John Deere 5126.04 – Engine Control Unit (ECU): Sensor Supply #8 Voltage Out of Range Low

The 5-volt reference circuit (Sensor Supply #8), which provides power to the Exhaust Pressure Sensor and the Engine Oil Pressure Sensor, has dropped below its lower acceptable limit. This condition is typically caused by a short circuit to ground in the wiring harness or an internal short within one of the sensors sharing this 5V supply circuit.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5085M
• 5100M
• 5100MH
• 5100ML
• 5115ML
• 6100D
• 6110D
• 6115D
• 6125D
• 6130D
• 6140D
• 8245R to 8400R
• 9470RT
• 9520RT
• 9570RT

1. 1

   Inspect the wiring harness and connectors associated with the Engine Oil Pressure Sensor and Exhaust Pressure Sensor for damaged, frayed, or loose wiring rubbing against the engine block or chassis (short to ground).

2. 2

   Perform a Sensor Isolation Test: Disconnect the Engine Oil Pressure Sensor and the Exhaust Pressure Sensor one at a time. Monitor active fault codes or use a multimeter to check the 5V supply line. If disconnecting a specific sensor causes the 5V reference voltage to return to normal (and the code changes to an open circuit code), that sensor is internally shorted.

3. 3

   Perform a Harness Continuity Test: If both sensors are disconnected and the 5V supply is still pulled low, the fault lies within the wiring harness between the ECU and the sensors. Use a digital multimeter with a flexible probe kit (e.g., JDG10466) to test the harness for continuity to ground.

4. 4

   Verify ECU Output: Verify that the ECU is outputting the correct 5V supply on the Sensor Supply #8 pins when the harness is completely disconnected.

1. 1

   Disconnect battery

   Turn off the machine and disconnect the battery if you are repairing the wiring harness.

2. 2

   Locate shorted component

   Based on the diagnostic isolation test, locate the shorted component (the wiring harness, the Engine Oil Pressure Sensor, or the Exhaust Pressure Sensor).

3. 3

   Replace faulty sensor

   If a sensor is faulty: Remove the internally shorted sensor and install the replacement part. (Note: Service manuals indicate exhaust pressure sensors should typically be torqued to 14 N·m / 10 lb-ft).

4. 4

   Repair or replace faulty harness

   If the harness is faulty: Repair the damaged wires using approved splicing methods and heat shrink, or replace the affected harness section. Ensure the harness is routed away from sharp edges or extreme heat sources.

5. 5

   Reconnect components

   Reconnect all sensors and the battery.

6. 6

   Reset ECU and test operation

   Reset the ECU to clear the stored fault code using a diagnostic tool (like John Deere Service ADVISOR) and run the engine to monitor the system and ensure normal operation.