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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 SLS 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 DTC P1820

Circuit Description

The transmission manual shift shaft switch assembly is a sliding contact switch attached to the lower control valve body within the transmission. The four inputs to the transmission control module (TCM) from the switch indicate which position is selected by the transmission selector lever. This information is used for engine controls as well as determining the transmission shift patterns. The input voltage at the TCM is high when the switch is open and low when the switch is closed to ground. The state of each input is displayed on the scan tool as IMS. The 4 IMS input parameters represented are Signal A, Signal B, Signal C and Signal P.

If the TCM detects a short to ground on the IMS signal A, then DTC P1820 sets. DTC P1820 is a type B DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1820 Internal Mode Switch A Circuit Low Voltage

Conditions for Running the DTC
    • Ignition voltage is between 8-18 volts.
    • The engine speed is between 450-6,800 RPM, for 5 seconds.
    • Engine torque default values are not being used.
    • Engine Torque is between 55-450 N·m (41-332 lb ft).
    • IMS range displays PARK for more than 1 second.
    • A transitional state of IMS lasts longer than 4 seconds.
    • The TCM detects a short to ground on the IMS Signal  A.

Conditions for Setting the DTC

The TCM detects no open circuit flag on the IMS Signal A.

Action Taken When the DTC Sets
    • The TCM requests the ECM to illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met.
    • The TCM commands maximum line pressure.
    • The TCM freezes transmission adaptive functions.
    • The TCM assumes a D shift pattern.
    • At the time of the first failure, the TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as a Failure Record.
    • At the time of the second failure, the ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as a Freeze Frame.
    • The TCM stores DTC P1820 in TCM history during the second consecutive trip in which the Conditions for Setting the DTC are met.

Conditions for Clearing the DTC
    • The ECM turns OFF the MIL after the sixth consecutive drive trip in which the TCM does not send a MIL illumination request.
    • A scan tool can clear the DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without a non emission related diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the ignition is OFF long enough in order to power down the TCM.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines if the fault is internal or external to the transmission.

    2.

  2. This step determines if the engine wiring harness circuit or the TCM is at fault.

    3.

  3. This step determines if the automatic transmission wiring harness or the IMS is at fault.

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.

    3. Important: 

       • Before clearing the DTC, use the scan tool in order to record the ECM Freeze Frame and the TCM Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the ECM and the TCM.
       • Using the Clear Info function erases stored DTCs in both the ECM and TCM.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Select Transmission Data on the scan tool.
  6. Select IMS range on the scan tool.
  7. Place the gear select lever in D.

Does the scan tool IMS range display Drive?

--

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the AT inline 20-way connector. Additional DTCs may set.
  3. Turn ON the ignition, with the engine OFF.
  4. Select IMS A/B/C/P on the scan tool.

Does the scan tool IMS Signal A display LOW?

--

Go to Step 4

Go to Step 5

4

Test the TR signal A circuit of the switch for a short to ground between the TCM connector and the AT inline 20-way connector.

Refer to Testing for Short to Ground and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 8

5

  1. Turn OFF the ignition.
  2. Connect the AT inline 20-way connector.
  3. Disconnect the automatic transmission wiring harness at the transmission manual shift shaft switch assembly.
  4. Turn ON the ignition, with the engine OFF.
  5. Select IMS A/B/C/P on the scan tool.

Is the IMS Signal A displaying LOW?

--

Go to Step 6

Go to Step 7

6

Replace the automatic transmission internal wiring harness. Refer to Transmission Internal Electrical Harness Replacement .

Did you complete the operation?

--

Go to Step 9

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7

Replace the transmission manual shift shaft switch assembly. Refer to Manual Shift Shaft Position Switch Replacement .

Did you complete the operation?

--

Go to Step 9

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8

Replace the TCM. Refer to Control Module References for replacement, setup and programming.

Did you complete the operation?

--

Go to Step 9

--

9

Perform the following procedure in order to verify the repair:

  1. Select the DTC.
  2. Select Clear Info.
  3. Operate the vehicle under the following conditions:
    4. • Drive the vehicle in D4.
    • IMS Signal A must be HI for 0.025 second.
  4. Select Specific DTC.
  5. Enter DTC P1820.

Has the test run and passed?

--

Go to Step 10

Go to Step 2

10

With the scan tool, observe the stored information, capture info and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK