Normal Operation

WARNING: This page is about a different car, the 2011 Mercury Milan, 2011 Lincoln MKZ, and 2011 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.

The vacuum sensor is used by the ABS module to determine if hydraulic pump intervention is necessary due to loss of vacuum. 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 commands the hydraulic pump motor to activate on the next (and any subsequent) brake pedal application to assist with vehicle braking. The ABS module continues to use the pump motor for braking assist as long as the low vacuum condition exists.

DTC C109D (Low Vacuum Condition Detected) - This DTC is set when the optimized hydraulic brake system is activated and does not necessarily indicate a concern with the vacuum sensor, the related wiring or the ABS module. This DTC most likely indicates a vacuum concern in the brake booster, engine and any related vacuum hoses/tubes. High altitude locations with low ambient air temperatures can cause a low vacuum condition to exist in the booster until the engine compartment warms up.
DTC C109E (Vacuum Sensor Circuit Fault) - If, during normal operation or the ABS module on-demand self test, the ABS module detects an open circuit, a short to voltage or a short to ground on the vacuum sensor circuits, DTC C109E sets. Also, a failure of the vacuum sensor causes DTC C109E to set.
DTC C109F (Vacuum Sensor Signal Fault) - The vacuum sensor has 4 ambient air pressure inlet ports (2 on each side). If these ports become restricted or clogged, DTC C109F sets.

This pinpoint test is intended to diagnose the following:

Wiring, terminals or connectors
Vacuum sensor
ABS module