idle speed control
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Transcript idle speed control
IDLE SPEED CONTROL
IDLE SPEED CONTROL
To comply with federal emissions
standards, idle speed control systems are
used
Idle speed controlled by electronic
module
Earlier systems, engines idled in open
loop
Idle systems depend on inputs to PCM
Throttle Kicker controlled by
Vacuum Diaphragm
Electrically controlled by solenoid
Operates during engine warm up, with air
conditioning on and engine overheating
Some act as a dashpot
327C
Throttle Kicker and Idle Stop
Solenoid
327C
Electrically
controlled
Vacuum operated
throttle kicker for
additional load
Allows curb idle
speed to be lower
Controlled by PCM
Must be adjusted
IDLE SPEED CONTROL
MOTOR (ISC)
325C
PCM controlled
Reversible gear drive
dc motor
PCM reverses
polarity
Has idle tracking
switch
Must be adjusted
ISC Schematic
IDLE AIR CONTROL (IAC)
Used with Fuel
Injection systems
PCM controlled
Stepper motor
Diverts air around
throttle plate
Do not over extend
when replacing
Not adjustable
Throttle Body IAC Location
Screws into throttle
body
Must be careful to
not over-tighten
Must keep passages
clean
Can use Scanner to
diagnose idle
problems
508L
IAC SCHEMATIC
Contains two motor windings
Can test with ohmmeter (GM are 40 - 80 ohms)
Use IAC Test tool to check operation
Can monitor operation with Scanner
IAC SCOPE PATTERNS
Typical patterns
Compare patterns to
known good patterns
Some IAC’s operate
on 12 volts and
others on less voltage
IAC operates with a
duty cycle
ADJUSTING CURB IDLE
(Idle Hardstop or Minimum
Idle speed)
Minimum idle rpm is
factory adjusted
Idle must be adjusted
correctly
Passages must be
clean
Some systems, you
must adjust TPS also
502L
IDLE SPEED ADJUSTMENT
Must shut off idle air
to adjust minimum
idle speed
Use idle plug on TBI
units
Other systems, you
just turn a screw to
control air flow (idle
rpm)
END