Code
P1820
FORD
P — Powertrain
Transmission Transfer Case Clockwise Shift Relay Coil Circuit Failure
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring between the transfer case control module (TCCM) and CW shift relay coil
- Failed CW shift relay (coil open or intermittent)
- Corroded, loose, or damaged relay connector or module connector
- Blown fuse or poor fuse connection feeding the relay coil circuit
- Faulty transfer case control module or relay driver output
- Water intrusion or corrosion in transfer case or harness
Symptoms
- Unable to shift into specific transfer case positions (4WD/4x4 modes) or intermittent shifting
- 4WD/transfer case malfunction warning lamp or message
- Stored P1820 in scan tool live data
- No audible click from relay when 4WD shift is commanded
- Inconsistent or delayed engagement of 4WD
What to check
- Scan for stored and pending codes; record freeze frame and related codes
- Verify battery voltage is within specification (key on and cranking)
- Inspect fuse(s) related to transfer case/relay coil for continuity and proper seating
- Visually inspect relay, relay socket, transfer case connectors and wiring for corrosion, damage, or water intrusion
- Backprobe relay coil pins and check for expected voltage/ground while commanding CW shift with a scan tool
- Measure coil resistance of the relay (compare to known-good or service data)
Signal parameters
- Relay coil continuity: typically tens to a few hundred ohms (model-dependent) — expect a finite resistance, not infinite or near zero
- Control voltage to relay coil: ~battery voltage (~12 V) when relay is commanded ON (key on/command active)
- Ground continuity: low resistance to chassis ground when relay is commanded
- No voltage (or very low) on coil when relay not commanded
Diagnostic algorithm
- Connect a professional scan tool, confirm P1820 and note any related transfer case or communications codes. Clear codes and attempt to reproduce.
- Visually inspect relay, relay socket, wiring harness, and transfer case connectors for corrosion, bent pins, water, or mechanical damage. Repair any obvious issues.
- Check related fuse(s) for continuity and proper voltage at the fuse while the circuit is powered.
- Remove the CW shift relay and measure coil resistance across the coil terminals. If coil is open or shorted, replace relay.
- With relay removed, backprobe the relay socket and command the CW shift using the scan tool. Verify the control side receives battery voltage and a ground/driver signal when commanded.
- If voltage or ground is missing or intermittent at the socket, trace wiring back to the transfer case control module looking for opens, shorts to ground or battery, or poor splices/connectors. Repair wiring as needed.
- If correct voltages are present at the socket but relay does not operate when commanded, replace the relay and retest.
- If relay and wiring are good but command output from the TCCM is absent or incorrect, test/replace the transfer case control module per manufacturer procedures.
- After repairs, clear codes and verify proper transfer case operation through multiple shift cycles and a road test if applicable.
Likely causes
- Damaged connector at the relay or transfer case control module (most common)
- Failed relay coil (relay stuck open or intermittent)
- Broken or chafed wiring harness to the transfer case
- Failed transfer case control module output driver
Fault status
Status
Transfer case clockwise shift relay coil circuit failure — open/short/incorrect voltage or ground detected. Transfer case shift may be unavailable or intermittent.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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