Normal Operation

WARNING: This page is about a different car, the 2012 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.

The ABS module uses the vacuum sensor to determine if hydraulic pump intervention is necessary due to a severe loss of vacuum in the booster. The ABS module sends a 5-volt reference voltage to the sensor and 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 severely below a predetermined level, the ABS module commands the hydraulic pump motor to activate on the next (and any subsequent) brake pedal application (with any vehicle speed) 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. Only the 3.5L engine uses an auxiliary vacuum pump to supplement the brake booster vacuum level when it is low. It assists the vacuum booster when vacuum levels drop below 57.56 kPa (17 in-Hg) of vacuum. DTCs set in the ignition cycle can only be cleared by cycling the ignition off and on once the fault has been corrected.

DTC C109D (Low Vacuum Condition Detected) - this DTC sets in continuous memory and every ignition cycle 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 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) - this DTC sets in continuous memory and every ignition cycle 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 or a failure of the vacuum sensor.
DTC C109F (Vacuum Sensor Signal Fault) - this DTC sets in continuous memory and every ignition cycle if the vacuum sensor ambient air pressure inlet ports, or the vacuum sensor ports become restricted or clogged.

This pinpoint test is intended to diagnose the following:

Wiring, terminals or connectors
Vacuum sensor
ABS module