PO751 code on 2003 GMC Sierra

[Brian Lee Rodgers]

I have 2003 gmc sierra 4x4 5.7 liter vortex engine with 4L60 transmission. I have a PO751 code. 1st gear and lockout gear works only at first start up. Once driven an few miles I loose 1st and overdrive. I've replaced with a rebuild transmission and problem reoccurring and had that one rebuilt and and replaced PCM. Code and symptoms are back.  Any suggestions???

[newdude]

50 minutes ago, Brian Lee Rodgers said:

I have 2003 gmc sierra 4x4 5.7 liter vortex engine with 4L60 transmission. I have a PO751 code. 1st gear and lockout gear works only at first start up. Once driven an few miles I loose 1st and overdrive. I've replaced with a rebuild transmission and problem reoccurring and had that one rebuilt and and replaced PCM. Code and symptoms are back.  Any suggestions???

 

 

Test the wiring for the 1-2 and 2-3 shift solenoids. 

 

The 1-2 solenoid is a light green wire, the pin at the PCM is pin 48 in connector C2 (green connector) and at the transmission its pin A in the round transmission connector.  

 

The 2-3 solenoid is a yellow wire with a black stripe.  Pin at the PCM is pin 47 in connector C2 (green connector) and at the transmission its pin B.  

 

Check for power and check terminal tension and corrosion.  Also, you have a 5.3 engine not a 5.7.