IDLE SPEED CONTROL

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IDLE SPEED CONTROL

• ENGINE PERFORMANCE
• COLUMBIA BASIN COLLEGE

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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

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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
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Throttle Kicker and Idle Stop Solenoid

• Electrically controlled
• Vacuum operated throttle kicker for additional
  load
• Allows curb idle speed to be lower
• Controlled by PCM
• Must be adjusted

327C
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IDLE SPEED CONTROL MOTOR (ISC)

• PCM controlled
• Reversible gear drive dc motor
• PCM reverses polarity
• Has idle tracking switch
• Must be adjusted

325C
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ISC Schematic
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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

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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

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IAC SCHEMATIC
508L

• 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

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IAC SCOPE PATTERNS

• Typical patterns
• Compare patterns to known good patterns
• Some IACs operate on 12 volts and others on less
  voltage
• IAC operates with a duty cycle

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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
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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)

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