Powertrain Controls - ECM/PCM

DTC 35 Chart:

Idle Speed Sensor Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) controls idle speed to a calculated, "desired" RPM based on sensor inputs and actual engine RPM. The PCM uses 4 circuits to move an Idle Air Control (IAC) valve, which allows varying amounts of air to flow into the intake manifold whenever the throttle plates are closed to control idle speed. A detailed description of the IAC system may be found in Fuel Delivery and Air Induction / Idle Air Control (IAC) Valve / Description and Operation.

DIAGNOSTIC TROUBLE CODE (DTC) 35 WILL SET WHEN
^ Engine speed is 175 RPM more or 250 RPM less than "desired idle."
^ Throttle Position (TP) sensor throttle angle is less than 1%.
^ IAC steps are less than 10, or greater than 140.
^ Above conditions are met for 10 seconds or more.

ACTION TAKEN (PCM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate, and the cooling fan will enable.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. The Tech 1 scan tool 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 valve movement in Step 1. Between 900 RPM and 1500 RPM, the engine speed should change smoothly with each flash of the test light in both extended and retracted position. 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 the IAC valve may cause the engine to stall. This may be normal.

Step 1 verifies proper IAC valve operation while this step checks the IAC valve circuits. Each lamp on the node light should flash red and green while the IAC valve is cycled. 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 contact.

DIAGNOSTIC AIDS
^ Check for vacuum leaks, disconnected or brittle vacuum hoses, cuts, etc. Examine manifold and throttle body gaskets for proper seal.
^ Check for cracked intake manifold.
^ Check for open, shorts, chafed insulation or poor connections to IAC valve in CKTs 1747,1748, 1749 and 444.
^ An IAC valve which is stopped and cannot respond to the PCM, a throttle stop screw which has been tampered with, a damaged throttle body or linkage could cause a DTC 35 to set.
^ If DTC 35 is set and speedometer is not accurately reflecting vehicle speed, refer to DTC 24 (vehicle speed sensor circuit) diagnostics.