JCB P0685 – ECU: ECM Power Relay Control Circuit Open

The fault code indicates an open circuit in the control circuit for the ECM / PCM power relay. This prevents the relay from functioning properly and supplying a stable voltage to the engine control module. As a result, power to the engine's computer is cut, which can severely affect the operation of the engine control system.

• 3CX
• 4CX
• 2CX
• 214
• 215
• 217
• 526
• 533-105
• 540
• TM320
• Fastrac
• Generators
• Robot

1. 1

   Use an OBD-II diagnostic scan tool to confirm P0685 is the active fault and note any accompanying codes.

2. 2

   Inspect the battery to ensure it is fully charged (resting voltage of 12.6V or higher) and check terminals and cables for corrosion, damage, or loose connections.

3. 3

   Visually inspect the wiring harness connected to the ECM power relay and the ECM itself for frayed wires, melted insulation, or corrosion.

4. 4

   Check all fuses related to the ECM and engine controls in both the cab's primary fuse box and the engine compartment's secondary fuse/relay boxes. Replace any blown fuses with new ones of the correct amperage.

5. 5

   Locate the 'Engine ECU Power Hold Relay' (typically in the engine fuse/relay box, engine pod, or near the battery) and inspect the relay and its socket for corrosion, burnt terminals, or a loose fit.

1. 1

   Test the power relay

   Swap the suspect relay with an identical, known-good relay from another circuit in the fuse box. If the fault clears, replace the relay. Alternatively, remove the relay, check continuity across the coil terminals, and apply a 12V power source to verify it clicks.

2. 2

   Test the relay control circuit

   With the relay removed and ignition on, use a multimeter to check for a constant 12V supply on one terminal and a switched 12V supply from the ignition on another at the relay socket. Verify good continuity to the chassis ground.

3. 3

   Check voltage at the ECM

   If the relay and circuit are functioning correctly, probe the ECM's power input pins to verify they receive close to battery voltage when the relay is energized. A voltage drop below 9V indicates a wiring issue between the relay and the ECM.

4. 4

   Repair wiring harness

   If an open circuit or high resistance is found, repair or replace the damaged section of wire. Ensure connections are clean, secure, and protected from abrasion or heat, and apply dielectric grease to connector pins to prevent future corrosion.

5. 5

   Verify repair

   Clear the P0685 fault code using a diagnostic tool, start the machine, and let it run to confirm the code does not reappear.

6. 6

   Test or replace ECM

   If all wiring and relay checks pass and the fault persists, the ECM itself may be faulty and require professional testing or replacement.