Help! Crazy! Showing codes; p0740, p0785, p2761. Also ABS , E- brake , 4x4 lights up. Won’t start “ most” times, but I’ve found by tapping fuse box under hood it starts. I’ve check all grounds I know of.
If you can get a hold of the wiring schematics for this truck, see which circuit is common to each of them.
Thank you!My bet is that it's your ignition switch. They're cheap (but it's considered an electrical part, so likely no 'return' privileges.) While you have a Sierra instead of a TB/Envoy, the symptoms look similar.
If you purchase one, make sure to get a genuine GM part (AC Delco, I believe; the other option would be Delphi). No others for this one.
Search the threads here for 'ignition switch', and see if you don't agree. You'll also find helpful tips for installing it (and some on diagnosing it, I believe.) Also, check here for a good YT video by one of our now inactive (I won't say 'former') members who has some really good diagnostic skills (he's a mechanic, by trade.)
It’s a new battery, but I’ll check it! 2004 5.3 270 thousand miles😳Also check your battery voltage. Should be 12.6 or higher. If it's closer to 12 or lower you could have a bad battery. That will make your computer go crazy because there isn't enough voltage to fully power the sensors. I bet if you try driving it, it's in limp mode.
What year, engine, and miles?
I’m going to try ignition switch in morning. I think I’ve check all grounds, but where about is the wheel well ground? Which side?--^-- +1.... and this video is one from some of our Hispanic Brothers (GM Mechanics) that... even in the complete absence of being able to speak the language... you will still be able to dope out how the VOP (Video Original Poster) performed his diagnostics on a similar class GM Truck by using a DMM first to check the fuses in the circuit and later to use a Snap-On Diagnostic Scanner to discover that the Ignition Switch was having sketchy voltage issues that caused the Transmission Control Solenoids to get "Wonky". I watched the entire hour + long video and have tried to distill the VOP's discovery down to this start location (around 50 Minutes along) of the critical stuff and have provided a single screen print image of the Suggested Repair Procedures. I hope that this is where the problem lies... and remember that last suggestion posted above here is for you to "Check The Grounds...":
From 50 minutes onward....
View attachment 86112
But... Please bear in mind that if the "problem cascade" is NOT located in the Ignition Switch... then this should lead you to getting under the vehicle to diagnose the "PINK" Wire inside the Transmission Connector Harness leading from the (Mickey Mouse) Ignition Switch Wiring Harness that services ALL of the Transmission Solenoids in the Shift Sequence... any one of which could ALSO have failed enough to make the Adjacent Solenoids inoperative. Make sure that if you "puncture probe" to examine voltages of any wires in that harness ...that you paint those areas over thoroughly with "Black Liquid Electrical Tape" to ensure that Road Salt does no enter into the wire insulation and cause it corrode the copper strands inside enough to fail over time.
The VOP shows just how thorough he was in keeping after the issues one at a time and finally "running these problems to GROUND". In this case... the Diagnosis lead to the need to run a Bad Wire By-Pass in the Ignition Switch Harness to finally solve the problem. We could use some help here from our Spanish speaking Members in translating exactly what the VOP said he did on specific wires nested inside that harness that became the focus of his solution.