Calais L4-151 2.5L VIN U TBI (1989)
Wiring Diagram For IAC System
CIRCUIT DESCRIPTION:
The ECM controls engine idle speed with the Idle Air Control (IAC) valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its
seat, allowing more air to pass by the throttle body. To decrease idle speed, it extends the IAC valve pintle towards its seat, reducing bypass air flow. A
"Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts indicate more air bypass (higher idle). The lower the counts
indicate less air is allowed to bypass (lower idle).
TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.
1.
Continue the test even if the engine will not idle. If the idle speed is to low, the "Scan" tool will display 80 or more counts or steps. If idle speed is
high, the displayed counts or steps will be 0. Occasionally an erratic or unstable idle may occur (engine speed may vary 200 rpm or more).
Disconnect the IAC valve. If the condition is unchanged, the IAC valve is not at fault.
2.
When the engine was not running, the IAC valve retracted (more air) to a fixed "Park" position for increased air flow. A "Scan" tool will display
100 or more counts.
3.
Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
4.
There is a remote possibility that one of the circuits is shorted to voltage, which may have been indicated by a steady light. Disconnect the ECM
and turn the ignition "ON." Probe the terminals to check for this condition.
DIAGNOSTIC AIDS:
A slow, unstable idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC "Scan" tool
counts will be above 60 if idle is too low, and "0" counts if the idle is too high. If the idle speed is too high, turn the ignition "OFF" to stop the engine.
Turn the ignition "ON" without the engine running. Wait 45 seconds for the IAC valve pintle to seat, then disconnect the IAC valve. The following
checks should be made to repair a non-IAC system problem:
a.
Vacuum leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 rpm,
locate and correct vacuum leak including PCV system. Also check for binding of throttle blade or linkage.
b.
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. A Code 44 may set. "Scan" O2 voltage will be less than 300 mv (0.3 volt). Check for low regulated fuel pressure
, or water in fuel. A fixed oxygen sonsor voltage above 800 mv (0.8 volts) may be contaminated by silicone. A Code 45 may be stored in the
computer.
c.
System Too Rich (Low Air/Fuel Ratio - The idle speed will be too low. Scaning IAC counts will usually be above 80. System is obviously
rich and may exhibit black smoke in exhaust. "Scan" tool O2 voltage will be fixed above 800 mv (0.8 volt). Check for high fuel pressure,
leaking or sticking injector. Silicone contaminated O2 sensors "Scan" voltage will be slow to respond.
d.
Throttle Body - Remove IAC valve and inspect bore for foreign material.
e.
IAC Valve Electrical Connections - IAC valve harness connections should be carefully checked for proper contact.
f.
PCV Valve - An incorrect or faulty PCV valve may result in an incorrect idle speed.
g.
Refer to "Rough, Unstable, Incorrect Idle or Stalling" in DIAGNOSIS BY SYMPTOM.
