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.

On vehicles equipped with a 3.5L engine, an electrically operated pump is used to supply additional vacuum to the brake booster. The ABS module controls the pump through a relay located in the Battery Junction Box (BJB). When activated, the relay supplies voltage to the brake vacuum pump. The ABS module uses a sensor mounted on the brake booster to monitor brake booster vacuum. The ABS module activates the pump when the vacuum in the brake booster is below 57.56 kPa (17 in-Hg) and deactivates the pump once the vacuum in the brake booster reaches a minimum of 71.11 kPa (21 in-Hg).

If the sensor on the brake booster suffers an internal failure, the vacuum pump may operate continuously (or it may fail to operate) and one or more DTCs is set in the ABS module. If a large vacuum leak exists in the brake booster or vacuum hoses the brake vacuum pump runs continuously. Areas of high elevation can also cause the brake vacuum pump to run continuously. Altitude affects vacuum readings, subtract 3.3863 kPa (1 in-Hg) from the reading for every 304.8 m (1, 000 ft) of elevation above sea level.

DTC P258A (Brake Booster Vacuum Pump Relay Circuit Failure) - This DTC sets in continuous memory if the ABS module detects a short to ground in the relay control circuit.
DTC P258B (Brake Booster Vacuum Pump Plausibility Failure) - This DTC sets in continuous memory if the ABS module detects that the desired vacuum level is not reached in a specified time in the brake booster when the brake vacuum pump is commanded on. This could be caused by one or more leaks in the vacuum system or an internal failure of the ABS module.
This pinpoint test is intended to diagnose the following:
Wiring, terminals or connectors
Brake booster vacuum sensor
Brake vacuum pump relay
Brake vacuum pump
ABS module