Powertrain Controls - ECM/PCM

Trouble Code 35 Chart:

Idle Air Control:

CIRCUIT DESCRIPTION
The Idle Air Control (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 Diagnostic Trouble Code (DTC) 35 will be set. Review the description of the IAC valve.

DTC 35 WILL SET WHEN
DTC 24 is not present. Power steering is not cramped. Closed throttle engine speed is 350 RPM above or below desired idle for 50 seconds.

POWERTRAIN CONTROL MODULE (PCM) WILL DEFAULT TO
The PCM will turn the Malfunction Indicator Lamp (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
Numbers below refer to circled numbers 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 scan tool H02S voltage reading will be less than 300 mV (0.3 volt). Check for low regulated fuel pressure, water in the fuel or a restricted injector or filter.

System Too Rich (low air/fuel ratio)
The idle speed will be too low. Tech 1 (or equivalent scan tool) IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. DTC 45 may set. Tech 1 (or equivalent) scan tool HO2S voltage will be fixed above 800 mV (0.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S will cause sluggish voltage variations on Tech 1 scan tool.

Throttle Body
Remove IAC valve and inspect bore or foreign material.

The IAC Valve Electrical Connections
The IAC valve connections should be carefully checked for proper contact. 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
If the A/C control relay drive circuit is shorted to ground or if the relay is faulty, an idle problem may exist. Refer to Computers and Controls / System Diagnosis / Procedures / Diagnostic Charts / Chart C-10. Chart C-10 A/C Clutch Control Circuit Diagnosis

Crankcase Ventilation Valve
An incorrect or faulty valve may result in an incorrect idle speed. For further diagnosis OR if above are OK, refer to Computers and Controls / System Diagnosis / Procedures / Diagnosis by Symptom / Rough, Unstable, Incorrect Idle, or Stalling. Rough, Unstable, or Incorrect Idle, Stalling