose and Troubleshoot

Proper diagnosis of P1820 requires a systematic approach using a diagnostic scanner, digital multimeter (DMM), and comprehensive wiring diagrams for the specific vehicle. The following steps outline a professional diagnostic procedure:

1. Retrieve and Document DTCs: Connect an OBD-II scanner and retrieve all stored Diagnostic Trouble Codes. Note any other related transfer case or powertrain codes, as they might provide additional context. Clear the codes after documentation.
2. Visual Inspection:
   • Inspect the transfer case shift relay and its connector for signs of corrosion, burning, or physical damage.
   • Trace the wiring harness from the TCCM/PCM to the relay and then to the transfer case shift motor. Look for chafed, pinched, or damaged wires.
   • Check all relevant fuses in the underhood and cabin fuse boxes that supply power to the TCCM and transfer case shift motor circuits.
3. Relay Functionality Test:
   • Locate the transfer case shift relays (there are usually two: one for clockwise, one for counter-clockwise rotation). Swap the suspected faulty clockwise shift relay with a known good, identical relay from a non-critical circuit (e.g., horn relay, if compatible) to see if the symptoms clear or the code shifts to the other direction.
   • Alternatively, use a DMM to test the relay’s coil resistance. Refer to service specifications for the correct ohmic value. An open circuit (OL) or very low resistance indicates a faulty coil.
   • Apply 12V and ground directly to the relay’s coil terminals (85 and 86) and listen for an audible click, then test for continuity across the switch terminals (30 and 87) when activated.
4. Circuit Integrity Testing (with DMM):
   • Power and Ground to Relay: With the ignition on, use a DMM to verify 12V power supply to the relay’s coil and switch sides, as well as a solid ground connection. Identify the relay’s control wire from the TCCM/PCM.
   • Control Circuit Voltage: Back-probe the TCCM/PCM connector at the terminal corresponding to the clockwise shift relay control wire. With the ignition on and the transfer case commanded to shift (using a bi-directional scanner or manually through the switch), observe if the TCCM/PCM attempts to send a voltage signal (usually a ground or 12V depending on the control strategy) to activate the relay. If no signal is present, suspect the TCCM/PCM or its input.
   • Continuity Test: Disconnect the TCCM/PCM and the relay. Perform a continuity test on the control wire from the TCCM/PCM connector to the relay socket. A reading of “OL” indicates an open circuit.
   • Short to Ground/Voltage Test: With the TCCM/PCM and relay disconnected, check for continuity between the relay control wire and a known good chassis ground. Any continuity indicates a short to ground. Also, check for voltage on this wire with the ignition on; any voltage indicates a short to voltage.
   • Resistance of Shift Motor: While not a direct cause of P1820, an excessively high resistance or internal short in the shift motor itself could overload the relay circuit, leading to relay failure or TCCM/PCM shutdown.
5. TCCM/PCM Diagnostic (Advanced): If all external wiring and relay checks pass, and the TCCM/PCM is not commanding the relay or generating the fault internally, further diagnostics on the TCCM/PCM itself may be necessary. This often involves specialized diagnostic tools or replacement for testing.

Recommended Repairs and Solutions

The resolution for P1820 typically involves addressing the specific electrical fault identified during diagnosis:

• Replace the Faulty Transfer Case Shift Relay: If the relay itself is found to be defective after testing, replacement with a genuine OEM or equivalent quality aftermarket part is the most common solution. Ensure the new relay has the correct electrical specifications.
• Repair or Replace Damaged Wiring: Any wiring exhibiting signs of chafing, shorting, or an open circuit must be professionally repaired using soldering and heat-shrink tubing, or by replacing the affected section of the harness. Ensure all connections are watertight and secure.
• Clean and Secure Electrical Connectors: If corrosion or loose terminals are found at any connector – including the relay socket, TCCM/PCM connector, or transfer case motor connector – clean them thoroughly with electrical contact cleaner and ensure a tight, secure fit. Pin-tensioning tools may be required for loose terminals.
• Replace Faulty TCCM/PCM: In rare cases where internal module failure is confirmed (after ruling out all external wiring and relay issues), the TCCM or PCM may need replacement. Note that module replacement often requires programming or “relearning” procedures using a factory-level scan tool.

Mechanic’s Tip: Always verify the repair by clearing the DTCs and performing a road test, attempting to shift through all transfer case modes multiple times. Pay close attention to the 4WD indicator lights and ensure no P1820 or related codes return. Given the exposed location of transfer case components, pay extra attention to environmental damage – water intrusion, road salt, and debris can cause significant corrosion over time, especially at connectors.