Case IH 518853-04 - VCM – VCM: VCM - Rail 12VM - Left And Right Brake Lights And Aux Extend Input Power Off

The 12VM voltage supply is low (Voltage Below Normal or Shorted to Low Source). The 12VM rail is a specific power circuit responsible for supplying 12-volt power to the left and right brake lights as well as the auxiliary hydraulic extend circuit. When this voltage drops below the operational limit, the module shuts off the input power to protect the circuit, triggering the code.

• MAGNUM AFS Connect
• SR130
• SR150
• SR160
• SR175
• SR200
• SR220
• SR250
• SV185
• SV250
• SV300
• TR270
• TR320
• TV380

1. 1

   Check the 12VM Fuse: Locate the VCM fuse block and check the specific 15A fuse designated for the VCM 12VM circuit (often labeled F-015).

2. 2

   Inspect for Shorts to Ground: A short to ground in the brake lights or aux extend circuit (or their wiring) will pull the voltage low and blow the fuse.

3. 3

   Inspect the Wiring Harness: Check the chassis wiring harness running to the rear brake lights and the auxiliary hydraulic solenoids for chafing, pinch points, or rodent damage.

4. 4

   Test the Brake Light Switch: A failed brake light switch that has shorted internally can cause the 12VM rail to drop voltage when the pedal is pressed.

5. 5

   Check Battery and Main Relays: Ensure the battery is fully charged and the battery cables are clean and tight. Verify that the Main PWR Relay and Acc PWR Relay are functioning correctly and not dropping voltage under load.

1. 1

   Disconnect Power

   Turn off the machine and manually trip the circuit breaker at the battery or disconnect the negative battery cable to ensure safety.

2. 2

   Locate and Inspect the Fuse

   Access the main fuse panel and locate the 15A fuse for the 12VM VCM rail. If it is blown, do not simply replace it yet, as the underlying short will just blow the new fuse.

3. 3

   Isolate the Circuits

   Disconnect the electrical connectors at the left brake light, the right brake light, and the auxiliary extend solenoid valve.

4. 4

   Test for Short to Ground

   Using a digital multimeter set to ohms/continuity, probe the power supply wire at the fuse block for the 12VM circuit and check for continuity to a known good chassis ground.

5. 5

   Trace the Fault

   If continuity to ground is present with the components unplugged, the wiring harness is shorted. Trace the harness along the chassis to find the rubbed or pinched wire and repair it using heat-shrink butt connectors. If the short disappears when the components are unplugged, test the brake light switch, the brake light bulb sockets, and the aux extend solenoid individually for internal shorts.

6. 6

   Replace Defective Components

   Replace the shorted switch, socket, or solenoid as necessary.

7. 7

   Reassemble and Test

   Install a new 15A fuse. Reconnect the battery, turn the key to the ON position, and clear the inactive fault codes using the EST (Electronic Service Tool) or the in-cab diagnostic display.

8. 8

   Verify Operation

   Operate the brake pedal and the auxiliary hydraulics to ensure the fault code does not return.