Massey Ferguson U 1079 (4) – Engine Control Unit (ECU) / EEM (Engine Electronic Module): Sensor Supply Voltage 1 monitoring: Shortcut to Ground

This code indicates a critical electrical fault where the 5-volt reference circuit ("Sensor Supply 1") is receiving full battery voltage (typically 12V). The system has detected a "Shortcut to Battery+" (short circuit to a positive power source), which will cause all sensors on this specific 5V circuit to read out of range or fail entirely.

• 8600 Series (8650
• 8660
• 8670
• 8680
• 8690)
• 7600 & 7700 Series (7614–7626
• 7714–7726)
• 5600 & 5700 Series (5608–5613
• 5710SL–5713SL)
• 4700 Series (4707–4710)

1. 1

   Ensure the machine is off and disconnect the main connector of the EEM control unit before major electrical work or welding.

2. 2

   Inspect the wiring harness between the EEM and the SCR/DEF sensors (especially the Urea/DEF pressure sensor) for chafing, rub points, or pinched wires.

3. 3

   Perform a sensor isolation drop test by disconnecting sensors on the Supply 1 circuit one at a time (starting with DEF pressure and tank sensors) while monitoring the 5V supply circuit with a multimeter to identify internal sensor shorts.

4. 4

   Verify ECU operation by temporarily swapping it with a known good EEM4 controller if the harness and sensors test good but the short remains.

1. 1

   Turn off the tractor and disconnect the negative (-) battery terminal to prevent electrical spikes.

   Ensure the machine is completely powered down before beginning repairs.

2. 2

   Based on the diagnostic isolation test, locate the shorted component.

   This will either be a section of the wiring harness or a specific sensor, commonly the DEF pressure sensor.

3. 3

   Repair or replace the damaged wiring harness if applicable.

   If the harness is damaged, repair the wire using OEM-approved waterproof splice connectors and heat shrink, or replace the affected harness section. Reroute the harness away from hot edges or vibration rub points.

4. 4

   Replace the shorted sensor if applicable.

   If a sensor is shorted, remove the defective sensor. Clean the mounting port (ensure no dirt enters the DEF or fuel system, as the SCR system is highly sensitive to contamination). Install the new OEM sensor and torque to spec.

5. 5

   Reconnect the sensor pigtails and the main EEM connectors.

   Ensure all connections are secure.

6. 6

   Reconnect the negative battery terminal.

   Restore power to the machine.

7. 7

   Turn the key to the ON position and use a diagnostic scan tool to clear the U 1079 (4) fault code.

   Clear the code from the ECU memory.

8. 8

   Start the engine and monitor the DEF system pressure and sensor supply voltage live data.

   Ensure the fault does not return during operation.