For 1990-2009 cars

## Electrical/Accessory Load Idle-Up Signal Diagnosis Automatic Transmission

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, exterior lighting, and the rear window defogger circuits. When the electrical load idle-up signal is present, the PCM increases the idle air control (IAC) valve opening. Increasing the IAC valve opening enables the engine to maintain the idle speed necessary to compensate for the additional electrical load. The idle-up signal of the rear window defogger circuit (4-door vehicles only) 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 .

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:

• | Any backed-out terminals

• | Any improper mating of terminals

• | Any broken electrical connector locks

• | Any improperly formed or damaged terminals

• | Any faulty terminal-to-wire connections

• | Any physical damage to the wiring harness

• | A broken wire inside the insulation

• | Any corrosion of the electrical connections, the splices, or the 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 rear window defogger, 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 a normal operating temperature is reached.
4. Turn OFF all accessories.
5. Monitor the IAC Duty Cycle parameter on the scan tool while turning each of the following electrical accessories ON and OFF.

• | The headlights

• | The HVAC blower motor

• | The rear window defogger

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

--

Go to Diagnostic Aids

Go to Step 3

3 | Is the vehicle equipped with a rear window defogger? | -- | Go to Step 4 | Go to Step 5
4 | Does the scan tool indicate an increase in the IAC Duty Cycle when the rear window defogger is turned ON? | -- | Go to Step 5 | Go to Step 6
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 C1-32, on the harness side with a test lamp connected to ground.
5. Observe the test lamp while operating the headlight switch.
6. Probe the idle-up signal circuit of the HVAC blower , terminal C1-16, on the harness side with a test lamp connected to B+.
7. Observe the test lamp while operating the HVAC blower switch.

Does the test lamp illuminate when the accessories are turned ON?

--

Go to Step 12

Go to Step 9

6 |

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, on the harness side with a test lamp connected to ground.
5. Observe the test lamp while operating the rear window defogger switch.

Does the test lamp illuminate when the rear window defogger switch is turned ON?

--

Go to Step 12

Go to Step 7

7 |

1. Check for an open 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 13

Go to Step 8

8 |

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

Does the test lamp illuminate when the rear window defogger is turned ON?

--

Go to Step 11

Go to Step 10

9 | Repair the open or the short in the exterior lighting circuit or the HVAC blower circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 13 | --
10 | Repair the open or the short in the rear defogger circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 13 | --
11 | Replace the electrical load diode. Is the action complete? | -- | Go to Step 13 | --
12 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 13 | --
13 | Operate the vehicle within the conditions under which the original symptom was noticed. Does the system now operate correctly? | -- | System OK | Go to Diagnostic Aids

## Electrical/Accessory Load Idle-Up Signal Diagnosis Manual Transmission

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, exterior lighting (headlights), and the rear window defogger circuits. 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 and rear window defogger (4-door vehicles only) circuits 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 .

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:

• | Any backed-out terminals

• | Any improper mating of terminals

• | Any broken electrical connector locks

• | Any improperly formed or damaged terminals

• | Any faulty terminal-to-wire connections

• | Any physical damage to the wiring harness

• | A broken wire inside the insulation

• | Any corrosion of the electrical connections, the splices, or the 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 determines 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.This step checks for each electrical input to the diode module.

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 rear window defogger, 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 attained.
4. Turn OFF all accessories.
5. Monitor the IAC Duty Cycle parameter on the scan tool while turning each of the following electrical accessories ON and OFF.

• | The headlights

• | The HVAC blower motor

• | The rear window defogger, if equipped

Does the scan tool indicate an increase in the IAC Duty Cycle when each of the accessories is turned ON?

--

Go to Diagnostic Aids

Go to Step 3

3 | Does the scan tool indicate an increase in the IAC Duty Cycle when the HVAC blower motor is 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, on the harness side with a test lamp connected to B+.
5. Observe the test lamp while operating the HVAC blower motor switch.

Does the test lamp illuminate when the HVAC blower motor switch is 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, on the harness side with a test lamp connected to ground.
5. Observe the test lamp while operating the headlight switch and the rear window defogger switch, if equipped.

Does the test lamp illuminate when the headlight switch and the rear window defogger switch are 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 and the rear defogger circuit, if equipped, at the electrical load diode with a test lamp connected to ground.
2. Observe the test lamp while operating the headlight switch and the rear window defogger switch.

Does the test lamp illuminate when the headlight switch and the rear window defogger switch are turned ON?

--

Go to Step 10

Go to Step 9

8 | Repair the open or the short 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 or the rear defogger 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 noted. Does the system now operate properly? | -- | System OK | Go to Diagnostic Aids