For 1990-2009 cars
Engine Controls Components
Cell 21: Idle-Up Signals
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The powertrain control module (PCM) receives an electrical load idle-up signal from the diode module. The electrical load idle-up signal is present when one or more of the following systems is turned ON:

• | Headlights

• | The Rear-Window Defogger

• | The Heater Blower Motor

The idle air control (IAC) valve increases the idle air passage opening for the additional electrical load compensation.

### Diagnostic Aids

A short to ground in the signal circuit between the diode module and the PCM may cause an open in the TAIL fuse.

Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the electrical load idle-up circuit. Inspect the wiring harness and the components for any of the following conditions:

• | Backed out terminals

• | Incorrect mating of terminals

• | Broken electrical connector locks

• | Incorrectly formed terminals or damaged terminals

• | Faulty terminal-to-wire connections

• | Physical damage to the wiring harness

• | A broken wire inside the insulation

• | Corrosion of electrical connections, of splices, or of 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 to store the applicable freeze frame data on the scan tool. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool stores the information for later reference.

2. This step checks for the electrical load idle-up signal to the PCM.

3. 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 (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Important: If the rear defogger, the headlights, or the blower motor do not operate, immediately diagnose those components.

1. Install a scan tool.
2. Start the engine.
3. Run the engine until it reaches the normal operating temperature.
4. Turn OFF all accessories.
5. Monitor the electrical load parameter on the scan tool while turning each of the following electrical accessories ON and OFF.

• | The rear-window defogger

• | The headlights

• | The heater blower motor

Did the scan tool indicate YES when each of the accessories was turned ON, and NO when the accessory was turned OFF?

--

Go to Diagnostic Aids

Go to Step 3

3 | Did the engine speed increase with at least one of the accessories turned ON? | -- | Go to Step 7 | 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 harness side of the electrical load idle-up circuit to ground with a test lamp.
5. Turn the lamp switch ON.

Does the test lamp illuminate?

--

Go to Step 5

Go to Step 6

5 |

1. Check for faulty terminal connections at the PCM.
2. Repair the connections as necessary. Refer to Connector Repairs or Wiring Repairs to in Wiring Systems.

Was a repair necessary?

--

Go to Step 10

Go to Step 9

6 |

1. Check the electrical load idle-up circuit for an open between the PCM and the diode module.
2. Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 10

Go to Step 8

7 | Check the applicable systems that did not cause an increase in the engine speed.

• | Check the circuit between the blower speed selector switch and the diode module for an open or for a faulty connection.

• | Check the circuit between the headlamp switch and the diode module for an open or for a faulty connection.

• | Check the circuit between the rear defogger switch and the diode module for an open or for a faulty connection. Refer to Wiring Repairs in Wiring Systems.

Were any repairs necessary?

--

Go to Step 10

Go to Step 8

8 | Replace the diode module. Refer to Diode Module Replacement . Is the action complete? | -- | Go to Step 10 | --
9 | Replace the PCM. Is the action complete? | -- | Go to Step 10 | --
10 | Operate the vehicle within the conditions under which the original symptom was noted. Does the system now operate correctly? | -- | System OK | Go to Diagnostic Aids