GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Geo 🢒 1996 🢒 Prizm 🢒 Engine 🢒 Engine Controls - 1.8L 🢒 Air Conditioning (A/C) Idle Circuit Diagnosis

Schematic Reference: Refer to Engine Controls Schematics

Circuit Description

The A/C compressor control module sends an A/C On signal to the powertrain control module (PCM) when the A/C is turned ON (requested). The PCM sends a signal to the idle air control (IAC) valve in order to increase the flow through the idle (bypass) air passage. The engine idle speed increases as more air flows through the idle air passage. The increase in the idle speed prevents a rough idle or stalling condition.

Diagnostic Aids

The scan tool will display A/C Switch On when the air conditioning control is turned ON (blower switch is also ON), and A/C Command On when the PCM is allowing A/C compressor engagement.

Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the A/C On electrical circuit. Inspect the wiring harness and components for any of the following conditions:

    • Backed out terminals.
    • Improper mating of terminals.
    • Broken electrical connector locks.
    • Improperly formed or damaged terminals.
    • Faulty terminal to wire connections.
    • Physical damage to the wiring harness.
    • A broken wire inside the insulation.
    • Corrosion of electrical connections, splices, or terminals.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

  1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

    2.

  2. This step checks for an increase in the IAC Duty Cycle with the A/C ON. The engine speed should increase 100-200 RPM when the A/C is ON.

    3.

  3. The A/C On Signal circuit at the PCM should indicate 0.0 volts with the A/C ON (operating).

    4.

  4. The A/C On Signal circuit at the PCM should indicate B+ with the A/C OFF (not operating).

    5.

  5. The A/C Cutout Control circuit at the PCM should indicate 0.0 volts with the A/C OFF (not operating).

    6.

  6. The A/C Cutout Control circuit at the PCM should indicate B+ with the A/C ON (operating).

    7.

  7. This step checks for a short to ground in the Diagnostic Request circuit. A short to ground in the Diagnostic Request circuit can cause the PCM to continually ground the A/C Cutout Control circuit at engine speeds below 1,200 RPM. A short in the Diagnostic Request circuit will prevent the A/C compressor from operating. A short in the Diagnostic Request circuit will also cause the IAC duty cycle and the ignition timing to remain fixed at engine speeds below 1,200 RPM.

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install a scan tool.
  2. Start the engine.
  3. Run the engine until normal operating temperature is reached.
  4. Turn OFF all accessories.
  5. Monitor the IAC Duty Cycle parameter on the scan tool.
  6. Turn the A/C switch ON and the blower speed selector switch to any position except OFF.

Did the IAC Duty Cycle reading increase within the specified value when the A/C was turned ON?

4-7%

A/C Idle Circuit is functioning normally

Go to Step 3

3

  1. Start the engine.
  2. Turn ON the A/C (A/C operating).
  3. Backprobe the A/C On Signal circuit at the PCM connector with a DMM to ground.
  4. Measure the voltage.

Is the voltage at the specified value?

0.0-1.0 volts

Go to Step 4

Go to Step 6

4

  1. Start the engine.
  2. Turn OFF the A/C.
  3. Backprobe the A/C On Signal circuit at the PCM connector with a DMM to ground.
  4. Measure the voltage.

Is the voltage at the specified value?

B+

Go to Step 7

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the PCM electrical connector C3.
  3. Back out the A/C On Signal wire, terminal C3-10, from the PCM electrical connector.
  4. Reconnect the PCM electrical connector C3.
  5. Turn ON the ignition, leaving the engine OFF.
  6. Backprobe through the PCM electrical connector slot C3-10 to the PCM terminal with a DMM connected to ground.

Is the voltage at the specified value?

B+

Go to Step 6

Go to Step 13

6
  1. Check for an open or a short in the A/C On Signal circuit between the PCM and the A/C compressor control module.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 14

Go to Step 11

7

  1. Start the engine.
  2. Turn OFF the A/C.
  3. Backprobe the A/C Cutout Control circuit at the PCM connector with the DMM to ground.
  4. Measure the voltage.

Is the voltage at the specified value?

0 volts

Go to Step 8

Go to Step 13

8

  1. Start the engine.
  2. Turn ON the A/C (A/C operating).
  3. Backprobe the A/C Cutout Control circuit, terminal C3-21, at the PCM connector with the DMM connected to ground.
  4. Measure the voltage.

Is the voltage at the specified value?

B+

Go to Diagnostic Aids

Go to Step 9

9
  1. Check for an open or a short to ground in the A/C Cutout Control circuit between the PCM and the A/C compressor control module.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 14

Go to Step 10

10
  1. Disconnect the A/C compressor control module electrical connector.
  2. Turn ON the ignition, leaving the engine OFF.
  3. Measure the voltage of the A/C Cutout Control circuit at terminal 3 of the A/C compressor control module with a DMM.

Is the voltage at the specified value?

B+

Go to Step 12

Go to Step 11

11
  1. The A/C compressor control module is malfunctioning. Inspect the A/C compressor control module and circuitry for a fault.
  2. Repair as necessary. Refer to HVAC System Check in HVAC.

Is the action complete?

--

Go to Step 14

--

12

  1. Check for a short to ground in the Diagnostic Request circuit from PCM terminal C2-15 to the underhood diagnostic link connector.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 14

Go to Step 13

13

Replace the PCM. Refer to Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 14

--

14

Operate the vehicle within the conditions under which the original symptom was specified.

Does the system now operate properly?

--

System OK

Go to Diagnostic Aids