FMVSS 121 Air Brake Systems Inspection Procedure

Tools Required

• Wheel Chocks

• Stopwatch or Timer

• Calibrated Air Pressure Gauges (Vehicle-mounted)

Vehicle Preparation

1. Positioning: Park the vehicle on a level surface.

2. Security: Chock the wheels and ensure the parking brake is applied.

3. System Charge: Start the engine (ICE) or turn the key to the ON position (EV). Allow the compressor to build pressure until the governor cut-out is reached (typically 125 PSI).

4. Initial Shutdown:

   • ICE Units: Shut off the engine. Turn the key back to the ON position.

   • EV Units: Locate the Momentary Air System Test Switch near the ignition. Press and release this switch to disable the High-Voltage (HV) system and the air compressor.

Procedure

Part 1: Air Leakage Test

1. Set State: Ensure the key is ON but the compressor/engine is OFF.

2. Release: Disengage the parking brake.

3. Application: Fully apply and hold the service (foot) brake pedal.

4. Observation: Monitor the gauges for 60 seconds.

   • Pass/Fail (Single Unit): Loss must not exceed 3 PSI per minute.

   • Pass/Fail (Combination): Loss must not exceed 4 PSI per minute.

5. Audit: Listen for any audible leaks throughout the chassis.

Part 2: Low Air Warning Device Test

1. Fanning: With the key ON, rapidly apply and release (fan) the service brakes to bleed air.

2. Verification: Confirm that the visual warning light and audible buzzer activate at or above 60 PSI.

Part 3: Spring Brake (Pop-Out) Test

1. Fanning: Continue fanning the service brake pedal to further deplete the secondary and primary tanks.

2. Verification: Observe the parking brake valve (yellow knob) and tractor protection valve (if equipped).

   • Pass/Fail: The valves must automatically "pop out" (close) between 20 and 45 PSI.

Post-Test Steps (EV Only)

1. Reactivate: Perform a full key cycle (Turn Key OFF, then back to ON).

2. Confirmation: Verify the HV system engages and the air compressor begins to recover the system pressure.

Diagnosis and Repair Summary