Circuit Description
The engine control module (ECM) can control idle speed by electrically controlling
the throttle plate, advancing/retarding spark timing, or increasing/decreasing the
air/fuel ratio. The ability for the ECM to control the throttle plate is performed
by the use of the throttle actuator control (TAC) module. This assembly, internal
to the throttle body assembly, houses a DC motor along with 2 independent throttle
position (TP) sensors, which relay the position of the throttle plate. The throttle
plate is held open 7 percent to a mechanical stop by the use of spring pressure
for minimum idle air and in the event of a motor circuit malfunction. The ECM monitors
the throttle actuator control system for faults in the TP sensors, TAC motor circuit,
and for a correlation between the actual and desired throttle plate position. The
ECM also compares its commanded idle speed to the actual idle speed to ensure idle
control is in check. DTC P0506 will set if the actual idle speed is less than
a calibrated amount from the ECM commanded idle speed after a certain period of time.
Conditions for Setting the DTC
DTC P0506 will set if the actual idle speed is less than 100 RPM
from the ECM commanded idle speed when:
| • | The condition exists for longer than 2 seconds after the ECM has
attempted to correct the condition within a limited range. |
| • | The engine coolant temperature (ECT) is greater than 80°C (176°F). |
| • | The intake air temperature (IAT) is greater than 34°C (93°F). |
| • | The vehicle speed is 0 mph. |
| • | The evaporative emission (EVAP) leak diagnostics are not running. |
| • | No ECT, EVAP leak, EVAP purge solenoid, IAT, TP, or vehicle speed DTCs
have been set. |
DTC P0506 diagnostic runs continuously once the above conditions have
been met.
DTC P0506 is a type B DTC.
Diagnostic Aids
Important: If an EGR, fuel trim, HO2S, MAF, misfire, system voltage, or throttle actuator
is also set, diagnose that DTC first.
Verify the ECM ability to control engine speed by using the scan tool to perform
the ENGINE SPEED CONTROL special function. The test will allow the RPM to be raised
or lowered in increments of 25, 100, and 500 RPM. Compare the actual ENGINE
SPEED to the DESIRED IDLE SPEED. Both should be within 100 RPM of each other.
Make an underhood visual inspection first. Possible causes of low idle speed:
| • | EGR valve stuck open of gasket leaking |
| • | Air induction tubing cracked, damaged, or loose |
| • | Carbon around throttle plate/bore |
| • | Accessory drive component loading engine |
