Opel Frontera UE. Manual - part 359

6E2–242

6VD1 3.2L ENGINE DRIVEABILITY AND EMISSIONS

Diagnostic Trouble Code (DTC) P1508 IAC System Low RPM

T321115

Circuit Description

The powertrain control module (PCM) controls engine
idle speed by adjusting the position of the idle air control
(IAC) motor pintle. The IAC is a bi-directional stepper
motor driven by two coils. The PCM applies current to the
IAC coils in steps (counts) to extend the IAC pintle into a
passage in the throttle body to decrease air flow. The
PCM reverses the current to retract the pintle, increasing
air flow. This method allows highly accurate control of idle
speed and quick response to changes in engine load. If
the PCM detects a condition where too low of an idle
speed is present and the PCM is unable to adjust idle
speed by increasing the IAC counts, DTC P1508 will set,
indicating a problem with the idle control system.

Conditions for Setting the DTC

No Tech 2 test is being run.

None of these DTCs are set: TP sensor, VSS, ECT,
EGR, fuel system, MAF, MAP, IAT, canister purge,
injector control or ignition control.

Barometric pressure is above 75 kPa.

Engine coolant temperature (ECT) is above 50

(120

F).

Vehicle speed is less than 1 mph.

The engine has been running for at least 125 seconds.

Canister purge duty cycle is above 10%.

Ignition voltage is between 9.5 volts and 16.7 volts.

The throttle is closed.

Engine speed is lower than desired idle.

Engine speed is more than 100-200 RPM lower than
desired idle, based upon coolant temperature.

All of the above conditions are met for 5 seconds.

Action Taken When the DTC Sets

The PCM will illuminate the malfunction indicator lamp
(MIL) after the second consecutive trip in which the
fault is detected.

The PCM will store conditions which were present
when the DTC was set as Freeze Frame and in the
Failure Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL “OFF” on the third
consecutive trip cycle during which the diagnostic has
been run and the fault condition is no longer present.

A history DTC P1508 will clear after 40 consecutive
warm-up cycles have occurred without a fault.

DTC P1508 can be cleared by using the Tech 2 “Clear
Info” function or by disconnecting the PCM battery
feed.

Diagnostic Aids

Check for the following conditions:

Poor connection at PCM or IAC motor – Inspect
harness connectors for backed-out terminals,
improper mating, broken locks, improperly formed or
damaged terminals, and poor terminal-to-wire
connection.

Damaged harness – Inspect the wiring for damage.

6E2–243

6VD1 3.2L ENGINE DRIVEABILITY AND EMISSIONS

Restricted air intake system – Check for a possible
collapsed air intake duct, restricted air filter element,
or foreign objects blocking the air intake system.

Throttle body – Check for objects blocking the IAC
passage or throttle bore, excessive deposits in the IAC
passage and on the IAC pintle, and excessive deposits
in the throttle bore and on the throttle plate.

Large vacuum leak – Check for a condition that causes
a large vacuum leak, such as an incorrectly installed or

faulty PCV valve or a disconnected brake booster
hose.

Reviewing the Failure Records vehicle mileage since the
diagnostic test last failed may help determine how often
the condition that caused the DTC to be set occurs. This
may assist in diagnosing the condition.

DTC P1508 –IAC System Low RPM

Step

Action

Value(s)

Yes

Was the “On-Board Diagnostic (OBD) System Check”
performed?

—

Go to

Step 2

Go to

OBD

System

Check

1. Start the engine.
2. Turn all accessories “OFF”(A/C, rear defroster,

etc).

3. Using a Tech 2, command RPM up to 1500, down to

500, and then up to 1500 while monitoring the
“Engine Speed” on the Tech 2.

NOTE: This Tech 2 command may cause the engine to
“cut out” when RPM goes above 1500. If this occurs,
the “cutting out” will stop when the Tech 2 command for
the test is discontinued, or if the Tech 2 command is
changed to less than 1500 RPM.

Does the “Engine Speed” remain within the specified
value of the “Desired Idle” for each RPM command?

50 RPM

No trouble

found. Go to

Diagnostic

Aids

Go to

Step 3

1. Disconnect the IAC.
2. Install IAC Noid Light 5–8840–2312–0 or

equivalent.

3. With the engine running, command RPM up to

1500, down to 500, and then up to 1500 while
observing the noid light.

Does each noid light cycle red and green (never
“OFF”)?

—

Go to

Step 5

Go to

Step 4

1. Check the following circuits for an open, short to

voltage, short ground, or poor connections at the
PCM:

IAC “A” Low.

IAC “A” High.

IAC “B” Low.

IAC “B” High.

2. If a problem is found, repair as necessary,

Was a problem found?

—

Verify repair

Go to

Step 8

6E2–244

6VD1 3.2L ENGINE DRIVEABILITY AND EMISSIONS

DTC P1508 –IAC System Low RPM

(Cont'd)

Step

Yes

Value(s)

Action

Visually/physically inspect for following conditions:

Restricted air intake system. Check for a possible
collapsed air intake duct, restricted air filter
element, or foreign objects blocking the air intake
system.

Throttle body. Check for objects blocking the IAC
passage or throttle bore, excessive deposits in
the IAC passage and on the IAC pintle, and
excessive deposits in the throttle bore and on the
throttle plate.

Do any of the above require a repair?

—

Refer to

appropriate

section for

on-vehicle

service

Go to

Step 6

1. Check for a poor connection at the IAC harness

connector.

2. If a problem is found, replace faulty terminals as

necessary.

Was a problem found?

—

Verify repair

Go to

Step 7

Replace the IAC valve.

Is the action complete?

—

Verify repair

—

Replace the PCM.

Is the action complete?

—

Verify repair

—

6E2–245

6VD1 3.2L ENGINE DRIVEABILITY AND EMISSIONS

Diagnostic Trouble Code (DTC) P1509 IAC System High RPM

T321115

Circuit Description

The powertrain control module (PCM) controls engine
idle speed by adjusting the position of the idle air control
(IAC) motor pintle. The IAC is a bi-directional stepper
motor driven by two coils. The PCM applies current to the
IAC coils in steps (counts) to extend the IAC pintle into a
passage in the throttle body to decrease air flow. The
PCM reverses the current to retract the pintle, increasing
air flow. This method allows highly accurate control of idle
speed and quick response to changes in engine load. If
the PCM detect a condition where too high of an idle
speed is present and the PCM is unable to adjust idle
speed by increasing the IAC counts, DTC P1509 will set,
indicating a problem with the idle control system.

Conditions for Setting the DTC

No Tech 2 test is being run.

None of these DTCs are set: TP sensor, VSS, ECT,
EGR, fuel system, MAF, MAP, IAT, canister purge,
injector control or ignition control.

Barometric pressure is above 75 kPa.

Engine coolant temperature is above 50

C (120

F).

Engine speed is more than 100-200 RPM higher than
desired idle, based upon coolant temperature.

The engine has been running for at least 125 seconds.

Vehicle speed is less than 1 mph.

Canister purge duty cycle is above 10%.

Ignition voltage is between 9.5 volts and 16.7 volts.

Engine speed is higher than desired idle.

All of the above conditions are met for 5 seconds.

Action Taken When the DTC Sets

The PCM will illuminate the malfunction indicator lamp
(MIL) after the second consecutive trip in which the
fault is detected.

The PCM will store conditions which were present
when the DTC was set as Freeze Frame and in the
Failure Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL “OFF” on the third
consecutive trip cycle during which the diagnostic has
been run and the fault condition is no longer present.

A history DTC P1509 will clear after 40 consecutive
warm-up cycles have occurred without a fault.

DTC P1509 can be cleared by using the Tech 2 “Clear
Info” function or by disconnecting the PCM battery
feed.

Diagnostic Aids

Check for the following conditions:

Damaged harness – Inspect the wiring for damage.

Vacuum leak – Check for a condition that causes a
vacuum leak, such as disconnected or damaged
hoses, leaks at the EGR valve and the EGR pipe to the

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