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Other Features
Additional Code Scanner Diagnostic Features
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Relay and Solenoid Circuit Test: You
can switch on most of the computer
controlled relay and solenoid circuits for checking relay operation or making
wiring voltage checks.
Field Service Test (Fuel Injected
Engines Only): A quick check of the
fuel control system to verify proper
operation.
Become familiar with Code Scanner
use (Section 3) before using the
following procedures.
Relay and Solenoid
Circuit Test
Computer controlled relay and solenoid
coils are commonly wired as follows:
• One side of the coil is connected to
a source of vehicle battery power.
• The other side of the coil is wired to
the computer.
Inside the computer housing is a
transistor switch (often called a
RELAY COIL
“driver”). The computer energizes the
coil by using the transistor switch.
Transistor ON:
– Transistor electrically connects end
of coil to circuit ground.
– Coil is ON because circuit is
complete. (Coil connected to battery
power and ground.)
Transistor OFF:
– Transistor disconnects end of coil
from circuit ground.
– Coil is OFF because circuit is open.
(Coil not connected to circuit
ground.)
You can switch on most of the
computer controlled relay and solenoid
circuits except the fuel pump relay and
fuel injectors. This is helpful for
checking relay operation or making
wiring voltage checks. Do the following:
1) Safety First!
• Set the parking brake.
• Put shift lever in PARK (automatic
transmission) or NEUTRAL
(manual transmission).
• Block the drive wheels.
• Make sure ignition key is in OFF
position.
COMPUTER
12 volts
or more
Less than
1.5 volts
Transistor ON*
RELAY ON
RELAY COIL
*Transistor action is
shown using a “switch”
representation for
clarity.
COMPUTER
12 volts
or more
12 volts
or more
Transistor OFF*
RELAY OFF
2) Plug the Code Scanner
into the Test Connector.
Put TEST switch on
ENGINE.
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