Download 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