Powertrain Controls - ECM/PCM

DTC 35 Chart:

Idle Air Control Circuit:

Circuit Description:
The IAC motor will constantly adjust its valve position to maintain the desired idle speed. If idle speed is below the desired idle, the valve will open to allow more air flow and increase RPM. If the idle is too high, the IAC will slowly close to limit airflow and therefore engine RPM will decrease. If engine RPM becomes out of control for a long enough period, then a DTC 35 will be set. Review the Description and operation of the IAC.

DTC 35 Will Set When:
Closed throttle engine speed is 300 RPM above or below desired idle for 50 seconds.

Action Taken (PCM will default to):
The PCM will turn the MIL "ON" and set a DTC 35. The RPM will remain controlling erratically until the problem is corrected.

DTC 35 Will Clear When:
A current DTC 35 will clear when the engine speed controls within a 350 RPM range of desired idle. A history DTC 35 will clear after 50 consecutive ignition key cycles without a current DTC 35 being stored.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. The Tech 1 RPM control mode is used to extend and retract the IAC valve. The valve should move smoothly within the specified range. If the idle speed is commanded (IAC extended) too low (below 700 RPM), the engine may stall. This may be normal and would not indicate a problem. Retracting the IAC beyond its controlled range (above 1500 RPM) will cause a delay before the RPMs start dropping. This too is normal.

2. This test uses the Tech 1 scan tool to command the IAC controlled idle speed. The PCM issues commands to obtain commanded idle speed. The node lights each should flash red and green to indicate a good circuit as the PCM issues commands. 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:
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of range IAC counts will be above 60 if idle is too low and zero counts if idle is too high. If idle speed is above 600-700 RPM in drive with an A/T, locate and correct vacuum leak. If RPM is below spec., check for foreign material around throttle plates. The following checks should be made to repair a non-IAC system problem.

- Vacuum leak (high idle). If idle is too high, stop the engine. Fully extend IAC with tester. Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation system. Also check for binding of throttle blade or linkage.

- System too lean (high air/fuel ratio). The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. DTC 44 may set. Tech 1 O2S voltage will be less than 300 mV (.3 volt). Check for low regulated fuel pressure, water in the fuel or a restricted injector.

- System too rich (low air/fuel ratio). The idle speed will be too low. Tech 1 IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. DTC 45 may set. Tech 1 scan tool O2S voltage will be fixed above 800 mV (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 will cause sluggish voltage variations on Tech 1 scan tool.

- Throttle body. Remove IAC valve and inspect bore for foreign material.

- IAC valve electrical connections. IAC valve connections should be carefully checked for proper contact.

- Crankcase ventilation valve. An incorrect or faulty valve may result in an incorrect idle speed.

- Refer to "Rough, Unstable, Incorrect Idle, or Stalling" in "Diagnosis By Symptom." Rough, Unstable, or Incorrect Idle, Stalling

- If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.

- A/C compressor or relay failure. Refer to CHART C-10 Chart C-10 A/C Clutch Control Circuit Diagnosis
if the A/C control relay drive circuit is shorted to ground or if the relay is faulty, an idle problem may exist.

- If above are OK see "Rough, Unstable, Incorrect Idle, or Stalling" in "Diagnosis By Symptom." Rough, Unstable, or Incorrect Idle, Stalling