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A/C:
Air Conditioning.
A/F:
Air/Fuel ratio. This refers to the proportion of
air and fuel delivered to the cylinder for com-
bustion. For example, if you have 14 times
more air than fuel (by weight) then the A/F
ratio is 14:1 (read as fourteen to one). The
ideal operating A/F ratio in an automotive
application is 14.7:1.
AC Clutch Relay:
The PCM uses this relay to energize the A/C
clutch, turning the A/C system on or off.
AC Pressure Sensor:
This sensor is connected to the A/C refriger-
ant line. It measures refrigerant pressure
and sends a voltage signal to the PCM. The
PCM will turn off the A/C system (by de-ener-
gizing the A/C Clutch Relay) to prevent com-
pressor damage if the pressure is too high
or low.
AC Pressure Switch:
This is a mechanical switch connected to
the A/C refrigerant line. The switch is acti-
vated (sending a signal to the PCM) when
the A/C refrigerant pressure becomes too
low. The PCM will turn off the A/C system (by
de-energizing the A/C Clutch Relay) to pre-
vent compressor damage. Some vehicles
have a second switch activated when the
refrigerant pressure is too high.
Actuator:
Devices that are powered by the PCM to con-
trol things. Actuator types include relays,
solenoids, and motors. Actuators allow the
PCM to control system operation.
Air Injection Reaction (AIR) System:
This is an emission control system oper-
ated by the PCM. During cold starts, an air
pump injects outside air into the exhaust
manifold to help burn hot exhaust gases.
This reduces pollution and speeds warm-
up of oxygen sensors and catalytic convert-
ers. After the engine is warm, the air will ei-
ther be dumped back to the atmosphere
(or into the air cleaner assembly) or sent to
the catalytic converter. There are several
versions of the AIR system, depending on
the vehicle.
Analog Signal:
A voltage signal which can have any volt-
age reading. For example, thermistors
send analog signals indicating small
changes in temperature. In contrast are
digital signals, which are either high or
low with no variations in between. Also
see Digital Signal definition.
BARO:
Barometric Pressure Sensor. See MAP
Sensor definition for full explanation.
Boost Control Solenoid:
Used on certain supercharger-equipped
engines. This solenoid is normally ener-
gized by the PCM, allowing the supercharger
system to operate normally. Under high en-
gine speed and load conditions, the PCM
de-energizes the solenoid to reduce boost
pressure.
Brake Switch Signal:
An input signal to the PCM indicating that
the brake pedal is being pressed. Vehicles
with Cruise Control Systems monitor the
brake switch to determine when to engage
or disengage the cruise control function.
The brake switch may also have a circuit
supplying power to the Torque Converter
Clutch (TCC) solenoid. This connection
insures the TCC solenoid will disengage
when the brake pedal is depressed. Also
see TCC definition.
CAM:
Camshaft Position Sensor. This sensor
sends a frequency signal to the PCM. Ve-
hicles with sequential fuel injection (SFI) use
this signal to synchronize the injector firing
order. Some DIS type ignition systems use
this signal to synchronize spark plug firing.
CARB:
California Air Research Board
Appendix C: Glossary of Terms