Circuit Test Description
Turning on A/C supplies battery voltage to circuit No. 59 A/C compressor clutch, and to terminal "B8" of ECM connector to increase idle air rate and maintain idle speed.
The ECM does not control the A/C compressor clutch; therefore, if A/C does not function, service A/C system.
If A/C is operating properly and idle speed dips too low when A/C compressor turns on, or flares too high when A/C compressor turns off, check for an open circuit No. 59 to ECM. If circuit is okay, it is a faulty ECM connector terminal "B8" or ECM.
A/C On Signal Diagnosis (4.3L, 5.0L, 5.7L, 7.4L Engines) - Flow Chart & Schematic. Scheme 106
The ECM sends voltage pulses to proper IAC motor winding that causes the motor shaft and valve to move "in" and "out" a given distance for each pulse (called counts) received. This movement controls airflow around the throttle plate, that in turn controls engine idle speed.
Note. Test numbers refer to test numbers on diagnostic chart.
- Continue with test even if engine will not idle. If idle is too low, "SCAN" will display 80 or more counts (steps). Occasionally an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If condition is unchanged, IAC is not at fault. There is a system problem. Proceed to step 3).
- When engine was stopped, IAC valve retracted (more air) to fixed park position to provide increased airflow during next engine start. "SCAN" will display 100 or more counts.
- IAC MUST be disconnected before performing this step. Test light will confirm ECM signals by a steady or flashing light on all circuits.
- There is a remote possibility that one of the circuits is shorted to voltage that would have been indicated by a steady light. Disconnect ECM and turn ignition on. Probe terminals to check for this condition.
A slow unstable idle may be caused by a system problem that cannot be overcome by the IAC. "SCAN" counts will be above 60 counts (steps) if too low, and zero counts (steps) if too high.
If idle is too high, stop engine. Turn ignition on. Ground diagnostic terminal. Wait a few seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is more than 750-850 RPM, locate and correct vacuum leak.
Idle speed may be too high or too low. Engine speed may vary up and down. Disconnecting IAC does not help. May set Code 44.
"SCAN" and/or voltmeter will read an oxygen sensor output of less than .30 volt (300 millivolts). Check for low regulated fuel pressure, or water contaminated fuel. A lean exhaust with an oxygen sensor output fixed above .80 volt (800 millivolts) indicates a contaminated sensor, usually silicon. This may set a Code 45.
Idle speed too low. "SCAN" counts usually above 80. System obviously rich, and may exhibit Black smoke exhaust.
"SCAN" and/or voltmeter will read an oxygen sensor signal fixed above .80 volt (800 millivolts). Check for high fuel pressure, and/or leaking or sticking injector.
Remove IAC and inspect bore for foreign material, or evidence of IAC valve dragging in bore.