For 1990-2009 cars
A/C Controls and Idle-Up Signals
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The powertrain control module (PCM) receives electrical load idle-up signals from the HVAC blower motor circuit and the exterior lighting (headlights) circuit. When the electrical load idle-up signal is present, the PCM increases the idle air control (IAC) valve opening. Increasing the idle air control valve opening enables the engine to maintain the idle speed necessary to compensate for the additional electrical load. The idle-up signal of the exterior lighting circuit passes through an electrical load diode. The electrical load diode is located below the glove box, behind the PCM.

### Diagnostic Aids

If the scan tool indicates that the IAC Duty Cycle is increasing, but the idle quality is unsatisfactory, inspect the IAC system for proper operation. Refer to Idle Air Control (IAC) System Diagnosis .

Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in one of the electrical load idle-up circuits. 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. This step checks whether the PCM is responding to the electrical load idle-up inputs by commanding the IAC valve to increase the IAC Duty Cycle percentage. The IAC Duty Cycle parameter on the scan tool should increase 2 to 3 percent when operating the accessory. If the scan tool does not indicate an increase in the IAC Duty Cycle when the accessory is turned ON, the PCM may not be receiving the idle-up signal.

3. This step checks for a fault in the circuit between the PCM and the electrical load diode. The electrical load diode is located below the glove box and behind the PCM.

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 | Important: If the headlights or the HVAC blower motor do not operate properly, diagnose those components first.

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 Engine Speed and the IAC Duty Cycle parameters on the scan tool while turning each of the following electrical accessories ON and OFF.

• | Headlights

• | HVAC blower motor

Did the scan tool indicate an increase in the IAC Duty Cycle when either of the accessories was turned ON?

--

Go to Diagnostic Aids

Go to Step 3

3 | Did the scan tool indicate an increase in the IAC Duty Cycle when the HVAC blower motor was turned ON? | -- | Go to Step 5 | Go to Step 4
4 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connectors.
3. Turn ON the ignition, leaving the engine OFF.
4. Probe the HVAC Blower Idle-Up Signal circuit, terminal C1-16 (harness side), with a test lamp connected to B+.
5. Observe the test lamp while operating the HVAC blower motor switch.

Did the test lamp illuminate when the HVAC blower motor switch was turned ON?

--

Go to Step 11

Go to Step 8

5 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connectors.
3. Turn ON the ignition, leaving the engine OFF.
4. Probe the Electrical Load Idle-Up Signal circuit, terminal C1-15 (harness side), with a test lamp connected to ground.
5. Observe the test lamp while operating the headlight switch.

Did the test lamp illuminate when the headlight switch was turned ON?

--

Go to Step 11

Go to Step 6

6 |

1. Check for an open or a short in the Electrical Load Idle-Up Signal circuit between the PCM and the electrical load diode.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 12

Go to Step 7

7 |

1. Probe the headlight circuit at the electrical load diode, with a test lamp connected to ground.
2. Observe the test lamp while operating the headlight switch.

Did the test lamp illuminate when the headlight switch was turned ON?

--

Go to Step 10

Go to Step 9

8 | Repair the fault in the HVAC blower circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 12 | --
9 | Repair the fault in the exterior lighting circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 12 | --
10 | Replace the electrical load diode. Is the action complete? | -- | Go to Step 12 | --
11 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 12 | --
12 | Operate the vehicle within the conditions under which the original symptom was noticed. Does the system now operate properly? | -- | System OK | Go to Diagnostic Aids