Circuit Description
The 1-2 Shift Solenoid (SS) valve is a normally closed exhaust valve that, in conjunction with the 2-3 Shift Solenoid (SS) valve, allows four different shifting combinations. The 1-2 SS valve is attached to the control valve body, within the transmission.
The Transmission Control Module (TCM) monitors the actual gear ratio and compares the actual gear ratio with the commanded gear ratio. DTC P0751 sets under the two conditions:
| • | A stuck on 1-2 SS valve. |
| • | A stuck on 1-2 shift valve. |
If the TCM detects a 2-2-3-3-3 shift pattern, then DTC P0751 sets, which is a type 'B' DTC.
Conditions for Running the DTC
| • | Ignition voltage is between 8.0 and 18.0 volts. |
| • | Engine speed is between 450 and 6,800 RPM for at least 5 seconds. |
| • | The following DTCs are not set: |
| - | P0120--Throttle Position Signal (No valid GMLAN signal) |
| - | P0716 or P0717--Transmission Input Speed Sensor. |
| - | P0722 or P0723--Transmission Output Speed Sensor. |
| - | P0742--TCC System Stuck ON. |
| - | P0973, P0974, P0976, P0977, P0979 or P0980--Shift Solenoids Electrical. |
| - | P01815, P1820, P1822, P1823, P1825 or P1826--Transmission Manual Shift Shaft Switch Assembly. |
| - | P02637--Engine Torque Signal Circuit - No Valid GMLAN Signal. |
| • | The transmission fluid temperature is between 20 to 130 degrees Celsius. |
| • | The transmission manual shift shaft switch assembly does not indicate Park, Neutral or Reverse. |
| • | The input shaft speed is 200-6,800 RPM. |
| • | The output shaft speed is greater than 100 RPM. |
| • | The throttle position is 10 percent or more. |
Conditions for Setting the DTC
Both of the following conditions must occur twice during the same ignition cycle.
| • | The TCM commands 1st gear when the engine torque is from 40 - 450 N·m and the reported gear ratio is 2.16:1- 2.27:1 for 1.25 seconds. |
| • | The TCM commands 4th or 5th gear when the engine torque is from 36 - 450 N·m and the reported gear ratio is 1.56:1- 1.64:1 for 5 seconds. |
Action Taken When the DTC Sets
| • | The TCM requests the Powertrain Interface Module (PIM) to illuminate the Check Powertrain Malfunction Indicator Lamp (MIL) in the instrument cluster during the second consecutive trip in which the 'Conditions for Setting the DTC' have been met. |
| • | At the time of the first failure, the TCM records the operating conditions when the 'Conditions for Setting the DTC' have been met. The TCM stores this information as a failure record. |
| • | At the time of the second failure, the ECM records the operating conditions when the 'Conditions for Setting the DTC' have been met. The ECM stores this information as a freeze frame. |
| • | The TCM also turns off the pressure control solenoid (causing maximum line pressure). |
| • | The TCM freezes the transmission adaptive learning function. |
| • | The TCM stores DTC P0751 into TCM history during the second consecutive trip in which the 'Conditions for Setting the DTC' have been met. |
Conditions for Clearing the MIL/DTC
| • | Tech 2 can clear the DTC. |
| • | The TCM clears the DTC from its memory (history DTC) if the vehicle completes 40 warm-up cycles without a non-emission related DTC occurring. |
| • | The TCM cancels the default actions when the ignition is off long enough to power down the TCM. |
| • | The PIM turns off the Check Powertrain MIL after the sixth consecutive drive trip in which the TCM does not send a Check Powertrain MIL illumination request. |
Diagnostic Aids
If you clear DTC P0751 and then cannot reset DTC P0751, the following conditions may exist:
| • | Fluid contamination |
| • | Plugged / blocked fluid circuits |
| • | Restricted fluid circuits |
Refer to Shift Solenoid Valve State and Gear Ratio chart in 1.4 Electronic Component Description - Shift Solenoid (SS) Valves in this Section.
Test Description
The following number(s) refer to the step number(s) on the diagnostic table.
3. This step verifies a 2-2-3-3-3 shift pattern.
4. Checks for short to ground or open circuit in solenoid supply circuit.
5. Checks for short to voltage on the solenoid supply circuit.
6. Checks for short to ground on the solenoid supply circuit.
7. Checks for open circuit in transmission internal wiring harness.
17. This step tests for a mechanical or hydraulic condition causing the 1-2 SS valve to be stuck on, or the circuit to be released.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Automatic Transmission Controls Schematics For information on testing wiring faults, refer to Wiring Systems. | ||||||||||||||
1 | Has the Diagnostic System Check - Automatic Transmission been completed? | -- | Go to Step 2 | |||||||||||
2 | Perform the transmission fluid checking procedure. Refer to Refer to Transmission Fluid Check Did you perform the fluid checking procedure and is level now OK? | -- | Go to Step 3 | -- | ||||||||||
3 |
Are the commanded gear and gear ratio within the specified range? | 1st 2.16:1-2.27:1 4th / 5th 1.56:1-1.64:1 | Go to Step 4 | Go to Testing for Intermittent Conditions and Poor Connections | ||||||||||
4 |
Is voltage reading greater than specified? | More than 6.0 Volts | Go to Step 5 | Go to Step 10 | ||||||||||
5 |
Is voltage reading less than specified? | Less than 0.3 volts | Go to Step 6 | Go to Step 12 | ||||||||||
6 |
Is resistance reading greater than specified? | More than 500 kΩ | Go to Step 7 | Go to Step 13 | ||||||||||
7 |
Is resistance reading within specification? | 15 - 17.0Ω @ 20° C 17.9 - 20.3Ω @ 70° C | Go to Step 8 | If reading is more than specified, go to Step 15. If reading is less than specified, go to Step 14. | ||||||||||
8 |
Does solenoid click as jumper wire is connected? | -- | Go to Step 9 | Replace faulty 1-2 shift solenoid. Refer to 1-2 Shift Solenoid Valve Replacement Go to Step 18 | ||||||||||
9 | Replace TCM. Refer to Transmission Control Module Replacement. Is the replacement complete? | -- | Go to Step 18 | -- | ||||||||||
10 | Using a digital ohmmeter, check wiring for short to ground condition between transmission wiring harness TCM connector X112-T68, terminal X1-8 and transmission wiring harness connector X121-X1, terminal X1-17. Does check indicate a short to ground in the wiring? | -- | Repair short to ground in relevant circuit. Go to Step 18 | Go to Step 11 | ||||||||||
11 | Using a digital ohmmeter, check wiring for open circuit condition between transmission wiring harness TCM connector X112-T68, terminal X1-8 and transmission wiring harness connector X121-X1, terminal X1-17. Does check indicate an open circuit or high resistance in the wiring? | -- | Repair cause of open circuit or high resistance in relevant circuit. Go to Step 18 | Go to Step 9 | ||||||||||
12 | Using a digital voltmeter, check wiring for short to voltage condition between transmission wiring harness TCM connector A112-T68, terminal X1-30 and transmission wiring harness connector X121-X1, terminal X1-14. Does check indicate a short to voltage in the wiring? | -- | Repair short to voltage in relevant circuit. Go to Step 18 | -- | ||||||||||
13 | Using a digital ohmmeter, check wiring for short to ground condition between transmission wiring harness TCM connector X112-T68, terminal X1-30 and transmission wiring harness connector X121-X1, terminal X1-14. Does check indicate a short to ground in the wiring? | -- | Repair short to ground in relevant circuit. Go to Step 18 | -- | ||||||||||
14 | Using a digital voltmeter, check wiring for internal short circuit condition between transmission wiring harness TCM connector X112-T68, and transmission wiring harness connector X121-X1, circuits 1525 and 1222. Does check indicate an internal short circuit condition in the wiring? | -- | Repair cause of internal short circuit condition in relevant circuits. Go to Step 18 | Go to Step 9 | ||||||||||
15 | Using a digital ohmmeter, check wiring for open circuit condition between transmission wiring harness TCM connector A112-T68, terminal X1-30 and transmission wiring harness connector X121-X1, terminal X1-14. Does check indicate an open circuit or high resistance in the wiring? | -- | Repair cause of open circuit or high resistance in relevant circuit. Go to Step 18 | Go to Step 16 | ||||||||||
16 |
Is transmission internal wiring harness OK? | -- | Replace faulty 1-2 shift solenoid, refer to 1-2 Shift Solenoid Valve Replacement. Go to Step 18 | Replace transmission internal wiring harness. Refer to Transmission Internal Electrical Harness Replacement. Go to Step 18 | ||||||||||
17 | With the fluid pan removed, inspect the rear control valve body for the following conditions:
Did you find any of the above conditions? | -- | Repair or replace components as necessary. Go to Step 18 | Go to Step 18 | ||||||||||
18 |
Has the test run and passed? | -- | System OK | Go to Step 2 | ||||||||||
When all diagnosis and repairs are completed, clear all DTCs and verify correct operation | ||||||||||||||
