GM Service Manual Online
For 1990-2009 cars only

Object Number: 230201  Size: LF
Automatic Transmission Components Gasoline

Circuit Description

The Transmission Control Module (TCM) monitors the engine speed and the transmission output shaft speed. The TCM calculates turbine shaft speed and Torque Converter Clutch (TCC) slip speed by using inputs from the transmission Input Shaft Speed (ISS) Sensor, Transmission Output Shaft Speed (OSS) Sensor and other transmission components. The forward clutch housing is used as the ISS Sensor rotor. Whenever the TCC is engaged, engine speed and turbine speed will closely match, indicating low TCC slip speed. In D3 with the TCC engaged, calculated transmission component slip can only occur in the torque converter. In D4 Overdrive with the TCC engaged, transmission component slip can occur in the TCC or the fourth clutch assembly.

If the TCM detects an excessive TCC slip speed in D4 Overdrive when the TCC should be engaged, then DTC 68 sets.

Conditions for Setting the DTC

    • No DTC 28.
    • No DTC 71.
    • No DTC 74.
    • Not in park/neutral.
    • Fourth gear ratio is indicated.
    • TCC slip speed greater than 200 RPM.
    • All conditions are met for 2 seconds.

Action Taken When the DTC Sets

    • The TCM does not illuminate the Malfunction Indicator Lamp (MIL).
    • The TCM inhibits TCC operation.
    • The TCM inhibits manual mode operation.
    • DTC 68 is stored in TCM history.

Conditions for Clearing the DTC

    • The TCM clears the DTC from Current to History when the fault condition(s) no longer exist and the ignition switch is cycled OFF and then ON.
    • A scan tool can clear the DTC from Current and History status. The TCM clears the DTC from the TCM history if the vehicle completes 40 ignition cycles without a failure reported.
    • The TCM cancels the DTC default actions when the fault no longer exists and the ignition is cycled OFF and then ON.

Diagnostic Aids

    • Check for deformed connectors at pass-thru connector.
    • DTC 68 will set when going to default (second gear).
    • An intermittent, incorrect engine speed signal will set a DTC 68 if the incorrect signal lasts for greater than 2 seconds.
    • A mechanical failure in the 1-2 shift solenoid valve (stuck OFF), 2-3 shift solenoid valve (stuck ON), or the TCC PWM Sol. Valve, could set DTC 68.
    • A TFP Val. Position Sw. malfunction can set DTC 68.
    • Internal transmission failures can result in a DTC 68.
    • Sticking shift valves or contamination may cause intermittent slipping in D4.
    • First diagnose and clear any engine DTCs that are present. Then inspect for any transmission DTCs that may have reset.

Test Description

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

  1. This step tests the indicated range signal to the actual selected range. A faulty TFP Valve Position Switch can set a DTC 68.

  2. This step tests for excessive TCC slip speed while in a commanded TCC lock-up state and in fourth gear; and confirms that the fault is present.

  3. This step tests for excessive TCC slip speed while in a commanded TCC lock-up state and in third gear.

  4. This step tests for a sticking TCC shift valve.

  5. This step tests for proper transmission line pressure.

DTC 68

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to On-Board Diagnostic System Check

2

Perform the Transmission Fluid Checking Procedure. Refer to Transmission Fluid Check .

Did you perform the procedure?

--

Go to Step 3

Go to Transmission Fluid Check

3

  1. Install the Scan Tool .
  2. With the engine OFF, turn the ignition switch to the RUN position.
  3. Important:: Before clearing the DTC's, record the Current and History DTC's information for reference. The Clear Info function erases the stored Current and History DTC from the TCM.

  4. Record Current and History DTC's, then clear the DTC.
  5. With the engine idling and at normal operating temperature, apply the brake pedal.
  6. Select each transmission range: D 1, D 2, D 3, D 4, N, R and P.

Does each selected transmission range match the Scan Tool TFP Switch A/B/C display? (Refer to the Range Signal table.)

--

Go to Step 4

Go to DTC P1810 TFP Valve Position Switch Circuit

4

Drive the vehicle under the following conditions:

  1. The TFT is + 20
  2. - 130° C (50 - 266° F)

  3. The transmission is in D 4.
  4. The TCC duty cycle is greater than 80%.
  5. The TP angle is 10 80%.

At any time is the TCC Slip Speed greater than the specified value for 10 seconds?

200 RPM

Go to Step 5

Go to Diagnostic Aids

5

Drive the vehicle under the following conditions:

  1. The transmission is in D 3.
  2. Command the TCC ON with the Scan Tool .
  3. The TP angle is 10 80%.

At any time is the TCC Slip Speed greater than the specified value for 10 seconds?

200 RPM

Go to Step 7

Go to Step 6

6

Repeat the procedure in Step 4. Drive the vehicle under the following conditions:

  1. The transmission is in D 4.
  2. The TCC duty cycle is greater than 80%.
  3. The TP angle is 10 80%.

Is the TCC Slip Speed greater than the specified value?

200 RPM

Go to Step 8

Go to Step 11

7

Refer to the System Diagnosis table: Torque Converter Clutch Slipping Slipping TCC. Did you find and correct the condition?

--

Go to Step 12

--

8

  1. Connect the J 21867 Pressure Gauge to the transmission line pressure tap.
  2. Perform the Line Pressure Check. Refer to Line Pressure Check .

Is the line pressure within specifications?

--

Go to Step 9

Go to Low Line Pressure

9

  1. Remove the transmission oil pan. Refer to Automatic Transmission Fluid/Filter Changing .
  2. Inspect for contaminated fluid and excessive material in the pan.

Is the fluid or the pan contaminated?

--

Go to Transmission Overhaul Procedure, in Unit Repair

Go to Step 10

10

  1. Inspect the 1-2 SS Valve for contamination or damaged seals.
  2. Inspect the 2-3 SS Valve for contamination or damaged seals.

Did you find the condition?

--

Go to Step 12

Go to Step 11

11

Inspect the following components for contamination or sticking:

  1. The 2-3 shift valve
  2. The 3-4 shift valve

Did you find the condition?

--

Go to Step 12

Go to Transmission Overhaul Procedure, in Unit Repair

12

In order to verify your repair, perform the following procedure:

  1. Select DTC.
  2. Select Clear Info.
  3. Operate the vehicle under the following conditions, with road conditions permitting:
  4. 3.1. Ensure that TFT is + 20° - + 130° C (50° - 266° F).
    3.2. Drive the vehicle in 4th gear, with the TCC commanded ON, up to 55 MPH.
    3.3. The TP Angle is 7 80%.

Does the Scan Tool indicate a slip of 20 to + 20 RPM for greater than 10 seconds?

--

System OK

Begin the diagnosis again

Go to Step 1