GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Geo 🢒 1997 🢒 Metro 🢒 DTC 🢒 DTC P0506 Idle Speed Low
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 decreasing the idle speed.

Condition for Running the DTC
    • The closed throttle position (CTP) switch is ON.
    • Battery voltage is greater than 12 volts.
    • Engine coolant temperature is greater than 80°C (176°F).
    • Intake air temperature is between -10°C to 50°C (14°F to 122°F).
    • Barometric Pressure is at least 560 mmHg (75 kPa).
    • Engine running for 5 minutes or more.

Conditions for Setting the DTC

Engine idle speed is less than the target idle speed by 100 RPM or more.

Action Taken When the DTC Sets
    • The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

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
    • Diagnose a DTC P0510 first if a DTC P0510 is set.
    • A stuck or binding throttle plate may cause a DTC P0506 to set. Check the throttle plate for smooth and even movement.
    • 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 TP sensor for accurate throttle position readings.
    • Check the ISC Duty cycle parameter on the scan tool. The scan tool parameter should read 20% to 40% duty cycle, at normal operating temperatures.
    • 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 P0506 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.

  2. This step determines if the fault is present.

    3.

  3. This step checks for an open circuit in the ignition feed to the ISC Relay.

    4.

  4. This step check for an open or a short in the ISC motor control circuit.

    5.

  5. This step checks the functionality of the ISC motor and the ISC motor control circuits.

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

Check for any DTCs other than a P0506, P0507, P0171, P0172, P0300, P0301, P0302, or a P0303.

Are there any DTCs that are NOT listed above?

--

Go to the Applicable DTC Table first

Go to Step 3

3

  1. Start the engine.
  2. Run the engine and obtain normal operating temperature.
  3. Turn the all electrical accessories OFF.
  4. Monitor the RPM parameter on a scan tool.

Does the scan tool display an engine speed that is below the specified value by 100 RPM or more?

800-900 RPM

Go to Step 5

Go to Step 4

4
  1. Perform the scan tool Clear DTC Information function.
  2. Operate the vehicle within the Freeze Frame data as noted.

Does a DTC P0506 set?

--

Go to Step 5

Go to Diagnostic Aids

5
  1. Check for any of the following conditions:
    2. • Cracks and splits in the vacuum hoses.
    • Misrouted vacuum hoses.
    • A restricted air intake system.
    • Misadjusted ignition timing.
  2. Repair as necessary.

Was a repair necessary?

--

Go to Step 19

Go to Step 6

6

  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 7

Go to Step 12

7

Measure the resistance of the ISC relay between terminals 3 and 4.

Is the resistance at the specified value?

Infinity

Go to Step 8

Go to Step 13

8

Measure the resistance of the ISC relay between terminals 1 and 2.

Is the resistance within the specified range?

75-95 ohms

Go to Step 9

Go to Step 13

9

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

Is the voltage at the specified value?

B+

Go to Step 10

Go to Step 14

10

  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.

    9. 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 11

Go to Step 15

11
  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. Probe the ISC motor connector ground circuit with a test lamp connected B+.

Did the test lamp illuminate?

--

Go to Step 17

Go to Step 16

12

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 19

--

13

Replace the ISC motor relay. Refer to ISC Motor Relay Replacement .

Is the action complete?

--

Go to Step 19

--

14
  1. Check for an open or a short in the ISC motor relay control circuit between the ISC motor relay and the PCM.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems

Is the action complete?

--

Go to Step 19

--

15

Repair the open or the 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. Refer to Wiring Repairs in Wiring Systems

Was a repair necessary?

--

Go to Step 19

Go to Step 18

16

Repair the open in the ISC motor ground circuit. Refer to Wiring Repairs in Wiring Systems

Is the action complete?

--

Go to Step 19

--

17

Replace the PCM. Refer to Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 19

--

18

Replace the ISC motor. Refer to ISC Motor Replacement .

Is the action complete?

--

Go to Step 19

--

19
  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