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For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Powertrain Control Module Controlled Air Conditioning (A/C) Circuit Diagnosis

Object Number: 370012 Size: LF
Engine Controls Components
A/C HVAC Control
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

When an A/C mode is selected, the HVAC control head supplies a battery positive voltage 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 positive 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 5440 RPM.
    • ECT is greater than 125°C (257°F).
    • 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 or the HVAC control head. 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 between the A/C relay control circuit at the PCM harness connector and ground 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 5440 RPM.
    • ECT greater than 125°C (257°F).
    • IAT less than 5°C (41°F).

Test Description

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

  1. 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.

    2.

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

    3.

  3. 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.

    4.

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

    5.

  5. Checks the ignition positive voltage circuit to the A/C relay.

    6.

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

    7.

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

    8.

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

    9.

  9. Determines if the A/C relay is malfunctioning or the A/C relay control circuit is grounded all the time.

    10.

  10. 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.

A/C Compressor Clutch Control Circuit Diagnosis

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to the A Powertrain On Board Diagnostic (OBD) System Check

2

Are any powertrain DTCs stored?

--

Diagnose DTC(s) first. Go to Powertrain Control Module Diagnosis

Go to Step 3

3

Important:: Before continuing with diagnosis, the following conditions must be met:

   • Intake air temperature greater than 9° C (48° F).
   • Engine coolant temperature less than 119° C (246° F).

  1. With the A/C OFF, start and idle the engine.
  2. Observe the A/C compressor.

Is the A/C compressor clutch engaged?

--

Go to Step 21

Go to Step 4

4

  1. Run the engine at 1000-1500 RPM.
  2. Select an A/C mode on HVAC control head.
  3. Observe the A/C compressor.

Is the A/C compressor clutch engaged?

--

Refer to Diagnostic Aids

Go to Step 5

5

With an A/C mode still selected, observe A/C Request on the scan tool.

Does A/C Request display Yes?

--

Go to Step 9

Go to Step 6

6

  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Turn ON the ignition.
  4. With an A/C mode still selected, measure voltage on the A/C request circuit at the PCM harness connector using a J 39200 Digital Multimeter that is connected to a good ground.

Is the voltage near the specified value?

B+

Go to Step 8

Go to Step 7

7
  1. Check the A/C request circuit for the following conditions:
    2. • Open.
    • Shorted to ground.
    • Poor connection at the A/C control head or A/C programmer (as applicable).
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 37

Go to Diagnosis in Manual Control Air Conditioning

8
  1. Check the A/C request circuit for a poor connection at the PCM.
  2. If a problem is found, repair as necessary. refer to Testing for Intermittent and Poor Connections .

Did you find and correct the condition?

--

Go to Step 37

Go to Step 35

9

With a scan tool, observe the A/C Refrigerant Pressure.

Is the pressure within the specified values?

0.35-4.27 V

Go to Step 12

Go to Step 10

10
  1. Connect a J 39500 Refrigerant Recovery, Recycling, and Recharging Station to monitor high side refrigerant pressure. Refer to Heater, Ventilation and Air Conditioning.
  2. Observe high side refrigerant pressure on the gauge.

Is pressure within the specified values?

234-2854 kPa

(34-414 psi)

Go To Step 11

Go to Refrigerant Recovery and Recycling, Adding Oil, Evacuating and Recharging Procedures in Heater, Ventilation and Air Conditioning

11
  1. Turn OFF the ignition.
  2. Disconnect the PCM and the A/C refrigerant pressure sensor.
  3. Measure the resistance of the following circuits between the PCM and the A/C refrigerant pressure sensor:
    4. • The A/C refrigerant pressure sensor signal circuit.
    • The 5 volt reference B circuit.
    • The sensor ground circuit.
  4. If any resistance is higher than the specified value, locate and repair cause of high resistance in circuit as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

5 ohms

Go to Step 37

Go to Step 18

12

  1. Disconnect the A/C relay.
  2. Probe both ignition positive voltage circuits at the A/C relay harness connector using a J 35616-200 Test Lamp that is connected to a good ground.

Does the test lamp illuminate when probing both circuits?

--

Go to Step 13

Go To Step 31

13

Connect a fused jumper between the ignition positive voltage circuit and A/C compressor clutch feed circuit at the A/C relay connector.

Does the A/C compressor clutch engage?

--

Go to Step 14

Go to Step 15

14
  1. Check for poor connections at A/C relay.
  2. If a problem is found, repair as necessary. Refer to Testing for Intermittent and Poor Connections .

Did you find and correct the condition?

--

Go to Step 37

Go to Step 32

15
  1. Disconnect the A/C compressor clutch (leave the ignition positive voltage circuit and A/C compressor clutch feed circuit jumpered at the A/C relay connector).
  2. Connect a test lamp between A/C compressor clutch feed circuit and the compressor ground circuit.

Does the test lamp illuminate?

--

Go to Step 16

Go to Step 17

16
  1. Check for poor connections at the A/C compressor clutch.
  2. If a problem is found, repair as necessary. Refer to Testing for Intermittent and Poor Connections .

Did you find and correct the condition?

--

Go to Step 37

Go to Step 33

17
  1. Check for an open or short to ground in the A/C compressor clutch feed circuit .
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 37

Go to Step 34

18
  1. Leave the PCM and the A/C refrigerant pressure sensor disconnected.
  2. Turn ON the ignition.
  3. Measure the voltage on the A/C refrigerant pressure signal circuit using a DMM that is connected to a good ground.

Is the voltage near the specified value?

0 V

Go to Step 19

Go to Step 20

19
  1. Check for the following conditions:
    2. • The 5 volt reference B circuit for a poor connection at the PCM.
    • The 5 volt reference B circuit for a poor connection at the A/C refrigerant pressure sensor.
    • The A/C refrigerant pressure signal circuit for a poor connection at the PCM.
    • The A/C refrigerant pressure signal circuit for a poor connection at the A/C refrigerant pressure sensor.
    • The sensor ground circuit for a poor connection at the A/C refrigerant pressure sensor.
  2. If a problem is found, repair as necessary. Refer to Testing for Intermittent and Poor Connections .

Did you find and correct the condition?

--

Go to Step 37

Go to Step 36

20

Locate and repair short to voltage in the A/C refrigerant pressure signal circuit. Refer to Wiring Repairs .

Is the action complete?

--

Go to Step 37

--

21

  1. Turn ON the ignition.
  2. A/C OFF
  3. With a scan tool, observe A/C Request.

Does A/C Request display Yes?

--

Go To Step 22

Go To Step 24

22

  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Turn ON the ignition.
  4. A/C OFF.
  5. Measure the voltage between the A/C request circuit at the PCM harness connector using a DMM that is connected to a good ground.

Is the voltage near the specified value?

B+

Go To Step 23

Go to Step 35

23
  1. Turn OFF the ignition.
  2. Disconnect the HVAC control head.
  3. Turn ON the ignition.
  4. Check the A/C request circuit for a short to B+.
  5. If a problem is found, repair as necessary. Refer to Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 37

Go to Diagnosis in Manual Control Air Conditioning

24
  1. Disconnect the A/C relay.
  2. Start the vehicle.
  3. Observe the A/C compressor.

Is the A/C compressor clutch engaged?

--

Go to Step 29

Go to Step 25

25

Probe the A/C compressor clutch control circuit at the A/C relay harness connector using a test lamp that is connected to a good ground.

Does the test lamp illuminate?

--

Go to Step 27

Go to Step 26

26

Replace the A/C relay.

Is the action complete?

--

Go to Step 37

--

27

  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Probe the A/C relay control circuit at the PCM connector with a test lamp that is connected to B+.

Does the test lamp illuminate?

--

Go to Step 28

Go to Step 35

28

Repair short to ground in the A/C compressor clutch relay control circuit. Refer to Wiring Repairs .

Is the action complete?

--

Go to Step 37

--

29
  1. Turn OFF the ignition.
  2. Disconnect the A/C compressor clutch.
  3. Start the engine.
  4. Observe the A/C compressor.

Is the A/C compressor clutch engaged?

--

Go To Step 33

Go to Step 30

30

Locate and repair short to voltage in the A/C compressor clutch control circuit. Refer to Wiring Repairs .

Is the action complete?

--

Go to Step 37

--

31

Repair open or short to ground in the ignition positive voltage circuit to the A/C compressor relay. Refer to Wiring Repairs .

Is the action complete?

--

Go to Step 37

--

32

Replace the A/C compressor relay.

Is the action complete?

--

Go to Step 37

--

33

Repair the A/C compressor clutch. Refer to Compressor Clutch Plate/Hub Assembly Install (V5 - Conventional Mount) .

Is the action complete?

--

Go to Step 37

--

34

Locate and repair open in the A/C compressor clutch ground circuit . Refer to Wiring Repairs .

Is the action complete?

--

Go to Step 37

--

35

Important:: The replacement PCM must be programmed. Refer to PCM Replacement/Programming .

Replace the PCM.

Is action complete?

--

Go to Step 37

--

36

Replace the A/C refrigerant pressure sensor.

Is the action complete?

--

Go to Step 37

--

37

Important:: Before continuing with diagnosis, the following conditions must be met:

   • Intake air temperature greater than 9° C (48° F).
   • Engine coolant temperature less than 119° C (246° F).

  1. With the A/C OFF, start and idle the engine.
  2. Observe the A/C compressor.

Is the A/C compressor clutch engaged?

--

Go to Step 21

Go to Step 38

38
  1. Run the engine at 1000-1500 rpm.
  2. Select an A/C mode on the HVAC control head.
  3. Observe A/C compressor.

Is the A/C compressor clutch engaged?

--

System OK

Go to Step 5