Case IH 1521 – UCM (Universal Control Module): Left Brake Light Open Circuit

The control module is sending voltage to the left brake light circuit but is not detecting the expected electrical current draw. An "open circuit" typically indicates a break in the electrical path, which is most commonly caused by a blown light bulb, a disconnected plug, a severed wire, or severe corrosion in the light socket.

• skid steer
• Skid Steer Loaders (SSL)
• Compact Track Loaders (CTL)
• SR130
• SR150
• SR160
• SR175
• SR200
• SR210
• SR220
• SR250
• SV185
• SV250
• SV300
• TR270
• TR320
• TV380

1. 1

   Inspect the left rear brake light to see if the internal filament is broken or burnt out (if using a halogen/incandescent bulb).

2. 2

   Unplug the wiring harness at the left tail light assembly and check the pins for pushed-out terminals, moisture, or green corrosion.

3. 3

   Using a digital multimeter, probe the power wire at the left brake light connector. Have an assistant press the brake pedal (with the key ON) and verify that 12V is reaching the socket.

4. 4

   Verify that the ground wire at the connector has good continuity (low resistance) to the machine's chassis.

5. 5

   If there is no voltage or ground, trace the wiring harness back toward the UCM, looking for pinch points, rodent damage, or chafed wires along the loader arms and chassis.

1. 1

   Prepare machine for service

   Turn off the machine, lower the loader arms (or secure them with safety struts if access requires it), and remove the key.

2. 2

   Locate tail light

   Navigate to the left rear tail light housing.

3. 3

   Disconnect wiring harness

   Disconnect the main wiring harness connector from the back of the light assembly.

4. 4

   Remove tail light assembly

   Remove the retaining screws or bolts securing the tail light assembly to the chassis.

5. 5

   Replace bulb or housing

   If the housing uses a replaceable bulb, twist the socket out and replace the blown bulb. If the socket is melted/corroded, or if the machine uses a sealed LED assembly, discard the old housing and prepare the new tail light assembly.

6. 6

   Install new assembly

   Mount the new tail light assembly into the chassis and tighten the retaining hardware.

7. 7

   Reconnect wiring harness

   Apply dielectric grease to the electrical connector to prevent future moisture intrusion, then plug the harness back in until it clicks.

8. 8

   Test brake lights

   Turn the ignition ON and test the brake lights. The UCM should detect the proper current draw, and the 1521 fault code will change to an inactive state or clear entirely.