Vehicles With Canister Purge And Vent Valves; Ect Sensor; App Sensor - Chevrolet Trailblazer Service Manual

evaporative pressure is being maintained in fuel tank. This is a test for gross evaporative
emissions leaks.

Vehicles with Canister Purge and Vent Valves

Some vehicles will route a small amount of engine vacuum through the canister purge
valve to the fuel tank. When both the canister purge valve and vent solenoid valve are
closed the vacuum is trapped between the fuel tank and canister purge valve. The fuel
tank pressure sensor signals the EMS that the vacuum is present. The vacuum in the
system should reach a certain level and remain for a pre-determined amount of time. This
is used to test for minute system pressure leaks.

ECT Sensor

The engine coolant temperature (ECT) sensor is a variable resistor that measures the
temperature of the engine coolant. When the engine coolant temperature is low, the
sensor resistance is high. When the engine coolant temperature is high, the sensor
resistance is low. The Engine Coolant Temperature Sensor (ECT) input to the PCM is a
primary input for calculation strategies, fuel delivery control and spark timing. The ECT is
also used to determine loop status timer requirements (inside of PCM) and to support
cooling fan operation. ECT failures can cause excessive rich conditions, increased injector
pulse-width and retarded spark timing.
A/F Sensor
The air fuel ratio (A/F) sensor like the oxygen sensor is used by the powertrain control
module to measure the amount of oxygen in the exhaust. The PCM uses this data to
control the air fuel mixture in the engine. The (A/F) sensor can meter fuel more accurately
than an oxygen sensor. The A/F sensor changes current output by measuring the amount
of oxygen in the exhaust. The A/F sensor operates at a much higher temperature than an
oxygen sensor. The PCM measures the amount of current that is output by the A/F sensor
and converts this current flow data to a voltage signal. When the mixture is lean and the
oxygen content is high the voltage signal will be above 3.3V. When the mixture is rich and
the oxygen content is low the voltage signal will be below 3.3V. When the mixture is at
14.7 to 1 the voltage signal will be at 3.3V. The heater circuit in the A/F sensor shortens
the time required for the sensor to reach operating temperature and provides a more
accurate signal.

APP Sensor

The Accelerator Pedal Position (APP) sensor contains 2 potentiometer sensors that
convert the accelerator pedal position into two signals. Each sensor has three circuits a 5V
reference, a signal and a low reference. The APP sensor provides the PCM with voltage
signals proportional to the pedal movement. The voltage signal differs by 1 volt between
the two sensors. When one APP sensor signal measures 0.5V the other TP sensor will
measure 1.5V.