Beretta L4-138 2.3L DOHC QUAD 4 (1991)
Chart C-2C Wiring Diagram
CIRCUIT DESCRIPTION:
The ECM controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve out, allowing more air to bypass the throttle plate.
To decrease rpm, it moves the IAC valve in, reducing air flow bypassing the throttle plate. A "Scan" tool will read the ECM commands to the IAC valve
in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).
TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.
1.
The 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. This step checks the quality of the IAC movement. Between 700 rpm and
about 1500 rpm, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is
retracted beyond the control range (about 1500 rpm), it may take many flashes in the extend position before the engine speed will begin to drop.
This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
2.
Step 1 verified proper IAC valve operation while this step checks IAC circuits. Each lamp on the node lights should flash red and green while the
IAC valve is cycled. 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 begining with poor terminal contacts.
DIAGNOSTIC AIDS:
^
A slow, unstable idle may be caused by a system problem that cannot be overcome by the IAC. Out of control range IAC "SCAN" tool counts will
be above 60 if idle is to low, and zero counts if idle is to high. The following checks should be made to repair a non-IAC system problem.
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Vacuum Leak (High Idle) - If idle is to high, stop the engine. Fully extend the IAC 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.
^
System lean (High Air Fuel Ratio) - 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 less than 300 mV (0.3 volt). Check for low regulated fuel pressure or
water in fuel. A lean exhaust, with an oxygen sensor output fixed above 800 mV (0.8 volt), will be a contaminated sensor, usually silicone. This
may also set Code 45.
^
System rich (Low Air/Fuel Ratio) - Idle speed too low. "Scan" counts usually above 80. System obviously rich and may exhibit black smoke
exhaust. "Scan" tool and/or voltmeter will read an oxygen sensor signal fixed above 800 mV (0.8 volt). Check for high fuel pressure or possibly an
injector leaking or sticking.
^
Throttle Body - Remove IAC and inspect bore for foreign material or evidence of IAC pintle dragging the bore.
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IAC Valve Electrical Connections - IAC Valve electrical connections should be checked for proper contact.
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PCV Valve - An incorrect or faulty PCV valve may result in an incorrect idle speed.
^
Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/STALLS-ROUGH UNSTABLE OR INCORRECT IDLE.
