Circuit Description
The powertrain control module (PCM) controls the air entering into the engine with an idle control (IAC) valve. In order to increase the idle RPM, the PCM commands the pintle inside the IAC valve away from the throttle body seat. This allows more air to bypass through the throttle blade. In order to decrease the RPM the PCM commands the pintle towards the throttle body seat. This reduces the amount of air bypassing the throttle blade. A scan tool will read the IAC valve pintle position in counts. The higher the counts, the more air that is allowed to bypass the throttle blade.
Conditions for Setting the DTC
| • | The IAC valve reads less than 2 counts on the scan tool. |
| • | The barometer is greater than 72 kPa. |
| • | The ECT sensor indicates temperature greater than 40°C (104°F). |
| • | The idle speed deviates below desired rpm by greater than 60 RPM for greater than 5 seconds. |
| • | The battery voltage is between 10 and 17 volts |
| • | The TP angle is less than 1%. |
| • | One or more of the following DTCs are not set: P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0335, P0341, P0342, P0502, P0503, P0562, P0563, P0600, P1133, P1520. |
Action Taken When the DTC Sets
| • | The Malfunction Indicator Lamp (MIL) illuminates after 2 consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The Freeze Frame and Failure Records buffers stores this information. |
| • | A history DTC is stored. |
Conditions for Clearing the MIL/DTC
| • | The malfunction indicator lamp (MIL) will turn OFF after 3 consecutive ignition cycles in which the diagnostic runs without a fault. |
| • | A history DTC will clear after 40 consecutive warm up cycles without a fault. |
| • | Use a scan tool to clear the DTCs. |
Diagnostic Aids
| • | Inspect the IAC valve electrical connection for proper mating. |
| • | Inspect the wiring harness for damage. |
| • | Inspect for a disconnected, cracked or cut vacuum hoses. |
| • | Inspect for the proper installation of the crankcase ventilation valve. |
| • | Inspect the intake manifold and throttle body for leaks. |
| • | Inspect the intake manifold for cracks. |
| • | Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
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The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
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A normally operating IAC system will be able to be extended and retracted by a scan tool and change the engine idle RPM. Valve movement is verified by an engine RPM change.
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If the scan tool was able to command the IAC valve smoothly, a malfunction may still exist internally within the IAC valve. This can be checked by checking the IAC valves internal resistance.
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The IAC circuits always have ground or voltage signals on them in pairs. If the test light illuminates on more or less than 2 terminals, 1 of the circuits is shorted to voltage or open.
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The IAC circuits always have ground or voltage signals on them in pairs. If the test light illuminates on more or less than 2 terminals, 1 of the circuits is shorted to ground or open.
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The IAC circuits are constantly switched between ground and voltage so the test light should blink on all circuits when connected to ground.
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Any circuitry, that is suspected as causing the intermittent complaint, should be thoroughly checked for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wiring connections or physical damage to the wiring harness.
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A test light that remains ON constantly, indicates that the circuit is shorted to voltage.
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Replacement PCMs must be reprogrammed. Refer to the latest Isuzu Technical Communication System (ITCS) information for programming procedures.
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If no malfunctions have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to | ||
Does the engine RPM change smoothly when commanded by the scan tool? | 900-2000 RPM | |||
Are the resistance's across terminals A and B and C and D within the specified valve. | 40-80 ohms | |||
4 |
Are the resistance's across terminals B and C and terminals A and D infinite? | -- | ||
Does the test light illuminate on 2 terminals? | -- | |||
Connect a test light to B+, probe the IAC valve electrical connector terminals. Does the test light illuminate on 2 terminals? | -- | |||
7 | Check for an open or a short to ground in the IAC valve hi and lo circuits and repair as necessary. Was a repair necessary? | -- | ||
Does the test light flash ON and OFF for all terminals? | -- | |||
9 | Check for an open or a short to voltage in the IAC valve hi and lo circuits and repair as necessary. Was a repair necessary? | -- | ||
Check the PCM electrical connector for poor connections and repair as necessary. Was a repair necessary? | -- | |||
11 | Inspect the IAC passages. Was a problem found? | -- | ||
Did the test light remain ON constantly for the terminals that did not blink? | -- | |||
13 | Check for poor connection. If OK, replace the IAC valve. Refer to Idle Air Control Valve Replacement . Is the action complete? | -- | -- | |
Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||
15 |
Does the scan tool indicate that this diagnostic has ran and passed? | -- | ||
Check if any additional DTCs are set. Are any DTCs displayed that have not been diagnosed? | Go to applicable DTC table | System OK Refer to Diagnostic Aids |
