John Deere 109.18 – Engine Control Unit (ECU): Coolant Pressure Signal Moderately Low

This code indicates a moderate deviation of the engine coolant pressure falling below the desired threshold set in the control unit. This condition typically points to a failing coolant pump, a substantial leak in the cooling system, a faulty radiator cap failing to hold pressure, or a malfunctioning coolant pressure sensor.

• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 6140R
• 6150R
• 7200R
• 7310R
• 8235R
• 8360R
• 9460R
• S650
• S660
• S670
• S680
• S690
• 7280
• 7380
• 8100
• 8600
• R4038
• R4044
• R4060
• CP690
• CS690
• 250D

1. 1

   Verify that the engine coolant is at the proper level in the expansion tank

2. 2

   Inspect all radiator hoses, the water pump weep hole, the radiator core, and the EGR cooler for visible signs of coolant leaks

3. 3

   Use a pressure tester to ensure the radiator filler cap is holding the correct system pressure (typically 15 PSI)

4. 4

   Inspect the coolant pressure sensor and its 3-pin wiring harness connector for corrosion, chafing, or loose connections, and verify the 5-volt reference signal

5. 5

   Check the water pump drive belt/pulley and verify that the pump is actively circulating coolant

1. 1

   Secure the Machine

   Park the equipment on level ground, turn off the engine, and allow the cooling system to depressurize and cool down completely to avoid burn injuries.

2. 2

   Address Leaks/Levels

   If the diagnostic checks revealed a leak or a faulty radiator cap, replace the damaged hose, cap, or leaking component. Top off the system with approved John Deere Cool-Gard coolant.

3. 3

   Replace the Sensor (If Faulty)

   If the system holds pressure but the code persists, the coolant pressure sensor is likely faulty. Locate the coolant pressure sensor on the engine block/cooling circuit. Disconnect the 3-pin electrical connector. Unthread the old sensor (be prepared to catch a small amount of spilled coolant). Thread in the new OEM sensor and tighten to factory specifications. Reconnect the wiring harness.

4. 4

   Bleed the System

   Run the engine with the cap off temporarily to bleed any trapped air from the cooling system.

5. 5

   Clear Codes

   Use a John Deere Service ADVISOR diagnostic tool to clear the 000109.18 fault code from the ECU. Run the machine under load to verify the code does not return.