John Deere 2931.04 – CCU: Hydraulic Brake Fluid Level Switch - Voltage Below Normal, or Shorted to Low Source

The controller is detecting a lower-than-expected voltage on the brake fluid level switch circuit. This typically means the brake fluid level in the reservoir has dropped below the minimum threshold, or there is a physical short to ground in the wiring harness or a failure within the switch itself.

• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 5050E
• 5058E
• 5067E
• 5075E
• 5075M
• 8130
• 8230
• 9120
• 9220
• CTS
• CSX7080
• 9600 Maximizer

1. 1

   Visual Fluid Check: Locate the brake fluid reservoir and check the physical fluid level. If it is low, inspect the entire brake hydraulic circuit for leaks.

2. 2

   Wiring & Connector Inspection: Inspect the wiring harness leading to the brake fluid level switch. Look for rubbed-through insulation, pinched wires, or corrosion in the connector that could be causing a short to ground.

3. 3

   Switch Continuity Test: The switch operates using a magnetic float. When the fluid is sufficient, the float is held above the reed switch (contacts open). When fluid drops, the float falls and magnetic attraction closes the contacts. Disconnect the switch and test it with a multimeter to see if it is stuck closed even when the reservoir is full.

4. 4

   Circuit Isolation: Disconnect the switch from the harness. If the fault code changes from FMI 04 (Low Volts/Shorted) to an open circuit code (usually FMI 03), the wiring to the CCU is good, and the switch itself is the culprit.

1. 1

   Park and Prep

   Park the machine on a level surface, engage the parking brake, and turn off the engine.

2. 2

   Address Fluid Levels

   If the diagnostic check revealed low fluid, fill the reservoir to the MAX line with the OEM-specified brake fluid/hydraulic oil. Cycle the key switch to see if the CCU clears the code.

3. 3

   Repair Wiring

   If a short to ground was found in the harness, splice and heat-shrink the damaged wire, or replace the affected pigtail connector.

4. 4

   Replace the Switch

   Place a catch pan under the brake fluid reservoir. Drain the reservoir or use a clean syringe to pull the fluid level below the switch mounting point to prevent spillage. Disconnect the electrical connector. Unthread/remove the faulty brake oil level switch from the reservoir. Install the new switch and tighten to OEM specifications. Refill the brake fluid reservoir to the proper level. Note: Depending on the machine, you may need to bleed the brake system if air entered the master cylinder lines during the repair.

5. 5

   Clear Codes

   Turn the key switch on. The CCU may require you to cycle the key or use the CommandCenter display to clear the stored inactive code.