Massey Ferguson U 1079 – Engine Control Unit (ECU) / EDC: Sensor Supply Voltage 1 monitoring

This code indicates a fault in the 5V DC reference voltage circuit (Supply 1) that powers a specific bank of engine or vehicle sensors. It typically appears with FMI 3 (Shortcut to Battery +) or FMI 4 (Shortcut to Ground).

• Massey Ferguson 8600 Series
• MF 8650
• MF 8660
• MF 8670
• MF 8680
• MF 8690
• Massey Ferguson 5400 Series
• Massey Ferguson 6100 Series

1. 1

   Check Battery Voltage: Ensure the main system voltage is not excessively high, which can cause overvoltage spikes in the ECU.

2. 2

   Isolate the Circuit: Identify which sensors are on the 'Supply 1' 5V circuit (commonly includes fuel pressure, boost pressure, or accelerator pedal position sensors depending on the exact model year).

3. 3

   Test for Shorts: Use a multimeter to check the wiring harness between the sensors and the EDC for continuity to ground (if FMI 4) or continuity to 12V power (if FMI 3).

4. 4

   Sensor Drop Test: Disconnect the sensors on the Supply 1 circuit one at a time. If the 5V reference voltage returns to normal at the harness connector after unplugging a specific sensor, that sensor is internally shorted.

1. 1

   Disconnect Battery

   Before servicing the EDC or main engine harness, disconnect the negative battery terminal.

2. 2

   Inspect the Harness

   Visually inspect the engine wiring harness routing for chafing, melted insulation, or pinch points against the engine block or frame. Repair any damaged wires using OEM-approved splicing methods.

3. 3

   Inspect Connectors

   Check the connectors at the EDC and the sensors for water ingress, bent pins, or corrosion. Clean with electrical contact cleaner and apply dielectric grease.

4. 4

   Replace Faulty Sensor

   If the diagnostic drop-test isolated a specific sensor that was pulling the 5V circuit down, replace that sensor.

5. 5

   Test or Replace the EDC

   If the wiring harness tests perfectly (no shorts) and all sensors are unplugged, but the EDC is still failing to output a clean 5V on the Supply 1 circuit, the EDC has suffered an internal electrical failure. The EDC must be replaced or sent out for remanufacturing.

6. 6

   Clear Codes

   Reconnect all components, power up the tractor, and use a diagnostic tool to clear the U 1079 code. Verify that the 5V supply remains stable under load.