07 Suburban C0379 , P0756 Trans in limp

[smokymtn65]

07 Burb guessing not super common don't find a lot posted. The service 4wd is on. All functions of 4wd systen fuction with bidirectional scanner. Trans will shift fine manually but defaults to 3rd gear. Looking for bad grounds now. Is there a"common" failure known?

 

[smokymtn65]

Showing no ignition voltage to trans module.

 

[smokymtn65]

Where does the TCM get it's input power from?

 

[NickTransmissions]

Assuming you have a 4L60E, P0756 is a fault code for shift solenoid B. If any one of those solenoids or the internal trans wiring harness fails or shorts you be put into limp mode. Drop the pan and ohm all your solenoids, starting with shift solenoid B, which is located at the 2-3 shift valve train (its the solenoid on the left side of the valve body closest to the 1-2 accumulator housing - which is the large, cylindrical object on the end.

 

[petethepug]

Whoa, that’s better than taking your truck to a good, reputable shop and getting a fair estimate for an in house rebuild, Jaspar or GM certified replacement.

That tidbit of info is more like the beer truck pulling up to your vehicle with a tv car repair show in tow. They all jump out, hand you a beer, circle your car and explain what’s going on.

 

[smokymtn65]

This trans had about 40k on rebuild
Mostly highway no towing. Have 12v to one of the pink wires to tcm and 12 to red/ white.
My diagram shows two pink on from fuse 19. Which i have 12v, to the one that shows it goes to tcm other pink shows it going to 3-2 solenoid. Question is does it go from fuse block to the tcn plug/junction block or directly to trans

 

[smokymtn65]

Went through cleaning grounds and connectors, and now have p0700, U0101, on ecm, and U0073 on tcm

 

[smokymtn65]

Dropped the pan, both shift solenoids ohm out good, then added a 9v battery both click. Harness ohms out good. Have 12v on E in run. The valve is extremely hard to move. Will a stuck valve set the code same as a dead solenoid?
@NickTransmissions

 

[smokymtn65]

Going to add a note here, when i dropped the pan i had found a pc of what looked like a piece of gasket material in behind first valve in 2-3 valve train. The back valve would not come out with any trick i found. It moved some but not more than 1 in. I pushed it back in place and buttoned everything back up. Truck was still in limp mode. 4 days later it started working properly. Very odd. I did find a gm tsb that the P0756 is mechanical related to 2-3 valve. I will update if it changes.

 

[NickTransmissions]

Going to add a note here, when i dropped the pan i had found a pc of what looked like a piece of gasket material in behind first valve in 2-3 valve train. The back valve would not come out with any trick i found. It moved some but not more than 1 in. I pushed it back in place and buttoned everything back up. Truck was still in limp mode. 4 days later it started working properly. Very odd. I did find a gm tsb that the P0756 is mechanical related to 2-3 valve. I will update if it changes.

Yea debris on the bore obstructing the valve's movement can potentially cause a dtc to set for that solenoid though ive heard of builders blocking that valve inboard for quicker 2-3 upshifts but have never done it myself. If it happens again, Id drop the valve body and go through it (all valves out, clean it in a parts washer or hot tank) and install a Transgo shift kit and convert the pwm tcc apply to an on-off config using Fitzall's valve kit ($15.00 or so).