P1509

DTC P1509 Idle Speed Higher Than Expected:

Idle Air Control:

CIRCUIT DESCRIPTION
The Vehicle Control Module-A (VCM-A) controls idle speed with an Idle Air Control (IAC) valve to a calculated "desired" RPM based on sensor inputs and actual engine RPM. The VCM-A used 4 circuits to move an IAC valve, allowing varying amounts of air flow into the intake manifold.

To increase idle RPM, the VCM-A moves the IAC valve out, allowing more air to bypass the throttle plate. To decrease RPM, the VCM-A will move the IAC valve in, reducing the amount of air bypassing the throttle plate. A Tech 1 scan tool reads the VCM-A commands to the IAC valve in counts. The higher the counts, the more air that 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). If the passive test fails and the active test fails then P1509 will set. DTC P1509 is a type "B" DTC.

DTC P1509 WILL SET WHEN
Passive Test
^ No Throttle Position (TP) sensor DTC's set.
^ No Engine Coolant Temperature (ECT) DTC's set.
^ No Vehicle Speed Sensor (VSS) DTC's set.
^ No Manifold Absolute Pressure (MAP) DTC's set.
^ ECT is greater than 72°C (161°F).
^ Intake Air Temperature (IAT) is greater than -25°C (-13°F).
^ Commanded IAC is greater than 93 counts.
^ Barometric Absolute Pressure (BARO) is greater than 70 kPa (21 Hg).
^ Vehicle speed is less than 2 mph.
^ System voltage is between 10 volts and 18 volts.
^ Engine run time is greater than 30 seconds.
^ TP is less than 1%.
^ Engine speed to at least 75 RPM below desired engine speed.
^ Above met for a time greater than 3 seconds.

If the passive test fails, the active test is run
^ Commanded IAC is between 36 counts to 116 counts.
^ Calculated air flow is between 17.5 grams per second - and 37.5 grams per second.
^ Vehicle speed is between 35 mph and 85 mph.
^ Change in TP sensor is less than 1%.
^ Change in engine speed is less than 30 RPM.
^ IAC motor commanded to 0 counts.
^ Change in calculated air flow is less than 2 grams per second.

ACTION TAKEN (VCM-A WILL DEFAULT TO)
VCM-A will turn "ON" Malfunction Indicator Lamp (MIL) (Service Engine Soon) after two consecutive driving cycles with the fault active.

DTC P1508 WILL CLEAR WHEN
The VCM-A will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

DTC CHART TEST DESCRIPTION
Number(s) below refer to step number(s) on the diagnostic chart.
1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
2. This step checks the quality of the IAC movement in Step 1. Between 700 RPM and about 1200 RPM, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check the circuits for faults beginning with poor terminal contacts.

DIAGNOSTIC AIDS
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

Check for:
^ Poor connection or damaged hamess - Inspect VCM-A harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
^ Intermittent - If connections and hamess check OK, monitor a digital voltmeter connected between VCM-A terminal and component terminal while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change. This may help to isolate the location of the malfunction.