03 Chevy Blazer 4x4. No power to door locks, radio, dash instruments, dome lights, 4x4 switch except when I turn on headlights & pull high beam switch to on position then everything works. Tried changing headlight switch and examined main circuit board with no change. Any suggestions would help me track this problem

 

It sounds like you are having trouble with one or more of the contacts inside the ignition switch and it is back feeding power from the headlight switch. You can find the relevant ignition switch wiring schematics in the following link as well as some pictures showing what goes wrong inside the ignition switch: Anatomy of the Ignition Switch

This is a very common issue in these trucks that can cause some very weird problems to occur. You can verify that this is the issue by testing for power at the appropriate pin(s) on the main ignition harness under the steering column using an appropriate back probes (link at bottom) so as to not damage the terminals while leaving the connections engaged. I would bet that you are not getting +12V power to the IGN0 & IGN3 circuits (cavities C1 & C6 in the center portion of the ignition harness connector). Test these and report back with your findings. You can go through all of the ignition sources and verify that they have +12V when they are supposed to and not when then are not supposed to while you are down there as well.

Since I hate how amazon links show up, I put them at the bottom instead of in-line like I normally would...

If you have a digital multimeter with test leads having removable probe tips, here is a set of back-probe tips that I have used in the past and like --
Using back probe tips like this allows you to probe into connectors while they are fully connected to see live power/ground conditions where a typical fat tipped probe would not be able to reach.

 

Thanks Swartlkk, I am going to try replacing the starter switch to see if that solves my problem. If not then I have to look deeper.
RickR

 

Have you made any progress in repairing your truck Rick?

 

I have connected power to a red wire under the dash that seemed dead after checking the ignition switch lines, the headlight wires the 4x4 wires and any wires I could see. Now I have power to the dash cluster, the dome lights, radio and 4x4 switch that I didn’t have before. The tachometer act erratically when I start the engine, flying all the way to the right then back to idle speed. Not sure why. Still no door locks or hatch switch. Generally pleased that at least I have a speedometer while driving. A bit concerned that my prodding could lead to more problems down the road. I’m going to see if I can trace the origin of this red wire to the source of my problems. With no training in this area I’m very weak at understanding wiring diagrams and how to translate their information to what I find under the dash. So many wires!!!

Thanks for your help & support so far. At least I have made a bit of progress on this issue. Sorry I can’t provide information that would be more valuable to others with electrical issues.