Circuit Description
The engine control module (ECM) controls the engine idle speed by adjusting the position of the idle air control (IAC) valve pintle. The IAC valve is a stepper motor driven by two internal coils. The movement of the IAC valve is controlled electrically by four circuits. Drivers within the ECM control the polarity of the two windings inside the IAC valve through these circuits. The ECM, by commanding the correct polarity in sequence, is able to command the motor within the IAC valve to rotate clockwise or counter clockwise in steps. In order for the armature of the stepper motor of the IAC valve to turn one revolution, it needs to move approximately twenty four steps. The IAC valve motor is linked by a gear drive to the IAC valve pintle. The electrical pulses sent to the IAC valve coils, by the ECM, allows the pintle to extend or retract into the passage in the throttle body. By retracting the pintle, air is allowed to bypass the throttle valve, which will increase air flow and raise engine speed. When the pintle is extended, bypass air is decreased which lowers engine speed. IAC valve movement is measured in counts on the scan tool. Each count is equivalent to a step of the IAC valve. When the IAC valve is fully extended, and seated in the throttle body bore, the scan tool displays zero and the engine speed is slow. As the IAC valve pintle retracts the counts will raise along with engine speed.
If the ECM detects that the engine speed is not within a predetermined amount of the desired engine speed, this DTC sets.
IAC Valve Reset
When the ignition switch is turned OFF for more than ten seconds an IAC valve reset occurs. At that time, the ECM commands the IAC valve to extend for a certain period which allows the IAC pintle to seat in the throttle body bore. This position is then sensed as zero count position for the IAC valve by the ECM. It must be observed that the IAC valve position is only sensed by the ECM measuring counts or steps of the driver circuit, there is no direct sensing of its exact position. When this extend time period ends, the ECM then commands the IAC valve to retract a predetermined amount. This will allow for a high engine speed on the next ignition cycle. If for any reason the IAC valve pintle moves after this reset, before the next ignition cycle, the ECM will not be able to detect it and will affect the ability to control engine idle. When the IAC valve is removed for any reason the reset must be performed. Refer to Idle Learn .
DTC Descriptor
This diagnostic procedure supports the following DTCs:
DTC P0505 Idle Control System
Conditions For Running the DTC
| • | DTCs P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0132, P0131, P0132, P0133, P0137, P0138, P0140, P0141, P0171, P0172, P0261, P0262, P02064, P0265, P0267, P0268, P0300, P0335, P0336, P0337, P0341, P0342, P0351, P0352, P0353, P0400, P0404, P0405, P0406, P0444, P0445, P0501, P1320, and P1321 are not set. |
| • | The engine coolant temperature (ECT) is more than 60°C (140°F). |
| • | The intake air temperature is more than -20°C (-4°F) |
| • | The barometric pressure (BARO) is more than 72 kPa. |
| • | The engine run time is more than 60 seconds. |
| • | The manifold absolute pressure (MAP) is less than 60 kPa. |
| • | The ignition voltage is between 11.0 and 16.0 volts. |
| • | This DTC runs continuously when the above conditions are met for 5 seconds. |
Conditions for Setting the DTC
| • | The actual engine speed is 100 RPM less than the desired engine speed, or the actual engine speed is 200 RPM more than the desired engine speed. |
| • | The conditions exists for 10 seconds. |
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) will illuminate. |
| • | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records. |
| • | A history DTC is stored. |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after 4 consecutive ignition cycles in which the diagnostic runs without a fault. |
| • | A history DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | DTCs can be cleared by using the scan tool. |
| • | Disconnect the ECM battery feed for more than 10 seconds. |
Diagnostic Aids
| • | A low, high or unstable idle condition may be caused by a non-IAC system condition that cannot be overcome by the IAC valve. Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
| • | If the condition is intermittent, refer to Intermittent Conditions . |
Step | Action | Values | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||||||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||||||||||
2 |
Important: Ensure that engine speed stabilizes with each commanded RPM change in order to determine if engine speed stays within the specified value of the commanded RPM.
Is the engine speed within the specified value of each command of the RPM? | 100 RPM | Go to Step 3 | Go to Step 4 | ||||||||||||
3 |
Does the DTC fail this ignition cycle? | -- | Go to Step 4 | Go to Diagnostic Aids | ||||||||||||
4 |
Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1,800 RPM when the IAC valve was commanded in and out? | -- | Go to Step 6 | Go to Step 5 | ||||||||||||
5 | Is the engine speed over 100 RPM less than the desired engine speed? | -- | Go to Step 11 | Go to Step 12 | ||||||||||||
6 |
Important: If the test lamp illuminates dimly on one or more of the IAC circuits, repair the high resistance in the appropriate IAC circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did the test lamp remain ON and never flash while the IAC Counts were incrementing at any of the IAC valve control circuits? | -- | Go to Step 10 | Go to Step 7 | ||||||||||||
7 | Did the test lamp remain OFF and never flash while the IAC Counts were incrementing at any of the IAC valve circuits during the above test? | -- | Go to Step 9 | Go to Step 8 | ||||||||||||
8 |
Did the test lamp stay illuminated and never flash while the IAC counts were incrementing during the above test? | -- | Go to Diagnostic Aids | Go to Step 16 | ||||||||||||
9 |
Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 14 | ||||||||||||
10 |
Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 14 | ||||||||||||
11 |
Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 13 | ||||||||||||
12 |
Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 13 | ||||||||||||
13 | Test for an intermittent and for a poor connection at the IAC Valve. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 15 | ||||||||||||
14 | Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 16 | ||||||||||||
15 |
Important: If the IAC valve is removed or replaced, perform the Idle Learn . Replace the IAC valve. Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||
16 |
Important: If the ECM power is removed or the ECM is replaced, perform the Idle Learn . Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||
17 |
Does the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 18 | ||||||||||||
18 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||||
