GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD 🢒 Transmission/Transaxle 🢒 Automatic Transmission - Allison 🢒 DTC P1688

Circuit Description

Vehicles currently using the 8.1L gas engine use a 3 wire system to communicate engine torque data/requests between the powertrain control module (PCM) and the transmission control module (TCM).

DTC P1688 is a type B DTC.

Conditions for Running the DTC
    • 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.
    • Pulse width modulation (PWM) driver demand torque option is selected in transmission calibration.

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.

  1. This step tests for a damaged terminal or connector in the TCM connector.

  2. This step tests for an open/short.

Step

Action

Value

Yes

No

Schematic Reference: Automatic Transmission Controls Schematics

1

Did you perform the Diagnostic System Check - Automatic Transmission?

--

Go to Step 2

Go to Diagnostic System Check - Automatic Transmission

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 Freeze Frame and Failure Records for reference. The Clear Info function will erase data.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Turn OFF the ignition.
  5. Disconnect the TCM connector C1 (GRAY).
  6. Inspect the C1 connector terminal 9 for a pulled back, unlocked, terminal or visible terminal damage.

Did you find and correct a condition?

--

Go to Step 8

Go to Step 3

3
  1. Disconnect the PCM connector C2 (RED).
  2. Inspect terminal 2 for a pulled back, unlocked, terminal or visible terminal damage.

Did you find and correct a condition?

--

Go to Step 8

Go to Step 4

4
  1. Turn OFF the ignition.
  2. Disconnect the TCM connector C1 (GRAY) and the PCM connector C2 (RED).
  3. Using a DMM, test the unmanaged torque signal circuit between terminal 9 of the TCM and terminal 2 of the PCM for a short to ground or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct a condition?

--

Go to Step 8

Go to Step 5

5

Verify that the PCM 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 PCM calibration. Refer to Powertrain Control Module (PCM) Programming.

Is the calibration complete?

--

Go to Step 8

--

7

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls - 8.1L.

Did you complete the replacement?

--

Go to Step 8

--

8

Perform the following procedure in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Operate the vehicle under the following conditions:
    4. • Vehicle at operating temperature
    • Engine speed greater than 200 RPM
    • All conditions are met for 2 seconds
  4. Select Specific DTC.
  5. Enter DTC P1688.

Has the test run and passed?

--

System OK

Go to Step 1