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PCM uses sensors to measure such things
as throttle position, coolant temperature,
engine speed, incoming air mass and tem-
perature, etc.
SFI or SEFI:
Sequential Fuel Injection or Sequential Elec-
tronic Fuel Injection. A fuel injection system
that uses one or more injectors for each cyl-
inder. The injectors are mounted in the in-
take manifold, and are fired individually.
Shift Solenoid:
Used in computer controlled transmissions,
the solenoids are located in the transmission
housing and are controlled by the PCM. The
PCM energizes the solenoids individually or in
combination to select a specific gear. The so-
lenoids control the flow of hydraulic fluid to the
transmission shifting valves. The PCM selects
the appropriate gear ratio and shift point based
on engine operating conditions.
Solenoid:
A solenoid is a device to convert an electri-
cal signal to mechanical movement. It con-
sists of a coil with a wire and a moveable
metal rod in the center. When the power is
applied to the coil, the resulting electromag-
netism moves the rod and performs some
mechanical action. The PCM often uses
solenoids to switch vacuum lines on and
off. This allows the PCM to control vacuum
operated devices such as the EGR valve.
Fuel injectors are operated by another type
of solenoid.
ST:
Short Term fuel trim.
Stepper Motor:
A special type of electric motor with a shaft
that rotates in small steps instead of con-
tinuous motion. A certain sequence of fre-
quency-type signals is required to step the
motor shaft. A different signal sequence will
step the motor in the opposite direction. No
signal maintains current shaft position A
constant signal drive will continuously ro-
tate the shaft. The shaft is usually connected
to a threaded assembly which moves back
and forth to control things such as idle speed
bypass air flow (see IAC definition).
TBI:
Throttle Body Injection. A fuel injection sys-
tem having one or more injectors mounted
in a centrally located throttle body, as op-
posed to positioning the injectors close to
an intake valve port. TBI is also called Cen-
tral Fuel Injection (CFI) in some vehicles.
TDC:
Top Dead Center. When a piston is at its
uppermost position in the cylinder.
Thermistor:
A resistor whose resistance changes with
temperature. Thermistors are used as sen-
sors for vehicle coolant and manifold air tem-
perature. The resistance decreases as tem-
perature goes up, sending a voltage signal
to the PCM where it is converted to a tem-
perature measurement.
Throttle Body:
A device which performs the same function
as a carburetor in a fuel injection system.
On a throttle body injection (TBI) system, the
throttle body is both the air door and the lo-
cation of the fuel injectors. On port fuel injec-
tion systems (PFI, MPFI, SFI, etc.) the throttle
body is simply an air door. Fuel is not added
until the injectors at each intake port are ac-
tivated. In each case, the throttle body is at-
tached to the accelerator pedal.
TPS:
Throttle Position Sensor. This is a rotary-type
potentiometer connected to the throttle shaft.
It has a voltage signal output which increases
as the throttle is opened. This sensor is used
by the PCM for idle speed, spark advance,
fuel delivery, emission systems, and elec-
tronic automatic transmission control.
TTS:
Transmission Temperature Sensor. A ther-
mistor, a sensor whose resistance de-
creases with increases in temperature, is
mounted in the transmission housing in
contact with the transmission fluid. It sends
the transmission temperature as a voltage
signal to the PCM.
VECI:
Vehicle Emission Control Information.