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 four inputs to the PCM 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 PCM 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.
Diagnostic Aids
Refer to Transmission Internal Mode Switch Logic .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests the IMS for an open ground circuit.
-
This step tests for correct reference voltage from the PCM.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
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 PCM. Does each range selected match the scan tool IMS display? | -- | Go to Intermittent Conditions in Engine Controls | Go to Step 2 |
2 |
Does the IMS A/B/C/P display match the specified value for each range selected? | HI/HI/HI/HI | Go to Step 12 | Go to Step 3 |
Does the scan tool IMS A/B/C/P parameter indicate HI for all range signal states? | -- | Go to Step 4 | Go to Step 8 | |
4 |
Refer to Automatic Transmission Inline 20-Way Connector End View . Does the voltage measure ignition voltage at all four terminals? | -- | Go to Step 5 | Go to Step 9 |
Connect a fused jumper wire from terminal F of the J 44152 , signal A circuit, to ground while monitoring the scan tool IMS A/B/C/P parameter. Refer to Automatic Transmission Inline 20-Way Connector End View . When the signal A circuit (CKT 771) is grounded, do any other signal circuits indicate LOW? | -- | Go to Step 10 | Go to Step 6 | |
6 | Connect a fused jumper wire from terminal G of the J 44152 , signal B circuit, to ground while monitoring the scan tool IMS A/B/C/P parameter. Refer to Automatic Transmission Inline 20-Way Connector End View . When the signal B circuit (CKT 772) is grounded, do any other signal circuits indicate LOW? | -- | Go to Step 10 | Go to Step 7 |
7 | Connect a fused jumper wire from terminal H of the J 44152 , signal C circuit, to ground while monitoring the scan tool IMS A/B/C/P parameter. Refer to Automatic Transmission Inline 20-Way Connector End View . When the signal C circuit (CKT 773) is grounded, do any other signal circuits indicate LOW? | -- | Go to Step 10 | Go to Step 11 |
8 | Test the signal circuits (CKTs 771, 772, 773, and 776) of the IMS that did not indicate HI for a short to ground between the PCM connector C1 and the AT inline 20-way connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 16 |
9 | Test the signal circuits (CKTs 771, 772, 773, and 776) of the IMS that did not indicate ignition voltage for an open between the PCM connector C1 and the AT inline 20-way connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 16 |
10 | Test the affected signal circuits of the IMS for a shorted together condition between the PCM connector C1 and the AT inline 20-way connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 16 |
11 | Test the signal circuits (CKTs 771, 772, 773, and 776) of the IMS for an open or shorted condition between the AT inline 20-way connector and the IMS. Refer to Circuit Testing in Wiring Systems. Did you find a condition? | -- | Go to Step 14 | Go to Step 15 |
12 | Test the ground circuit (CKT 1550) of the IMS for an open between the AT inline 20-way connector and the ground distribution block. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 13 |
13 | Test the ground circuit (CKT 1050) of the IMS for an open between the AT inline 20-way connector and the IMS. Refer to Circuit Testing in Wiring Systems. Did you find a condition? | -- | Go to Step 14 | Go to Step 15 |
14 |
Important: The automatic transmission wiring harness consists of two separate harnesses, the Automatic Transmission Wiring Harness Assembly, GM P/N 15320475, and the Automatic Transmission Wiring Extension Harness Assembly, GM P/N 15320478. 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 17 | -- |
15 | 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 17 | -- |
16 | Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 17 | -- |
17 |
Does each selected transmission range match the scan tool IMS display? | -- | System OK | Go to Step 2 |
