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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1350 (Engine Control System (Troubleshooting & Diagnosis) -- 1.8L (LNK)) >> Troubleshooting >> Idle Air Control (IAC) System Diagnosis >> Test Description

Test Description

WARNING: This page is about a different car, the 2004 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

  1. 1:  A Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
  2. 4:  This step tests whether the PCM can operate the IAC valve with the scan tool. The IAC valve can be commanded to increase and/or decrease engine speed by using the scan tool RPM Control function. Remember to clear any DTCs and run the engine until the engine reaches a normal operating temperature before attempting RPM Control with the scan tool.
  3. 5:  This step checks the operation of both electrical load idle-up circuits. If there is no increase in the IAC Motor Command percentage when the electrical accessory is turned ON, the idle-up circuit may be inoperative or always ON.
  4. 6:  This step checks the operation of the A/C idle-up circuit. If there is no increase in the IAC Motor Command percentage when the A/C compressor is engaged, the idle-up circuit may be inoperative or always ON.
  5. 10:  This step simulates the pulse width modulated (PWM) signal supplied to the IAC valve by the PCM by rapidly grounding and un-grounding terminal 1 of the IAC valve.
  6. 12:  The most likely cause of no ignition positive voltage is an open circuit condition between the IAC valve connector terminal 2 and the splice. A shorted or inoperative EFI relay circuit will cause a no-start condition.
Idle Air Control (IAC) System Diagnosis

Step Action Value(s) Yes No
Schematic Reference:  Engine Controls Schematics 
1  Did you perform A Diagnostic System Check-Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Engine Controls
2 Is there a DTC P0505 set? - Go to DTC P0505 Go to Step 3 
3
  1. Start the engine.
  2. Run the engine until a normal operating temperature of at least 80° C (174° F) is achieved.
  3. Allow the engine to idle for at least 15 minutes after reaching operating temperatures.
  4. Operate the engine at 2,500 RPM for 90 seconds.
  5. Release the throttle and wait 30 seconds while the engine idle stabilizes.
  6. Observe the Engine Speed and the IAC Motor Command parameters on the scan tool with the transmission in Park or Neutral.
Are the Engine Speed and the IAC Motor Command within the specified ranges?		 750-850 RPM M/T / 700-800 RPM A/T
27-47% IAC Motor Command		 Go to Step 5  Go to Step 4 
4 
  1. Operate the engine at a normal operating temperature.
  2. Select RPM Control, under IAC System, in the Special Functions menu of the scan tool.
  3. Command the IAC valve to increase and decrease the engine speed with the scan tool.
Did the engine speed increase and decrease as commanded by the scan tool?		 - Go to Step 5  Go to Step 8 
5 
  1. Start the engine.
  2. Place the transmission in Park or Neutral.
  3. Operate the following accessories, one at a time, while observing the Engine Speed and the IAC Motor Command parameters on the scan tool:
    • Turn ON the headlights.
    • Turn ON the rear window defogger, if equipped.
Did the IAC Motor Command increase within the specified range?		 1-3% IAC Motor Command Go to Step 6  Go to Electrical/Accessory Load Idle-Up Signal Diagnosis 
6  Turn ON the HVAC blower motor and the air conditioning, if equipped.
Did the IAC Motor Command increase within the specified range?		 4-14% IAC Motor Command Go to Step 7  Go to Electrical/Accessory Load Idle-Up Signal Diagnosis 
7
  1. Turn OFF all accessories.
  2. Turn the steering wheel to the right and to the left.
  3. Return the steering wheel to the center, straight ahead position.
Did the IAC Motor Command increase within the specified range, when the steering wheel was turned all the way in either direction?		 2-10% IAC Motor Command Go to Step 16  Go to Electrical/Accessory Load Idle-Up Signal Diagnosis 
8
  1. Turn OFF the ignition.
  2. Disconnect the IAC valve electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Using a test lamp connected to ground, probe terminal 2 of the IAC valve connector on the harness side.
Does the test lamp illuminate?		 - Go to Step 9  Go to Step 12 
9 Using a test lamp connected to B+, probe terminal 3 of the IAC valve connector.
Does the test lamp illuminate?		 - Go to Step 10  Go to Step 13 
10 
  1. Turn OFF the ignition.
  2. Jump terminal 2 of the IAC valve connector on the harness side to terminal 2 of the IAC valve connector with a fused jumper wire.
  3. Jump terminal 3 of the IAC valve connector on the harness side to terminal 3 of the IAC valve connector with a fused jumper wire.
  4. Connect a fused jumper wire to terminal 1 of the IAC valve-not the harness. Insulate the terminal 1 jumper from contact to ground.
  5. Start and idle the engine.
  6. With the engine idling, repeatedly touch the terminal 1 jumper to battery ground with a rapid ON-OFF motion.
Does engine speed increase as the terminal 1 jumper is rapidly grounded ON and OFF?		 - Go to Step 11  Go to Step 14 
11
  1. Turn OFF the ignition.
  2. Remove all jumpers.
  3. Disconnect the PCM connector containing the IAC valve open control circuit.
  4. Check terminal 1 of the IAC valve open control circuit for a grounded, open, or shorted to voltage circuit.
  5. Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.
Was a circuit repair made?		 - Go to Step 17  Go to Step 15 
12 
  1. Locate the cause of no power to the IAC valve connector. Refer to the Test Description.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.
Is the action complete?		 - Go to Step 17  -
13 Repair the open IAC valve ground circuit. Refer to Wiring Repairs in Wiring Systems.
Is the action complete?		 - Go to Step 17  -
14
IMPORTANT: The IAC valve seals the coolant passage of the throttle body.
Replace the IAC valve. Refer to Idle Air Control (IAC) Valve Replacement .Is the replacement complete?		 - Go to Step 17  -
15 Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement .
Did you complete the repair?		 - Go to Step 17  -
16
  1. Turn OFF all accessories.
  2. Ensure that the engine is operating at a normal temperature.
  3. Observe the Engine Speed and the IAC Motor Command parameters on the scan tool with the transmission in Park or Neutral.
Are the Engine Speed and the IAC Motor Command within the specified ranges?		 750-850 RPM M/T / 700-800 RPM A/T
27-47% IAC Motor Command		 System OK Go to Rough, Unstable, or Incorrect Idle and Stalling 
17
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Are any DTCs displayed on the scan tool?		 - Go to Step 18  Go to Step 2 
18 Operate the vehicle within the conditions in which the customer concern was first noticed.
Is the concern still present?		 - Go to Diagnostic Trouble Code (DTC) List System OK
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