NEED HELP 2004 GMC Sierra 2500 6.0L 4x4

[JSM_1985]

I have had an issue with this truck over the last two years that I have been trying to fix and I am now out of ideas.

Symptom:
Truck will not turn over from ignition switch, I can start by jumping the ignition relay under the hood but truck only drives in 2nd gear.

Brand new battery.
New ignition switch.
New transmission solenoids.
New neutral safety switch.

Cleared codes after neutral safety switch was installed (these were A&B solenoid codes) but issue still persists. Not sure where to go next.

 

[Mooseman]

If it's only running in second gear, that's a limp home mode. Are there still any codes?

For the no crank from the ignition switch, you have to get the schematics and check where the power is missing. Need to find if power is being cut by the neutral safety switch or somewhere on the other side of the relay. If the trans is also in limp mode, sounds like a general power issue. Check what circuit feeds the trans and starter relay.

Check my signature for the manuals for the full size trucks. Should have schematics in there.

 

[JSM_1985]

Thank you for info. I will get on that this weekend.

 

[mrrsm]

If your Truck is equipped with an On-Star Module... Check Out THIS Video for an interesting No Crank No Start solution on a Later Model 2500::

And eyeball the Ground Wire at the Passenger Side Rear of the Engine Block for having a "Chewed Through" Ground Wire for Rodent Activity:

So... You Need 12-14 Volts DC Power EVERYWHERE, Right? Well Make DAMNED SURE to Check and Repair ALL the Engine and Body Ground Straps. They are Delicate .... They Corrode...They can BREAK... and if they Break... ALL SORTS OF WEIRD PROBLEMS ELECTRICAL PROBLEMS WILL HAPPEN:

More On Grounds:

Remember... Once you have performed all of these "Ground Improvements"... You MUST "Clear All Codes" afterwards in order to remove the PCM Limp Mode Status and ensure that it remains operational during and after your Test Drive. Closely Observe your Dashboard Lighting for ANY MIL or Code Lights during your Drive and jot them (all) down if further Diagnostic Actions become necessary.