GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1999 🢒 Catera 🢒 Engine 🢒 Engine Controls - 3.0L 🢒 Air Conditioning (A/C) Request Circuit Diagnosis

Refer to Electrical Schematic Diagrams for wiring view.

A/C Operation Description

The ECM controls the A/C clutch relay. ECM A/C operation is controlled by two input circuits, A/C request and A/C load, and one output circuit, the A/C compressor relay control.

The heater and A/C control sends an A/C request signal (approx. B+) whenever the operator depresses the A/C selector switch on the control panel. When the ECM receives this signal, the ECM will turn the A/C clutch relay ON unless one of the following conditions is observed:

    • The engine is not running.
    • Ignition voltage is less than 9 volts.
    • A wide open throttle (WOT) condition is present.

The ECM will NOT detect the A/C request signal if the A/C system pressure is too high or too low. The A/C request circuit is in series with a portion of the A/C compressor refrigerant pressure switch. The pressure activated switch contacts will open if the A/C pressure is not correct for proper A/C operation. Refer to the electrical schematic diagrams for the specific operating pressures. The heater and A/C control monitors the A/C request circuit for shorts. A shorted condition will cause an HVAC DTC to set.

The ECM also monitors the A/C load switch circuit. This circuit is designed to increase the engine speed by about 50 RPM whenever a high temperature/pressure condition causes the A/C load switch contacts to close.

The ECM controls clutch operation by supplying a ground path for the A/C compressor relay. The A/C compressor relay control circuit is monitored for opens and shorts. An open or shorted condition will cause an ECM DTC to set. ECM control of the A/C clutch relay can be tested using a scan tool.

Diagnostic Aids

Verify A/C system pressure is sufficient for A/C operation. Refer to HVAC diagnosis in Body and Accessories.

An intermittent may be caused by the following:

    • Corrosion
    • A mis-routed harness
    • A rubbed through wire insulation
    • A broken wire inside the insulation

Refer to Symptoms for Intermittents.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. The Heater and A/C Control will set a DTC if the A/C Request circuit is shorted to ground or to battery voltage.

    2.

  2. If all circuit and terminal inspections are OK the fault must be in the Heater and A/C Control.

A/C Request Circuit Diagnosis

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Inspect the Heater and A/C Control for DTCs using the scan tool.
  2. Inspect the ECM for DTCs using the scan tool.

Are any DTCs set?

--

Go to the applicable DTC table

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Disconnect the ECM.
  3. Connect the J 39200 DMM between the A/C Request Input circuit in the ECM harness connector and ground in order to measure for voltage.
  4. Turn the ignition ON.
  5. Turn the A/C ON at the Heater and A/C Control Panel.

Is the voltage near the specified value?

B+

Go to Step 4

Go to Step 6

4
  1. The A/C Request Input circuit is OK. Inspect the ECM connector for proper terminal contact. Repair the terminal if necessary.
  2. Turn the ignition OFF.
  3. Connect the ECM.
  4. Turn the ignition ON leaving the engine OFF.
  5. Command the A/C Clutch relay ON using the scan tool.

Does the A/C CLUTCH engage and disengage as the A/C Clutch relay is commanded ON and OFF?

--

Go to Step 5

Go to HVAC Compressor Clutch Circuit Diagnosis

5
  1. Clear ECM and HVAC DTC Information using the scan tool.
  2. Operate the A/C system with the engine running. Ensure proper A/C system pressure is present. Refer to HVAC.
  3. Observe the ECM A/C Clutch relay display on the scan tool while the engine is running and A/C is requested.

Does the ECM A/C Clutch relay display OFF while A/C system pressures remain OK and A/C is requested?

--

Go to Step 10

System OK

6
  1. Disconnect the A/C compressor refrigerant pressure switch.
  2. Connect the J 39200 DMM between the A/C Request Input circuit at the switch harness connector terminal 2 and ground in order to measure for voltage.
  3. Turn the ignition ON.
  4. Turn the A/C ON at the Heater and A/C Control Panel.

Is the voltage near the specified value?

B+

Go to Step 7

Go to Step 8

7

Connect the J 39200 DMM between the A/C Compressor Refrigerant pressure switch connector terminal 2 and terminal 1 in order to measure for continuity. The DMM should measure very low resistance.

Is continuity measured through the A/C Compressor Refrigerant Pressure switch?

--

Go to Step 9

Go to HVAC Compressor Controls System Check in HVAC

8

  1. Turn the ignition OFF.
  2. Disconnect the Heater and A/C Control connector that contains the A/C Request circuit.
  3. Using the J 39200 DMM check the A/C Request circuit for continuity between the Heater and A/C Control connector and the A/C Compressor Refrigerant Pressure switch. The DMM should indicate very low resistance.
  4. Inspect for proper terminal contact at the Heater and A/C Control connector.
  5. Repair the wiring as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found and repaired?

--

Go to Step 11

Go to HVAC Compressor Controls System Check in HVAC

9

Repair the open/high resistance in the A/C Request Input circuit between the A/C Compressor Refrigerant Pressure switch and the ECM or repair the poor terminal contact at the A/C Compressor Refrigerant Pressure switch. Refer to Wiring Repairs in Wiring Systems for the proper procedure.

Is the action complete?

--

Go to Step 11

--

10

Important: This vehicle is equipped with a Theft Deterrent System that interfaces with the Engine Control Module (ECM). Reprogram the new ECM with the frequency code of the theft deterrent module currently on the vehicle.

Replace the ECM. Refer to Engine Control Module Replacement/Programming .

Is the action complete?

--

Go to Step 11

--

11
  1. Turn the ignition ON leaving the engine OFF.
  2. Clear the DTC Information from the ECM and the Heater and A/C Control.
  3. Start the engine.
  4. Turn the A/C ON.

Does the A/C system function normally?

--

System OK

Go to HVAC Compressor Controls System Check in HVAC