For 1990-2009 cars

Table 1: | ISC Relay Circuit

Schematic Reference: Engine Controls Schematics

### Circuit Description

The idle speed control (ISC) motor opens and closes the throttle valve according to signals from the powertrain control module (PCM). The ISC motor will extend the plunger of the motor when the motor receives the correct signals from the PCM. Extending the plunger of the ISC motor opens the throttle plates and increases the idle speed. When a higher idle speed is no longer required, the PCM will reverse the polarity of the signals to the ISC motor. The change in the polarity of the signals reverses the direction of the ISC motor. The plunger of the ISC motor then retracts, causing the throttle plates to close and decrease the idle speed.

### Condition for Running the DTC

• | The engine is running.

• | The closed throttle position (CTP) switch is ON.

• | Battery voltage is greater than 12 volts.

• | Engine coolant temperature is greater than 30°C (84°F).

### Conditions for Setting the DTC

Throttle valve feed back check:

• | The movement of the ISC motor is less than 0.244 inches.

• | The above diagnostic will run 10 times per continuous drive cycle.

ISC motor voltage check:

• | No operation signal from the PCM.

• | The above diagnostic will run 2 times per continuous drive cycle.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL).

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

• | The PCM enters the Fail-Safe Function and stops ISC operation.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after three consecutively passing trips without a fault present.

• | A History DTC clears after 40 consecutive warm-up cycles without a fault.

• | Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Diagnose a DTC P0510 first if a DTC P0510 is set.

• | A stuck or binding throttle plate may cause a DTC P0505 to set. Check the throttle plate for smooth and even movement.

• | A short to ground in the CTP switch circuit may cause a DTC P0505 to set.

• | Check the adjustment of the accelerator cable. An improperly adjusted accelerator cable may prevent the throttle plate from closing completely. Refer to Accelerator Cable Adjustment .

• | A poor connection at the PCM. Inspect the harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal to wire connection.

• | A damaged wiring harness. Inspect the wiring harness for damage and repair as necessary.

• | Inspect the engine for a large vacuum leak. Check for a condition that could cause a large vacuum leak, such as a incorrectly installed or faulty crankcase ventilation valve.

• | Check the resistance of the ISC motor relay. The resistance across terminal 1 and terminal 2 is 79-95 ohms at 20°C (68°F). The resistance across terminal 3 and terminal 4 is infinite.

• | The ISC motor relay's electrical contacts may be pitted or sticking. Replace the ISC motor relay if tapping gently on the relay or wiggling the relay causes a change in the relay's operation.

• | The performance of the ISC motor relay may be affected by temperature. Check the ISC motor relay after sitting outside overnight and after running the engine 30 minutes.

An intermittent malfunction may be caused by a problem in the ISC motor 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.

If a DTC P0505 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

Use the following relay cavity table in order to locate the correct cavities to probe during diagnosis. The table layout corresponds to the cavity layout of the relay block.

#### ISC Relay Circuit

Relay Cavity Identification
To PCM
To ISC Motor
Coil Power | Coil Ground

### 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 the operation of the ISC motor. The scan tool output control feature for the ISC motor is RPM Control. RPM Control is disabled by the PCM until the engine has reached a normal operating temperature of at least 80°C (176°F) and there are no DTCs are stored in the PCM.

3. This step determines if the condition that set the DTC P0505 is still present.

4. Check for an open or short in the ISC motor control circuits or for a faulty ISC motor.

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. Start the engine.
2. Run the engine and obtain normal operating temperature.
3. Clear the DTC.
4. Install a scan tool and select the RPM Control.
5. Increase and than decrease the RPM with the scan tool output control, while monitoring the ISC plunger.

Does the ISC plunger extend out with an RPM increase and then retract when the RPM is decreased?

--

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Review the Freeze Frame data.
3. Clear the scan tool information.
4. Operate the vehicle within the Freeze Frame data as noted.

Is a DTC P0505 set?

--

Go to Step 4

Go to Diagnostic Aids

4 |

1. Disconnect the ISC motor relay.
2. Turn ON the ignition, leaving the engine OFF.
3. Probe the ISC relay connector cavity 1 with a test lamp connected to ground. Refer to the Relay Cavity Identification table in Diagnostic Aids.

Did the test lamp illuminate?

--

Go to Step 5

Go to Step 11

5 | Measure the resistance of the ISC relay between terminals 3 and 4. Is the resistance at the specified value? | Infinity | Go to Step 6 | Go to Step 12
6 | Measure the resistance of the ISC relay between terminals 1 and 2. Is the resistance within the specified value? | 75-95 ohms | Go to Step 7 | Go to Step 12
7 |

1. Disconnect the PCM.
2. Connect one end of a fused jumper wire to B+.
3. Connect the other end of the fused jumper wire to the ISC motor relay cavity 2.
4. Measure the voltage from the ISC motor relay control circuit at the PCM harness connector with a DMM.

Is the voltage within the specified value?

B+

Go to Step 8

Go to Step 13

8 |

1. Turn OFF the ignition.
2. Connect the ISC motor relay.
3. Disconnect the PCM electrical connector C1 and back out terminals C1-11 GRY/YEL wire, C1-16 GRY/BLK wire and C1-24 GRY/RED wire.
4. Reconnect the PCM electrical connector C1.
5. Connect a fused jumper wire to terminal C1-16 GRY/BLK wire and ground.
6. Turn ON the ignition, leaving the engine OFF.
7. Check that the throttle lever is in contact with the plunger of the ISC motor actuator.
8. Connect four new 1.5 volt batteries in series.

Important: DO NOT apply the voltage (6.0 volts) for longer than 1 second, or a faulty condition may occur. Check that the CTP switch parameter on the scan tool reads IDLE. If the scan tool display does not read IDLE, move the ISC actuator in order to turn the CTP switch ON (IDLE).

9. Connect terminal C1-11 GRY/YEL wire to the positive side of the batteries.
10. Connect terminal C1-24 GRY/RED wire to the negative side of the batteries.

Did the ISC motor plunger extend out?

--

Go to Step 9

Go to Step 14

9 |

1. Turn ON the ignition, leaving the engine OFF.
2. Check that the throttle lever is in contact with the plunger of the ISC motor actuator.

Important: DO NOT apply the voltage (6.0 volts) for longer than 1 second, or a faulty condition may occur. Check that the CTP switch parameter on the scan tool reads IDLE. If the scan tool display does not read IDLE, move the ISC actuator in order to turn the CTP switch ON (IDLE).

3. Connect terminal C1-11 GRY/YEL wire to the negative side of the batteries.
4. Connect terminal C1-24 GRY/RED wire to the positive side of the batteries.

Did the ISC motor plunger retract?

--

Go to Step 10

Go to Step 14

10 |

1. Turn OFF the ignition.
2. Disconnect the four 1.5 volt batteries and reinstall all PCM terminals in their proper places.
3. Turn ON the ignition, leaving the engine OFF.
4. Disconnect the ISC motor electrical connector.
5. Probe the ISC motor electrical connector ground circuit with a test lamp connected to B+.

Did the test lamp illuminate?

--

Go to Step 16

Go to Step 15

11 | Repair the open in the ignition feed circuit between the Main relay and the ISC motor relay. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 18 | --
12 | Replace the ISC motor relay. Refer to ISC Motor Relay Replacement . Is the action complete? | -- | Go to Step 18 | --
13 | Repair the open or the short in the ISC motor relay control circuit between the ISC motor relay and the PCM. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 18 | --
14 |

1. Check for an open or short in the ISC motor control circuits GRY/YEL wire, GRY/RED wire and the GRY wire between the ISC motor relay and the ISC motor.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 18

Go to Step 17

15 | Repair the open in the ISC motor ground circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 18 | --
16 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 18 | --
17 | Replace the ISC motor. Refer to ISC Motor Replacement . Is the action complete? | -- | Go to Step 18 | --
18 |

1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
2. Review the scan tool data and check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to the Applicable DTC Table

System OK