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Installation of FMVSS 121 Air System Test Switch

Instructions for the field installation and verification of the ABS Diag/Regen momentary switch. This switch is required on electric vehicles to deactivate the air compressor, allowing technicians to perform FMVSS 121 air brake tests in a simulated "Key On, Engine Off" state

Tools and Materials Required

  • Phillips screwdriver

  • Wire strippers and crimpers

  • Drill with 15/32" drill bit

  • ABS Diag/Regen Switch: Part number 308E-0009

  • Ring Terminals: (2) 18 AWG 6/32 size

  • Safety glasses and gloves

Vehicle Preparation

  1. Power Down: Ensure the ignition is OFF and all 12V chassis power is disconnected to prevent accidental shorts.

  2. PPE: Wear appropriate eye protection while drilling and working behind the dashboard.

Procedure

Part 1: Cable and Switch Preparation

  1. Access the area behind the center dashboard.

  2. Locate two blunt-cut wires taped to the main harness:

    • Wire R-G: Red insulation.

    • Wire R-S: Black insulation.

Part 2: Terminal Attachment

  1. Strip approximately 1/4" of insulation from wires R-G and R-S.

  2. Crimp a 6/32 ring terminal onto each wire.

  3. Perform a "pull test" to ensure the crimps are mechanically secure.

Part 3: Physical Installation

  1. Select a visible, accessible location on the dashboard for the switch.

  2. Drill a 15/32" hole at the selected location.

  3. Insert the switch (308E-0009) into the hole and secure the mounting nut.

Part 4: Final Wiring

  1. Connect the R-G (Red) wire terminal to Terminal B on the switch.

  2. Connect the R-S (Black) wire terminal to Terminal C on the switch.

  3. Tighten terminals and ensure no stray wire strands are present.

System Verification (Post-Installation)

  1. Charge System: Turn the key ON and build air pressure to the governor cut-out (approx. 125 PSI).

  2. Trigger Test Mode: With the key remaining in the ON position, press and release the new momentary switch.

  3. Confirm HV Shutdown: Verify that the High-Voltage system shuts down and the air compressor stops running.

  4. Validation: Perform the standard FMVSS 121 leakage, low-air warning, and spring brake pop-out tests.

  5. Reset: Cycle the ignition key (OFF then ON) to restore normal HV and compressor operation.