GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

DTC Descriptors

DTC P2101: Idle Speed Control (ISC) Motor Control Circuit

DTC P2118: Idle Speed Control (ISC) System

DTC P2119: Idle Speed Control (ISC) System Performance

Diagnostic Fault Information

Circuit

Short to Ground

High Resistance

Open

Short to Voltage

Signal Performance

5-Volt Reference

P0122, P0222

P0510, P2119

P0122, P0222

P0123, P0223

--

Idle Motor Position Signal

P0222

P2118

P0222

P0223

--

Idle Motor Switch Signal

P0510

--

P0510

--

--

Throttle Position Sensor Signal

P0122

P0510

P0122

P0123

--

Idle Motor High Control

P2101, P2118

P2101, P2118

P2101, P2118

P2101, P2118

--

Idle Motor Low Control

P2101, P2118

P2101, P2118

P2101, P2118

--

--

Low Reference

--

--

P0123, P0223

--

--

Typical Scan Tool Data

ISC Command

Circuit

Short to Ground

Open

Short to Voltage

Operating Conditions: Engine operating in closed loop

Parameter Normal Range: ISC Command 30%

Idle Motor High Control

-40%

0%

0%

Idle Motor Low Control

0%

0%

-40%

Circuit/System Description

The throttle body assembly uses an idle speed control (ISC) system that contains motor, throttle position (TP) sensor, motor position sensor, and an idle switch. The ISC system is used to control engine speed at idle. The engine control module (ECM) uses high and low control circuits to control the ISC motor. A 5-volt reference and low reference supplied by the ECM is shared by both the TP and motor position sensors. The TP and motor position sensors each have a signal circuit to feedback voltage to the ECM. The ECM commands the ISC motor by pulse width modulating the voltage on the high and low control circuits to adjust the throttle angle for idle speed control. The motor position sensor is used by the ECM to monitor the position of the ISC motor for idle operation. The TP sensor is used by the ECM to monitor the position of the throttle blade. An idle switch is used by the ECM to monitor when the throttle blade is at it rest position to allow the ISC motor to control idle operation. With the throttle blade at the rest position, the idle switch will be closed. When the throttle blade is opened, the idle switch will be open.

The ISC system consist of the following circuits:

    • A 5-volt reference
    • Idle motor high control
    • Idle motor low control
    • Idle motor position signal
    • Idle motor switch signal
    • Low reference
    • Throttle position sensor signal

Conditions for Running the DTC

P2101 and P2118

    • The engine is running.
    • The ISC motor angle is between 0.235-5.6 degrees.
    • This DTC runs continuously when the above conditions are met.

P2119

    • Ignition is ON.
    • The DTC runs continuously when the above condition is met.

Conditions for Setting the DTC

P2101 or P2118

The ECM has commanded a duty cycle of the ISC motor greater than 59.5 percent or less than or equal to -70 percent for 10 seconds.

P2119

The ECM detects the learned lower and upper limits exceed the actual lower and upper limits.

Action Taken When the DTC Sets

DTC P2101, P2118, and P2119 are Type B DTCs

Conditions for Clearing the DTC

DTC P2101, P2118, and P2119 are Type B DTCs

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification

  1. Ignition ON, with a scan tool, command the ISC motor ON. The ISC motor position sensor percentage should fluctuate between 3-5 percent.
  2. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

Circuit/System Testing

  1. Verify that the following conditions do not exist:
  2. • The throttle body is not damaged or modified.
    • Vacuum leaks
    • A restricted air intake system
    • Excessive deposits on the throttle plate or bore
    • The throttle shaft for binding
    • The throttle linkage or the throttle cable for binding
    • Incorrectly installed positive crankcase ventilation (PCV) valve or hose
    • A malfunctioning PCV valve
         ⇒If you find any of the above conditions, repair as necessary.
  3. Ignition OFF, disconnect the harness connector of the ISC motor.
  4. Important: The test lamp is used to load the circuit and may not illuminate.

  5. Connect a test lamp between the high control circuit of the ISC motor and the negative terminal of the battery.
  6. Ignition ON, test for less than 0.3 volt between the high control circuit of the ISC motor and the positive terminal of the battery.
  7. If greater than 0.3 volt, test the high control circuit of the ISC motor for a short to ground or an open/high resistance. If the circuit/connections test normal, replace the ECM.

    Important: Use the AC volt scale on the DMM. The millivolt scale will produce an inaccurate reading.

  8. Engine running, test for 30-40 hertz on the AC scale between the high control circuit of the ISC motor and the negative terminal of the battery.
  9. If frequency is not between 30-40 hertz, test the high control circuit of the ISC motor for a short to voltage. If the circuit/connections test normal, replace the ECM.
  10. Ignition OFF, connect a test lamp between the low control circuit of the ISC motor and the positive terminal of the battery.
  11. If test lamp illuminates, test the low control circuit of the ISC motor for a short to ground. If the circuit/connections test normal, replace the ECM.
  12. Ignition ON, test for less than 0.3 volt between the low control circuit of the ISC motor and the negative terminal of the battery.
  13. If greater than 0.3 volt, test the low control circuit of the ISC motor for a short to voltage or an open/high resistance. If the circuit/connections test normal, replace the ECM.
  14. Ignition ON, connect a test lamp between the 5-volt reference circuit of the ISC motor and the negative terminal of the battery.
  15. Test for 4.8-5.2 volts between the 5-volt reference circuit of the ISC motor and the negative terminal of the battery.
  16. If less than 4.8 volts, test the 5-volt reference circuit of the ISC motor for a short to ground or open/high resistance. If the circuit/connections test normal, replace the ECM.
  17. If all circuits/connections test normal, test or replace the ISC motor.

Component Testing

  1. Ignition OFF, disconnect the harness connector of the ISC motor.
  2. Measure for 3-6 ohms of resistance between the high and low control circuit terminals of the ISC motor.
  3. If the resistance is out of the specified range, replace the ISC motor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Throttle Body Assembly Replacement
    •  Control Module References for ECM replacement, setup, and programming