C109D FORD - Low Vacuum Condition Detected

Repair Importance Level: 3/3
Repair Difficulty Level: 3/3   
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Possible causes

  • Loose Brake Booster Vacuum Hose
  • Faulty Brake Booster Vacuum Sensor
  • Brake Booster Vacuum Sensor harness is open or shorted
  • Brake Booster Vacuum Sensor circuit poor electrical connection

Possible symptoms

  • Anti-Lock Brake System (ABS) Warning Light ON
  • Engine Light ON (or Service Engine Soon Warning Light)

C109d Ford Description

The vacuum sensor is used by the Anti-Lock Brake System (ABS) Module to determine if hydraulic pump intervention is necessary due to loss of vacuum in the booster. The ABS module sends a 5-volt reference voltage to the sensor, the sensor compares the pressure in the booster to the ambient air pressure in the engine compartment and sends a varying voltage back to the ABS module based on the difference between the pressures. Normally, the pressure in the booster is much lower than ambient air pressure. As the pressure in the booster gets higher (closer to matching ambient air pressure), the lower the voltage sent back to the ABS module becomes.

When the vacuum in the brake booster falls below a predetermined level, the ABS module will command the hydraulic pump motor to activate on the next (and any subsequent) brake pedal application to assist with vehicle braking. The ABS module will continue to use the pump motor for braking assist as long as the low vacuum condition exists.

OBDII Code C109d Ford - Low Vacuum Condition Detected - AutoCodes.com

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