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 P1820: Internal Mode Switch A Circuit Low Voltage

DTC P1822: Internal Mode Switch B Circuit High Voltage

DTC P1823: Internal Mode Switch P Circuit Low Voltage

DTC P1825: Internal Mode Switch - Invalid Range

DTC P1826: Internal Mode Switch C Circuit High Voltage

DTC P1915: Internal Mode Switch Does Not Indicate Park/Neutral (P/N) During Start

Diagnostic Fault Information

Circuits

Short to Ground

Open/High Resistance

Short to Voltage

TR Signal A

P1820, P1825, P1915

P1825, P1915

--

TR Signal B

P1825, P1915

P1822, P1825, P1915

--

TR Signal C

P1825, P1915

P1826, P1825, P1915

--

TR Signal P

P1823, P1825, P1915

P1825, P1915

--

Typical Scan Tool Data

Internal Mode Switch A/B/C/P

Gear Selector Position

A

B

C

P

Park

LOW

HI

HI

LOW

Park/Reverse

LOW

LOW

HI

LOW

Reverse

LOW

LOW

HI

HI

Reverse/Neutral

HI

LOW

HI

HI

Neutral

HI

LOW

HI

LOW

Neutral/Drive 4

HI

LOW

LOW

LOW

Drive 4

HI

LOW

LOW

HI

Drive 4/Drive 3

LOW

LOW

LOW

HI

Drive 3

LOW

LOW

LOW

LOW

Drive 3/Drive 2

LOW

HI

LOW

LOW

Drive 2

LOW

HI

LOW

HI

Drive 2/Drive 1

HI

HI

LOW

HI

Drive 1

HI

HI

LOW

LOW

Invalid

HI

HI

HI

HI

LOW

HI

HI

HI

HI

HI

HI

LOW

HI = Ignition Voltage

LOW = 0 Voltage

Circuit/System Description

The internal module switch (IMS) is a sliding contact switch attached to the selector detent inside the transmission side cover. The 4 inputs to the transmission control module (TCM) indicate the transmission range selector position. The input voltage level at the TCM is high, B+, when the IMS is open and low when the switch is closed to ground. The 4 input parameters represented are Signals A, B, C and P - Parity. Refer to Transmission Internal Mode Switch Logic .

Conditions for Running the DTC

P1820, P1822, P1823, P1825

    • The engine is running for 5 seconds.
    • The IMS indicates PARK for 1 second or greater.
    • The engine torque is 50 N·m (37 lb ft) or greater.
    • No engine torque malfunction.

P1826

    • No OSS DTC P0722 or P0723.
    • DTC P1826 has not passed this key cycle.
    • The engine is running for 5 seconds.
    • The IMS indicates PARK for 1 second or greater.
    • The engine torque is 50 N·m (37 lb ft) or greater.
    • Vehicle speed is 16 km/h (10 mph) or greater.
    • No engine torque malfunction.

P1915

    • The engine speed is greater than 560 RPM for 5 seconds.
    • A CRANK request signal has been requested for 409 seconds or greater.

Conditions for Setting the DTC

P1820

The IMS signal A is continuously low this ignition cycle, and the IMS indicates transitional position D4-D3 for 8 seconds or greater.

P1822

The IMS signal B is continuously high this ignition cycle, and the IMS indicates transitional position D2-D1 for 8 seconds or greater.

P1823

The IMS signal P is continuously low this ignition cycle, and the IMS indicates transitional position N-D4 for 8 seconds or greater.

P1825

The TCM detects an invalid combination of IMS signals for 8 seconds or greater.

P1826

The TCM detects IMS signal C is high, while the gear radio indicates first, second, third, or fourth gear for 8 seconds or greater.

P1915

The IMS does not indicate PARK or NEUTRAL for 2 seconds or greater when attempting to start the vehicle.

Action Taken When the DTC Sets

    • DTC P1820, P1822, P1823, P1825, P1826, and P1915 are type B DTCs.
    • The TCM commands maximum line pressure.
    • The TCM assumes a D4 shift pattern.
    • The TCM freezes transmission adaptive functions.

Conditions for Clearing the DTC

DTC P1820, P1822, P1823, P1825, P1826, and P1915 are type B DTCs.

Reference Information

Schematic Reference

Automatic Transmission Controls Schematics

Connector End View Reference

Component 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

Control Module References for scan tool information

Special Tools

    • J 35616 GM Terminal Test Kit
    • J 44152 Jumper Harness (20 pins)

Circuit/System Verification

Start and idle the engine. Using the scan tool, observe the IMS A/B/C/P parameter. The scan tool should display LOW/HI/HI/LOW. Move the gear selector through all ranges and ensure the IMS A/B/C/P parameter matches the gear selector for each selected range.

Circuit/System Testing

  1. Using the scan tool, does the IMS A/B/C/P parameter display HI/HI/HI/HI for each range selected.
  2. If the IMS A/B/C/P parameter displays HI/HI/HI/HI for each range selected, repair the open in the IMS ground circuit.
  3. Turn OFF the ignition. Disconnect the automatic transmission inline 20-way connector. Additional DTCs may set. Turn ON the ignition, with the engine OFF. Observe the IMS A/B/C/P parameter on the scan tool. The parameter should display HI/HI.
  4. If the IMS A/B/C/P parameter does not display HI/HI/HI/HI, repair the short to ground in the TR signal circuit that displayed LOW between the TCM and the automatic transmission 20-way inline connector.
  5. Turn OFF the ignition. Install the J 44152 jumper harness (20 pins) on the engine side of the automatic transmission inline 20-way connector. Turn ON the ignition, with the engine OFF. Using the DMM and the J 35616 GM terminal test kit, measure the voltage at each of the TR signal circuits at the J 44152 .
  6. If the voltage is less than battery voltage, test the affected TR signal for an open between the TCM and the automatic transmission 20-way inline connector.
  7. Turn OFF the ignition. Install the J 44152 on the transmission side of the automatic transmission inline 20-way connector. Measure the resistance for each TR signal circuit between the J 44152 circuit and the transmission case. The resistance should be greater than 10 ohms.
  8. If the resistance is less than 10 ohms, test the affected TR signal for a short to ground between the automatic transmission 20-way inline connector and the IMS switch.
  9. Connect the automatic transmission 20-way inline connector. Turn ON the ignition, with the engine OFF. Place the range selector in D3. Observe the IMS A/B/C/P parameter on the scan tool. The IMS A/B/C/P parameter should display LOW/LOW/LOW/LOW.
  10. If any of the TR signal circuits do not display LOW when the range selector is placed in D3, test the affected TR signal for an open or high resistance between the automatic transmission 20-way inline connector and the IMS switch.
  11. If all circuits and connections test normal, test or replace the IMS switch.

Component Testing

  1. Disconnect the automatic transmission inline 20-way connector. Install the J 44152 on the transmission side of the connector and use J 35616 . Referencing Transmission Internal Mode Switch Logic , test for continuity between the IMS ground circuit and each TR signal circuit at the transmission side of the automatic transmission inline 20-way connector, when Transmission Internal Mode Switch Logic indicates the signal circuit should be LOW.
  2. If any TR signal circuits do not display continuity when the table indicates it should be LOW, test the affected TR signal circuit for an open or high resistance between the automatic transmission inline 20-way connector and the IMS switch.
  3. Measure the resistance between each of the TR signal circuits and the transmission case. The resistance should be greater than 10 ohms.
  4. If any of the TR signal circuits display less than 10 ohms, test the affected TR signal for a short to ground.
  5. If all circuits and connections test normal and the IMS switch tests normal, replace the TCM switch.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for TCM replacement, setup, and programming.