Focus ZTS L4-2.0L DOHC VIN 3 (2001)
The master cylinder attached to the booster functions the same as on vehicles not equipped with the stability assist system. It differs in that there are
two pressure transducers that are installed in spare outlet ports. The values measured by each pressure transducer should be similar. The pressure
transducer is a diaphragm type unit. When a pressure is seen the diaphragm is stretched, causing a voltage to be output. As the pressure increases or
decreases, the voltage also increases or decreases with it. The configuration of the pressure transducers, which transducer is connected to the primary
port and which is connected to the secondary port, is not known by the stability assist module. The harness connectors can be connected to either
transducer, therefore, diagnosis of pressure faults requires some care to identify exactly which transducer is failed.
Modules which are normally connected to the BPP switch are connected to a signal from the stability assist module. This signal is called driver brake
application. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not
electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake applied when either the BPP switch or
the release switch built into the brake booster indicate that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the
driver brake application signal will indicate a brake apply only when the BPP switch indicates that the driver is applying the brake pedal. When the
ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy will try to keep the driver brake
application signal accurate. If the stability assist system detects faults on the driver brake application signal line it will also attempt to maintain correct
stoplamp operation by communicating on the Standard Corporate Protocol (SCP) network. Control of the stoplamp operation is also required because
the BPP switch can change states during an stability assist event when the driver is not applying the brake pedal. The switch can also change states
during the system check of the brake booster. Only the stability assist system can interpret these state changes and determine which ones are actually
due to the driver applying the brake.
Similarly, the stability assist system can take advantage of its other sensors to detect failures in the BPP switch.
