Filter
Top Products
38
DiagnosticTrouble Codes:
Diagnostic Trouble Codes (DTC) indicate
a malfunction flagged by a vehicle com-
puter. The computer will display a corre-
sponding code based on a lookup table in
the program of the on-board computer.
Most systems have the ability to store codes
in memory, which are commonly referred
to as history codes or soft codes. Mal-
functioning circuits will generate continu-
ous Check Engine lamp illumination, called
current codes or hard codes. OBD II
Systems will transmit many more DTCs
than the past systems, and therefore will
allow a technician the ability to better pin-
point failures and past events. The only
way to clear codes on OBD II systems will
be with a scan tool that has the proper pro-
gramming to perform the function.
Digital Signal:
An electronic signal which has only two (2)
voltage values: a low value (close to zero)
and a high value (usually 5 volts or more).
Sometimes the low voltage condition is
called OFF and the high voltage condition
is called ON. Signals which can have any
voltage value are called analog signals.
DIS:
Distributorless Ignition System or Direct
Ignition System. A system that produces
the ignition spark without the use of a dis-
tributor.
Driver:
A transistor switch inside the PCM used
to apply power to an external device. This
allows the PCM to control relays, solenoids,
and small motors.
Duty Cycle:
A term applied to frequency signals those
which are constantly switching between a
small voltage value (close to zero) and a
larger value (usually 5 volts or more). Duty
cycle is the percentage of time the signal
has a large voltage value. For example, if
the signal is high (large voltage) half of
the time, the duty cycle is 50%. If the signal
is high only one fourth of the time, then
the duty cycle is 25%. A duty cycle of 0%
means the signal is always at a low value
and not changing. A duty cycle of 100%
means the signal is always at a high
value and not changing. The PCM uses
duty cycle type signals when it wants more
than just on-off control of an actuator. This
is how it works: 50% duty cycle signal go-
ing to a vacuum switching solenoid means
the solenoid will be on (passing full
vacuum) half the time, and off (passing
no vacuum) half the time. The average
amount of vacuum passing through the
solenoid will be one half the full value be-
cause the solenoid is only on for half the
time. This signal changes at a rapid rate,
as often as ten times per second. Thus
the PCM can get a vacuum controlled ac-
tuator to move halfway between no
vacuum and full vacuum. Other positions
can be achieved by changing the duty cycle
of the control signal which in turn changes
the average amount of control vacuum.
DVM:
Digital Volt Meter. An instrument using a
numeric readout to display measured volt-
age values as opposed to a moving needle
on a gauge face. Usually the instrument
has other measuring capabilities, such as
resistance and current, and may be called
a Digital Multi-Meter (DMM). Most DVMs
have 10 Megohm input impedance. This
means the circuit under test will not be
electronically disturbed when the DVM is
connected for a measurement.
ECT:
Engine Coolant Sensor. A thermistor a
sensor whose resistance decreases with
increases in temperature is threaded
into the engine block, contacting the engine
coolant. It sends a voltage signal to the PCM
indicating the temperature of the coolant.
The PCM uses this signal for control of fuel
delivery, spark advance, and EGR flow.
EFI:
Electronic Fuel Injection. A term applied to
any system where a computer controls
fuel delivery to the engine by using fuel
injectors.