Circuit Description
Vehicles currently using the 8.1 liter gas engine use a 3 wire system to communicate engine torque data/requests between the engine control module (ECM) and the transmission control module (TCM).
DTC P1688 is a type B DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1688 Unmanaged Engine Torque Delivered to Transmission Control Module (TCM) Signal
Conditions for Running the DTC
| • | The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts. |
| • | Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds. |
Conditions for Setting the DTC
DTC P1688 sets if unmanaged engine torque, gross, signal is less than 1.5 percent or greater than 98.5 percent for greater than a 2 seconds.
Action Taken When the DTC Sets
| • | The TCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met. |
| • | DTC 1688 is stored in TCM history. |
| • | The TCM defaults to a calculated unmanaged torque, gross, input value using throttle and engine speed. |
| • | The TCM inhibits torque converter clutch (TCC) engagement. |
Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.
Diagnostic Aids
| • | Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions: |
| - | A bent terminal |
| - | A backed-out terminal |
| - | A damaged terminal |
| - | Poor terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. |
| • | You may have to drive the vehicle in order to experience a condition. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests for a damaged terminal or connector in the TCM connector.
-
This step tests for an open/short.
Step | Action | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Automatic Transmission Controls Schematics | |||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||
Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase data. Did you find and correct a condition? | Go to Step 8 | Go to Step 3 | |||||||
3 |
Did you find and correct a condition? | Go to Step 8 | Go to Step 4 | ||||||
Test the unmanaged torque signal circuit between the TCM and ECM for an open or short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct a condition? | Go to Step 8 | Go to Step 5 | |||||||
5 | Verify that the ECM has the proper model year software and calibration. This wiring option is only used with the 8.1L gas engine. Was the software and calibration current and proper for the engine? | Go to Step 7 | Go to Step 6 | ||||||
6 | Install the correct ECM calibration. Refer to Control Module References for replacement, setup, and programming. Is the calibration complete? | Go to Step 8 | -- | ||||||
7 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 8 | -- | ||||||
8 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | Go to Step 9 | Go to Step 2 | ||||||
9 | 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 | |||||||
