unplug the IAT sensor. what should happen?

[commodoredave]

vt II v6 no turbo or scharge.
Engine is running ( warming up but not at full operating temp) unplug IAT sensor.
What is suppose to happen? To compare...if maf sensor is unplugged engine stalls/stops. should same happen when IAT sensor is unlugged?

 

[Ginger Beer]

No

 

[Immortality]

No. The PCM will default to predetermined table/numbers and continue to run as well as set a check engine light. It probably won't run quite right but it should continue to run.

Basically the Holden/GM engineers built a lot or redundancy into the tune so that if some sensors fail you can still continue to drive the vehicle safely and not leave you stranded.

 

[BlackVXGTS]

From the Holden VT/VX Service Manual for your future information:

The purpose of the Idle Air Control (IAC) valve, is to control engine idle speed, and prevent stalls due to changes in engine load at idle.
The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. By extending the pintle (to decrease air flow) or
retracting the pintle (to increase air flow), a controlled amount of air can move around the throttle valve. If RPM is too low, more air is
bypassed around the throttle valve to increase RPM.
If RPM is too high, less air is bypassed around the throttle valve to decrease RPM.
The IAC Valve moves in small steps numbered from 0 (extended pintle, bypass air passage fully shut) to 255 (retracted pintle, maximum bypass
airflow) as commanded by the PCM.
At idle, the desired position of the IAC valve is calculated by the PCM based on coolant temperature, actual engine RPM, engine load, and
battery voltage.
If the IAC Valve is disconnected or reconnected with the engine running, the PCM can "lose track" of the actual position of the IAC. This also happens when PCM's keep alive memory voltage, i.e., PCM connectors, ENGINE fuse F31, or battery cables, are disconnected. If this happens, the PCM will "reset" the IAC. After the engine has been run for at least 5 seconds, then upon ignition "OFF" the IAC will be reset.
The "reset" procedure is as follows:
The PCM commands the IAC to shut the idle air passageway in the throttle body. It does so by issuing enough "extend" pulses to move the IAC
pintle fully shut in the bore, regardless of where the actual position was. Then, the PCM calculates the IAC is at a fully shut position, and calls that position "0 steps."
Next, the PCM issues "retract" steps to properly position the pintle.
The IAC can also be reset with the engine running by a special command on the Tech 2 scan tool.
The IAC valve affects only the idle RPM of the engine. If it is open fully, too much air will be allowed into the manifold and idle speed will be too
high. If it is stuck closed, too little air will be allowed into the intake manifold, and idle speed will be too low.

 

[Immortality]

From the Holden VT/VX Service Manual for your future information:

The purpose of the Idle Air Control (IAC) valve, is to control engine idle speed, and prevent stalls due to changes in engine load at idle.
The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. By extending the pintle (to decrease air flow) or
retracting the pintle (to increase air flow), a controlled amount of air can move around the throttle valve. If RPM is too low, more air is
bypassed around the throttle valve to increase RPM.
If RPM is too high, less air is bypassed around the throttle valve to decrease RPM.
The IAC Valve moves in small steps numbered from 0 (extended pintle, bypass air passage fully shut) to 255 (retracted pintle, maximum bypass
airflow) as commanded by the PCM.
At idle, the desired position of the IAC valve is calculated by the PCM based on coolant temperature, actual engine RPM, engine load, and
battery voltage.
If the IAC Valve is disconnected or reconnected with the engine running, the PCM can "lose track" of the actual position of the IAC. This also happens when PCM's keep alive memory voltage, i.e., PCM connectors, ENGINE fuse F31, or battery cables, are disconnected. If this happens, the PCM will "reset" the IAC. After the engine has been run for at least 5 seconds, then upon ignition "OFF" the IAC will be reset.
The "reset" procedure is as follows:
The PCM commands the IAC to shut the idle air passageway in the throttle body. It does so by issuing enough "extend" pulses to move the IAC
pintle fully shut in the bore, regardless of where the actual position was. Then, the PCM calculates the IAC is at a fully shut position, and calls that position "0 steps."
Next, the PCM issues "retract" steps to properly position the pintle.
The IAC can also be reset with the engine running by a special command on the Tech 2 scan tool.
The IAC valve affects only the idle RPM of the engine. If it is open fully, too much air will be allowed into the manifold and idle speed will be too
high. If it is stuck closed, too little air will be allowed into the intake manifold, and idle speed will be too low.

IAT not IAC

 

[Lex]

From the Holden VT/VX Service Manual for your future information:

The purpose of the Idle Air Control (IAC) valve, is to control engine idle speed, and prevent stalls due to changes in engine load at idle.
The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. By extending the pintle (to decrease air flow) or
retracting the pintle (to increase air flow), a controlled amount of air can move around the throttle valve. If RPM is too low, more air is
bypassed around the throttle valve to increase RPM.
If RPM is too high, less air is bypassed around the throttle valve to decrease RPM.
The IAC Valve moves in small steps numbered from 0 (extended pintle, bypass air passage fully shut) to 255 (retracted pintle, maximum bypass
airflow) as commanded by the PCM.
At idle, the desired position of the IAC valve is calculated by the PCM based on coolant temperature, actual engine RPM, engine load, and
battery voltage.
If the IAC Valve is disconnected or reconnected with the engine running, the PCM can "lose track" of the actual position of the IAC. This also happens when PCM's keep alive memory voltage, i.e., PCM connectors, ENGINE fuse F31, or battery cables, are disconnected. If this happens, the PCM will "reset" the IAC. After the engine has been run for at least 5 seconds, then upon ignition "OFF" the IAC will be reset.
The "reset" procedure is as follows:
The PCM commands the IAC to shut the idle air passageway in the throttle body. It does so by issuing enough "extend" pulses to move the IAC
pintle fully shut in the bore, regardless of where the actual position was. Then, the PCM calculates the IAC is at a fully shut position, and calls that position "0 steps."
Next, the PCM issues "retract" steps to properly position the pintle.
The IAC can also be reset with the engine running by a special command on the Tech 2 scan tool.
The IAC valve affects only the idle RPM of the engine. If it is open fully, too much air will be allowed into the manifold and idle speed will be too
high. If it is stuck closed, too little air will be allowed into the intake manifold, and idle speed will be too low.

He is talking about the iat (intake air temperature) that lives in top of the airfilter box.

 

[Lex]

IAT not IAC

Beat me to it.

 

[BlackVXGTS]

Oops, my mistake:

INTAKE AIR TEMPERATURE (IAT) SENSOR
The Intake Air Temperature (IAT) sensor is a thermistor (a resistor that changes resistance with changes in temperature) mounted in an air cleaner
housing of the intake system. Low intake air temperature produces high resistance in the sensor (100,866 ohms at -40 degrees C), while high intake
air temperature causes low sensor resistance (78 ohms at 130 degrees C).
The PCM:
1. Supplies a 5 volt signal voltage to the sensor through a resistor in the PCM, and
2. Monitors the intake air temperature circuit voltage, which will change when connected to the intake air temperature sensor.
The circuit voltage will vary depending on the resistance of the IAT sensor. The voltage will be close to the 5 volt level when the sensor is cold,
and will decrease as the sensor warms. The input intake air temperature signal voltage is used by the PCM to assist in calculating the fuel injector pulse width.
A failure in the IAT sensor circuit should set either a Diagnostic Trouble Code (DTC) 23 or DTC 25. An intermittent failure in the IAT sensor circuit should set DTC 26.

 

[commodoredave]

IAT not IAC

The IAT sesnsor can be seen just below the upper right hand corner of the photo.

 

[Immortality]

Nope, that is the IAC, idle air control stepper motor, as per the post above, it controls the engine idle speed.