Circuit Description
The VCM controls the idle speed with an Idle Air Control (IAC) valve to a calculated desired RPM based on the sensor inputs and the actual engine RPM. The VCM uses 4 circuits in order to move an Idle Air Control (IAC) valve, allowing a varying amount of air flow into the intake manifold.
In order to increase the idle RPM, the VCM moves the IAC valve out, allowing more air to bypass the throttle plate. In order to decrease the RPM, the VCM moves the IAC valve in, reducing the amount of air bypassing the throttle plate. A scan tool reads the VCM commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate (higher idle). The lower the counts, the less air that is allowed to bypass the throttle plates (lower idle). This DTC is a type B DTC.
Conditions For Setting DTC
Non-intrusive
| • | No TP sensor DTCs set |
| • | No ECT DTCs set |
| • | No VSS DTCs set |
| • | No MAP DTCs set |
| • | ECT is greater than 50°C |
| • | IAT is greater than -25°C |
| • | BARO is graeter than 75 kPa (10,300 ft.) |
| • | Vehicle Speed is less than 2 mph |
| • | System voltage is between 10 volts and 16 volts |
| • | Engine run time is greater than 30 seconds. |
| • | TP is less than 1% |
| • | Above met for a time greater than 3 seconds |
If the non-intrusive test fails, the intrusive test is ran.
Intrusive
| • | Calculated Air Flow is between 17.5 grams per second and less than 50 grams per second |
| • | Vehicle Speed is between 25 MPH and 85 MPH |
| • | Change in TP Sensor is less than 1% |
| • | Change in engine speed is less than 50 RPM |
| • | IAC motor commanded to 10% for 100 msec |
Action Taken When the DTC Sets
The VCM will turn ON the MIL after 2 consecutive driving cycles with the fault active.
Conditions for Clearing the MIL/DTC
The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.
Diagnostic Aids
The following items could cause this DTC:
| • | An IAC valve which is stopped and cannot respond to the VCM |
| • | A throttle stop screw which has been tampered with |
| • | A damaged throttle or linkage |
A slow, unstable, or fast idle may be caused by a non - IAC valve system problem that cannot be overcome by the IAC valve. Out of range control IAC scan tool counts will read above 60 if the idle is too low and zero if the idle is too high. Check for the following:
| • | Fuel system too lean and rich |
| • | Throttle body - inspect the bore for foreign material. |
| • | Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
| • | IAC reset is done with the scan tool misc. functions mode. |
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing DTCs, use the scan tool in order to record the freeze frame and the failure records for reference because the Clear Info function will lose the data. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to A Powertrain On Board Diagnostic (OBD) System Check | |
2 | Install the scan tool. Are any other DTCs set? | -- | Go to The Applicable DTC Table | |
3 | Is the DTC P0506 active this ignition cycle? | -- | ||
4 | This DTC is an intermittent. Refer to the Diagnostic Aids. Is the action complete? | -- | -- | |
5 | Problem is engine mechanical related. Refer to Engine Mechanical. Is the action complete? | -- | -- | |
6 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
7 | Using the scan tool, select the Capture Info and the Review Info. Are any DTCs displayed that have not been diagnosed? | -- | Go to The Applicable DTC Table | System OK |
Circuit Description
The control module controls idle speed with an idle air control (IAC) valve. The desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used in order to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the control module will command the valve pintle to move away from its seat, allowing more air to bypass the throttle plate. To decrease RPM, the control module commands the IAC pintle to move closer to its seat, causing a lower airflow to bypass the throttle plate. A scan tool will display the control module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicate that less air is commanded by the control module.
Conditions For Running DTC
| • | No active TP sensor DTCs |
| • | No active VS sensor DTCs |
| • | No active ECT sensor DTCs |
| • | No active MAP DTCs |
| • | No active MAF DTCs |
| • | No active IAT DTCs |
| • | No active transmission DTCs |
| • | No active PRNDL DTCs |
| • | No active misfire DTCs |
| • | The ECT is more than 50°C (122°F) |
| • | The system voltage is between 10 and 16 volts |
| • | The IAT is more than -25°C ( -13°F) |
| • | The engine has been running for more than 30 seconds |
| • | BARO is more than 75 kPa |
| • | The MAF is between 17.5 g/s and 50 g/s |
| • | The vehicle speed is between 25 MPH and 85 MPH |
| • | Any change in throttle position is less than 1 percent |
| • | Any change in engine speed is less than 50 RPM |
| • | The IAC valve was commanded to move specified number of counts |
| • | The above conditions present for more than 3 seconds |
Conditions For Setting DTC
The change in MAF is less than 6 g/s.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) if a failure is detected during 2 consecutive key cycles. |
| • | The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame/Failure Records. |
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
The following items could cause this DTC:
| • | A stuck IAC valve cannot respond to the VCM |
| • | A throttle stop screw which has been tampered with |
| • | A damaged throttle shaft, plate, or linkage |
A slow, unstable, or fast idle may be caused by a non - IAC valve system problem that cannot be overcome by the IAC valve. Out of range control IAC scan tool counts will read above 60 if the idle is too low and zero if the idle is too high. Check for the following:
| • | Fuel system too lean or rich |
| • | Throttle body - inspect the bore for foreign material. |
| • | Vacuum leaks |
| • | Refer to Symptoms . |
| • | IAC reset is done with the scan tool. |
An intermittent may be caused by any of the following conditions:
| • | A poor connection |
| • | Rubbed through wire insulation |
| • | A broken wire inside the insulation |
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
-
The IAC Reset function used in this step only energizes the circuit for 2 seconds, so this function will have to be used repeatedly during this step. Also it is recommended that the J 35616 connector test adapter kit is used during this procedure to prevent terminal damage to the IAC harness connector. This step can also be performed with the ignition ON and engine OFF.
-
If the test lamp remained ON at any of the IAC valve harness connector terminals during the IAC valve reset function the circuit is shorted to voltage. If these circuits are checked before or after the IAC valve reset function it is possible that any combination of IAC circuits could light a test lamp, this would be a normal condition.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure Records for reference. The control module's data is deleted once the Clear Info function is used. Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
2 |
Does the engine speed correspond within 100 RPM of the commanded speeds? | 1200 RPM 500 RPM | Go to Diagnostic Aids | |
3 |
Are the resistances across terminals A and B, and terminals C and D within the specified value? | 40-80 ohms | ||
4 |
Are the resistances across terminals B and C, and terminals A and D at the specified value? | ∞ | ||
Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury. Important:: Command ON the Reset IAC function for each circuit tested. This function will only flash each circuit for 2 seconds. Does the test lamp flash ON and OFF at all terminals? | -- | |||
Did the test lamp remain ON at any of the terminals during the IAC valve reset function? | -- | |||
7 | Check for an open or short to ground in the IAC valve circuit that did not flash the test lamp. Did you find a problem? | -- | ||
8 | Check for a short to voltage in the IAC valve circuit where the test lamp remained ON. Did you find a problem? | -- | ||
9 |
Did you find a problem? | -- | Go to Diagnostic Aids | |
10 | Check for a poor connection at the VCM. Did you find a problem? | -- | ||
11 | Repair the circuit as necessary. Refer to Wiring Repairs or Connector Repairs in Wiring Systems. Is the action complete? | -- | -- | |
12 | Clean the IAC valve and passages. Refer to the Cleaning and Inspection Procedure in the Idle Air Control Valve Replacement . Is the action complete? | -- | -- | |
13 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Is the action complete? | -- | -- | |
14 |
Is the action complete? | -- | -- | |
15 |
Does the scan tool indicate the diagnostic Passed? | -- | ||
16 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
