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Visual Inspection and Diagnostic Pre-checks

WARNING: This page is about a different car, the 2019 Ford F-550 Super Duty, 2019 Ford F-450 Super Duty, 2019 Ford F-350 Super Duty, and 2019 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Use the correct probe adapter(s) when making measurements. Failure to use the correct probe adapter(s) may cause damage to the connector.
Fig 1: Brake Booster Vacuum Sensor Connector - C149
G13342427Courtesy of FORD MOTOR CO.

Harness Side

Circuit Pin
SRef (Signal Reference Voltage) - VCA38 1 (BROWN/VIOLET)
SIGRTN (Signal Return) - RCA37 2 (WHITE/BLUE)
VREF (Reference Voltage) - LCA37 3 (YELLOW/ORANGE)
  1. C1: VERIFY ALL WIRING CONNECTIONS 
    • Ignition OFF.
    • Brake Booster Vacuum Sensor connector disconnected. (C149)
    • Using a good light source, inspect each connector terminal for any corrosion.
    • Inspect both connectors for any pushed-out pins. Refer to OEM Connector Repair Procedures article for schematic and connector information.
    • Are both connectors free of corrosion, pushed-out pins and deformed or enlarged contacts? 
    Yes No
    Go to  C2. REPAIR the terminals in question.
  2. C2: CHECK THE ABS MODULE DTCS 
    • Brake Booster Vacuum Sensor connector connected. (C149) Make sure the connector seats and latches correctly.
    • Ignition ON, engine OFF.

      Carry out the ABS self-test.

    • Are any DTCs from the DTC Fault Trigger Conditions Table present and active in the ABS module? 
    Yes No
    Go to  C3. Go to  C4.
  3. C3: CHECK THE BRAKE BOOSTER AND VACUUM HOSES FOR LEAKS 
    • Inspect the vacuum hoses, brake booster and brake booster seals for leaks or evidence of leaks.
    • Are the vacuum hoses, brake booster and brake booster seals free from leaks? 
    Yes No
    Go to  C4. REPAIR or INSTALL new components as necessary.
  4. C4: CHECK THE VACUUM SENSOR WIRING FOR A SHORT TO VOLTAGE 
    • Ignition OFF.
    • Brake Booster Vacuum Sensor connector disconnected. (C149)
    • Ignition ON, engine OFF.
    • Measure the voltage between:
      (+) ABS Module (C135) Connector, Harness Side (-)
      VREF - LCA37 (YELLOW/ORANGE) - Pin 5 Ground
      SIGRTN - RCA37 (WHITE/BLUE) - Pin 10 Ground
      SRef - VCA38 (BROWN/VIOLET) - Pin 37 Ground
    • Measure the voltage between:
      (+) PCM_B (C175B) Connector, Harness Side (-)
      VAC GND Ground
      VAC SIG Ground
      VAC SNS P Ground
    • Is any voltage present? 
    Yes No
    REPAIR the circuit in question. Go to  C5.
  5. C5: CHECK THE VACUUM SENSOR WIRING FOR A SHORT TO GROUND 
    • Ignition OFF.
    • Measure the resistance between:
      (+) ABS Module (C135) Connector, Harness Side (-)
      VREF - LCA37 (YELLOW/ORANGE) - Pin 5 Ground
      SIGRTN - RCA37 (WHITE/BLUE) - Pin 10 Ground
      SRef - VCA38 (BROWN/VIOLET) - Pin 37 Ground
    • Measure the resistance between:
      (+) PCM_B (C175B) Connector, Harness Side (-)
      VAC GND Ground
      VAC SIG Ground
      VAC SNS P Ground
    • Are the resistances greater than 100, 000 ohms? 
    Yes No
    Go to  C6. REPAIR the circuit in question.
  6. C6: CHECK THE VACUUM SENSOR WIRING FOR AN OPEN 
    • Measure the resistance between:
      (+) ABS Module (C135) Connector, Harness Side (-) Brake Booster Vacuum Sensor (C149) Connector, Harness Side
      VREF - LCA37 (YELLOW/ORANGE) - Pin 5 VREF - LCA37 - Pin 3
      SIGRTN - RCA37 (WHITE/BLUE) - Pin 10 SIGRTN - RCA37 - Pin 2
      SRef - VCA38 (BROWN/VIOLET) - Pin 37 SRef - VCA38 - Pin 1
    • Are the resistances less than 3 ohms? 
    Yes No
    Go to  C8. REPAIR the circuit in question.
  7. C7: CHECK THE VACUUM SENSOR WIRING FOR AN OPEN 
    • Measure the resistance between:
      (+) PCM_B (C175B) Connector, Harness Side (-) Brake Booster Vacuum Sensor (C149) Connector, Harness Side
      VAC SNS P VAC SNS P
      VAC SIG VAC SIG
      VAC GND VAC GND
    • Are the resistances less than 3 ohms? 
    Yes No
    Go to  C8. REPAIR the circuit in question.
  8. C8: CHECK THE VACUUM SENSOR WIRING FOR A SHORT TOGETHER 
    • Measure the resistance between:
      (+) Brake Booster Vacuum Sensor (C149) Connector, Harness Side (-) Brake Booster Vacuum Sensor (C149) Connector, Harness Side
      SRef - VCA38 (BROWN/VIOLET) - Pin 1 SIGRTN - RCA37 - Pin 2
      SRef - VCA38 (BROWN/VIOLET) - Pin 1 VREF - LCA37 - Pin 3
      SIGRTN - RCA37 (WHITE/BLUE) - Pin 2 VREF - LCA37 - Pin 3
      VAC GND VAC SIG
      VAC GND VAC SNS P
      VAC SIG VAC SNS P
    • Are the resistances greater than 100, 000 ohms? 
    Yes No
    Go to  C9. REPAIR the circuits in question.
  9. C9: CHECK THE MODULE OUTPUT 
    • Ignition ON, engine OFF.
    • Measure the voltage between:
      (+) Brake Booster Vacuum Sensor (C149) Connector, Harness Side (-)
      VREF - LCA37 (YELLOW/ORANGE) - Pin 3 Ground
      VAC SNS P Ground
      Yes No
      INSTALL a new brake booster vacuum sensor. Go to  C10.
    • C10: VERIFY ALL WIRING CONNECTIONS 
      • Ignition OFF.
      • Using a good light source, inspect all disconnected electrical connectors for the following:
        • corrosion - install new connector or terminal and clean the module pins.
        • damaged or bent pins - install new terminals or pins.
        • pushed-out pins - install new pins as necessary.
        • spread terminals - install new terminals as necessary.
      • Are the connectors free of corrosion, damaged pins, bent pins, pushed-out pins and spread terminals? 
      Yes No
      Go to  C11. REPAIR the connector or terminals. Refer to OEM Connector Repair Procedures article for schematic and connector information.
    • C11: CHECK FOR CORRECT ABS MODULE OPERATION 
      • Ignition OFF.
      • Brake Booster Vacuum Sensor connector connected. (C149) Make sure the connector seats and latches correctly.
      • ABS Module connector connected. (C135) Make sure the connector seats and latches correctly.
      • Operate the system and determine if the concern is still present.
      • Is the concern still present? 
      Yes No
      INSTALL a new ABS module.
      CHECK OASIS for any applicable service articles: TSB, GSB, SSM or FSA.
      If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions.
      If no service articles address this concern, INSTALL a new ABS module. REFER to the Anti-Lock Brake System (ABS) and Stability Control article.
      ORDER a replacement ABS module and INCLUDE the RVC on the form.
      If this is a Ford paid repair, INCLUDE the RVC on the warranty claim form.      		 The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. ADDRESS the root cause of any connector or pin issues.
    • C12: CHECK FOR CORRECT PCM OPERATION 
      • Ignition OFF.
      • Brake Booster Vacuum Sensor connector connected. (C149) Make sure the connector seats and latches correctly.
      • PCM_B connector connected. (C175B) Make sure the connector seats and latches correctly.
      • Operate the system and determine if the concern is still present.
      • Is the concern still present? 
      Yes No
      INSTALL a new PCM.
      CHECK OASIS for any applicable service articles: TSB, GSB, SSM or FSA.
      If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions.
      If no service articles address this concern, INSTALL a new PCM. REFER to the appropriate Electronic Engine Controls article.      		 The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. ADDRESS the root cause of any connector or pin issues.
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