For 1990-2009 cars

### 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 provides an overview of each diagnostic category.

### DTC Descriptor

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

### 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.

### Conditions for Running the DTC

#### P1820, P1822

• | No engine torque signal DTC P2637.

• | The engine run time is greater than 5 seconds.

• | The engine torque is greater than 55-450 N·m (41-332 lb ft).

#### P1823

• | No engine torque signal DTC P2637.

• | The engine run time is greater than 5 seconds.

• | The engine torque is greater than 25 N·m (18 lb ft).

#### P1825

The engine is running for 5 seconds.

#### P1826

• | No OSS DTCs P0722 or P0723.

• | No engine torque signal DTC P2637.

• | Vehicle speed is greater than 8 km/h (5 mph).

• | Engine torque is greater than 20 N·m (15 lb ft).

• | DTC P1826 has not passed during the current ignition cycle.

• | The gear ratio is within one of the following ranges:

- | 3.33:1 to 3.50:1 for first gear

- | 2.16:1 to 2.27:1 for second gear

- | 1.56:1 to 1.64:1 for third gear

- | 0.98:1 to 1.03:1 for fourth gear

- | 0.73:1 to 0.77:1 for fifth gear

#### P1915

• | No OSS DTC P0722 or P0723.

• | The transmission output shaft speed is less than 100 RPM.

### Conditions for Setting the DTC

#### P1820, P1822

• | The switch indicates PARK for 1 second, then;

• | The switch indicates a transitional state for greater than 4 seconds.

#### P1823

• | The switch indicates PARK for 1 second, then;

• | The switch indicates a transitional state for greater than 5 seconds.

#### P1825

The TCM detects an invalid switch range for 5 seconds.

#### P1826

The TCM detects Signal C is high, while the gear ratio indicates first, second, third, fourth, or fifth gear for 3 seconds.

#### P1915

The manual shift shaft switch assembly indicates a transitional state during the following sequence:

• | The engine speed is 60 RPM or less for 0.25 second or greater.

• | The engine speed is 81-625 RPM for 0.15 second or greater.

• | The engine speed is 651 RPM or greater for 1.5 seconds or greater.

• | The transmission input shaft speed is 200 RPM or greater for 1.5 seconds or greater.

### Action Taken When the DTC Sets

• | DTCs P1820, P1822, P1823, P1825, P1826, P1915 are Type B DTCs.

• | The TCM commands maximum line pressure.

• | The TCM freezes transmission adaptive functions.

• | The TCM assumes a D5 shift pattern.

### Conditions for Clearing the DTC

DTCs P1820, P1822, P1823, P1825, P1826, P1915 are Type B DTCs.

### Reference Information

#### Schematic Reference

Automatic Transmission Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Transmission General Description

#### 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

Transmission Control Module Scan Tool Information

#### Special Tools

• | J 35616 GM-Approved Terminal Test Kit

• | J 45681 Jumper Harness

### 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.

⇒ | 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.

2. 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.

⇒ | 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.

3. Turn OFF the ignition. Install the J 45681 jumper harness, 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-approved terminal test kit, measure the voltage at each of the TR signal circuits at the J 45681 .

⇒ | 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.

4. Turn OFF the ignition. Install the J 45681 on the transmission side of the automatic transmission inline 20-way connector. Measure the resistance for each TR signal circuit between the J 45681 circuit and the transmission case. The resistance should be greater than 10 ohms.

⇒ | 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.

5. 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.

⇒ | 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.

6. 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 45681 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.

⇒ | 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.

2. Measure the resistance between each of the TR signal circuits and the transmission case. The resistance should be greater than 10 ohms.

⇒ | If any of the TR signal circuits display less than 10 ohms, test the affected TR signal for a short to ground.

3. 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 repair.

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