Circuit Description
The transmission internal mode switch (IMS) is a sliding contact switch attached to the lower control valve body, within the transmission. The manual valve link assembly then connects the IMS to the manual valve. The 4 inputs to the transmission control module (TCM) from the IMS 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 level at the TCM is high, ignition voltage, when the IMS is open and low when the switch is closed to ground. The state of each input is available for display on the scan tool. The four input parameters represented are Signal A, Signal B, Signal C and Signal P, Parity.
If the TCM detects an IMS transitional state between D4 and D3 while the vehicle is in motion, 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
| • | No MAF DTCs P0101, P0102 or P0103. |
| • | No MAP DTCs P0106, P0107 or P0108. |
| • | The engine run time is greater than 5 seconds. |
| • | The engine torque is 50-1492 N·m (37-1100 lb ft). |
| • | DTC P1820 did not pass this cycle. |
Conditions for Setting the DTC
- The IMS indicates PARK for 1 seconds, then;
- The IMS indicates a transitional state between D4 and D3 for 8 seconds.
Action Taken When the DTC Sets
| • | The TCM requests the engine control module (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 ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as Freeze Frame and Failure Records and the TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records. |
| • | 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 MIL/DTC
| • | The ECM turns OFF the MIL after the third consecutive drive trip in which the TCM does not send a MIL illumination request. |
| • | The TCM clears the DIC message when the condition no longer exists. |
| • | A scan tool can clear the MIL/DTC. |
| • | The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring. |
| • | The TCM cancels the DTC default actions when the ignition switch 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.
-
This step determines if the fault is internal or external to the transmission.
-
This step determines if the engine wiring harness circuit is at fault or the TCM.
-
This step determines if the automatic transmission wiring harness is at fault or the IMS.
Step | Action | Values | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||
2 |
Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the TCM. Does the scan tool IMS display Drive 4? | -- | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 3 | ||||||
Is signal A LOW? | -- | Go to Step 4 | Go to Step 5 | |||||||
Test the signal A circuit of the IMS 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 | |||||||
Is signal A LOW? | -- | Go to Step 6 | Go to Step 7 | |||||||
6 |
Important: The automatic transmission wiring harness consists of 2 separate harnesses, the Automatic Transmission Wiring Harness Assembly and the Automatic Transmission Wiring Extension Harness Assembly. In order to avoid the unnecessary replacement of parts, determine which harness contains the fault and replace only that harness. Replace the automatic transmission wiring harness. Refer to Input Speed Sensor and Wiring Harness Removal in the 4T80-E section of the Transmission Unit Repair Manual. Did you complete the replacement? | -- | Go to Step 9 | -- | ||||||
7 | Replace the transmission internal mode switch. Refer to Control Valve Lower Body Disassemble in the 4T80-E section of the Transmission Unit Repair Manual. Did you complete the replacement? | -- | Go to Step 9 | -- | ||||||
8 | Replace the TCM. Refer to Control Module References . Did you complete the replacement? | -- | Go to Step 9 | -- | ||||||
9 | Perform the following procedure in order to verify the repair:
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? | -- | System OK |
