PCM Controlled Air Conditioning Diagnosis

PCM Controlled Air Conditioning Diagnosis:

PCM Controlled Air Conditioning Diagnosis (continued):

Engine Controls (A/C Control:

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 motor driven by two coils. The PCM pulses 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 pulses to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load.

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 harness for damage.

^ 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.

Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which will not allow the IAC pintle to fully extend.

Vacuum leak. Check for a condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at Exhaust Gas Recirculation (EGR) valve and EGR pipe to intake manifold, leaks at throttle body, faulty or incorrectly installed PCV valve, leaks at intake manifold brake booster hose disconnected, etc.

Reviewing the Fail 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.

TEST DESCRIPTION

Number(s) below refer to the step number(s) on the Diagnostic Table.

11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.

CIRCUIT DESCRIPTION

When an A/C mode is selected, the A/C control head or the A/C programmer supplies a 12 volt signal to the PCM A/C request input. The A/C compressor clutch relay is controlled by the PCM. The PCM provides a ground path for the A/C relay coil. When the A/C compressor relay is engaged battery voltage is supplied to the compressor clutch coil. The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested unless one or more of the following conditions are present:

^ Throttle angle is greater than 90%.

^ A/C head pressure is greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage is below 10.5 volts.

^ Engine speed is greater than 4500 RPM for 5 seconds or 5400 RPM.

^ Engine Coolant Temperature (ECT) is greater than 125°C (257°F).

^ Intake Air Temperature (IAT) is less than 5°C (41°F).

Any of the conditions described above will inhibit A/C compressor relay operation.

DIAGNOSTIC AIDS

Check for the following conditions:

^ Poor connection at the PCM. 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 harness for damage.

^ If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected to the A/C relay control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the A/C relay. A change in voltage will indicate the location of the malfunction.

Check for conditions that will disable A/C operation:

^ Throttle angle greater than 90%.

^ A/C head pressure greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage below 10.5 volts.

^ Engine speed greater than 4500 RPM for 5 seconds or 5400 RPM.

^ Engine Coolant Temperature (ECT) greater than 125°C (257°F).

^ IAT less than 5°C (41°F).

TEST DESCRIPTION

Number(s) below refer to Step number(s) on the diagnostic table:

2. Stored diagnostic trouble codes may disable the A/C compressor. This diagnostic table may lead to improper diagnosis and replacement of good parts if diagnostic trouble codes are present.

3. The A/C compressor clutch should engage if the appropriate conditions have been met.

4. The A/C compressor clutch should not be engaged with the engine running if an A/C mode is not selected at the control head.

6. Checks for an A/C request signal from the control head to the PCM.

12. Checks the ignition feed circuit to the A/C relay.

13. Determines whether the A/C relay or the circuitry is at fault.

21. Determines if the PCM is turning the A/C relay ON due to a false A/C request.

22. Isolates the problem to the A/C control head or the PCM.

27. Determines if the A/C relay is faulty or the A/C relay control circuit is grounded all the time.

35. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.