P0756. p0700 need help

Need help. P0700 p0756
06 Chevy express 6.6 Duramax 4l85e trans

Trans goes right in reverse no problem
Delayed engagement into 123D and when engaged transmission will not shift up or down

Replaced internal harness all sensors internally replaced input output speed sensors
Traced wires looking for and broken chaffed or shorted to ground found nothing

Transmission has pressure 88 psi at idle 78 psi at idle in D
115 psi in reverse at idle
Pressure increases nicely on acceleration
Replaced TCM

Checked all fuses and relays

Ecm has power and ground also has 5v while back pinning the connectors

Need some help or suggestions

Engine starts and runs fine will it shift

Could you post a wiring diagram if you have one? Can you also post what it takes to set the p0756 if you have that information?

Doing some reading it sounds like a mechanical problem and not an electrical problem.

hate to say it, but may have to open up valve body and free the stuck valve...

Did either of the shift solenoids get changed while you had the pan off?

The code for SSB likely means that you're starting out in 2nd or 3rd gear, does that sound correct?

If you manually power each solenoid, can you hear them click? That'd give you a good indication if mechanical movement is happening in the solenoids.

Yes I replaced the solenoid and I did ohm the new ones as well as tested with 12v to make sure they were operating correctly also just as I did to the original parts pressure switch solenoid a & b tcc pwm. I believe that’s the correct names

I will drop the pan tomorrow and see if I can figure out or find anything obvious

Someone else told me it’s a Failed oring inside the trans leaking the pressure from one of the pistons

P0756 (6.6L)




Circuit Description
The 2-3 shift solenoid (SS) valve controls fluid flow acting on the 2-3 shift valve. The 2-3 SS valve is a normally open exhaust valve used with the 1-2 SS valve in order to allow for 4 different shift combinations.

When the transmission control module (TCM) detects a 4-3-3-4 gear ratio, then DTC P0756 sets. DTC P0756 is a type C DTC. For California emissions vehicles, DTC P0756 is a type A DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:

DTC P0756 2-3 Shift Solenoid (SS) Valve Performance - No First or Second Gear

Conditions for Running the DTC
No MAF sensor DTCs P0101, P0102 or P0103.
No MAP sensor DTCs P0106, P0107, or P0108.
No TP sensor DTCs P0120 or P0220.
No VSS DTCs P0502 or P0503.
No AT ISS DTCs P0716 or P0717.
No TCC stuck ON DTC P0742.
No shift solenoid electrical DTCs P0753 or P0758.
No transmission component slipping DTC P0894.
No TFP manual valve position switch DTC P1810.
No TCC PWM electrical DTC P2761.
The engine is running for more than 7 seconds.
The AT OSS is greater than 7 RPM.
Not in 4WD Low.
The AT ISS is greater than 7 RPM.
The system voltage is 10 - 18 volts .
The TP angle is greater than 10 percent.
The transmission fluid temperature (TFT) is greater than 20°C (68°F) and less than 130°C (266°F) .
The engine torque must be between 108 Nm (80 ft. lbs.) and:
4.8L: 542 Nm (400 ft. lbs.)
6.0L: 576 Nm (425 ft. lbs.)
Conditions for Setting the DTC
The commanded gear equals 1st and the ratio equals 4th for greater than 2.5 seconds.
The commanded gear equals 2nd and the ratio equals 3rd for greater than 2.7 seconds.
All conditions are met for 2 occurrences. Action Taken When the DTC Sets
The TCM illuminates the malfunction indicator lamp (MIL) for California emissions vehicles.
The TCM commands a soft landing to 2nd gear.
The TCM commands maximum line pressure.
The TCM freezes transmission adaptive functions.
The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Freeze Frame, California only, and Failure Records, California and Federal.
The TCM stores DTC P0756 in TCM history.
Conditions for Clearing the MIL/DTC
For California emissions, the TCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
A scan tool can clear the MIL/DTC.
For California emissions, the TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission related diagnostic fault occurring.
For Federal emissions, the TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without a non-emission related diagnostic fault occurring.
The TCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the TCM.
Diagnostic Aids
Verify that the transmission meets the specifications in the Shift Speed table. Refer to Shift Speed.
Other internal transmission failures may cause more than one shift to occur.
The customer may have a concern with an engine over-rev condition or neutral condition in 4th gear.
DTC P0756 defaults to an elevated line pressure condition which may result in partial TCC apply. This may produce an idle surge that could stall the engine.
Test Description
The number below refers to the step number on the diagnostic table.
This step tests that the TCM commanded all shifts, that all shift solenoid valves responded correctly, but that all the shifts did not occur.

Step 1 - Step 5