1993 GMC Sierra Power Door Locks

First check for broken or frayed wires between the door hinges.

Do you have a voltmeter or test light and know how to use them?

Dandy. I'm not a fan of auto-ranging meters. I have over a dozen digital meters for tv and car radio repair, and most cost less then 40 bucks each, and none are auto-ranging.

Pop the driver's switch assembly out and look for the orange / black wire. That one, it appears, should have 12 volts all the time. If it does not, I'll have to search through the wiring diagrams to see where it comes from. Use something on the body, like an unpainted bolt head, for the ground for your meter.

If you have that 12 volts, move the meter's ground lead from the body to one of the black wires in that plug. If you still find 12 volts, move the ground lead to the other black wire and try again. Something there has to be missing. If you have all those 12 volts, measure again with a test light or while you're activating the switch one way or the other. We're still looking for one of those voltages that is missing. If one goes missing only when under load, (activating the switch or when using the test light), there is a break in the circuit and a carbon track has been left behind. Carbon tracks will pass just enough current for a digital voltmeter to falsely say the circuit is okay. This is where a test light can be more accurate.

Look at the fuse box inside the truck. You should see four columns of fuses and five in each column. In the last column to the far right, look at the second one from the bottom. That should be a 30 amp circuit breaker. It feeds the "Convenience Center". Pull that breaker out, then measure for voltage on the left terminal. You should find 12 volts there.

If you have that, reinstall the circuit breaker, then you have to find the Convenience
Center. I don't know where that is, but it has the signal flasher and horn relay in it. I've never seen a diagram so impossible to read, but it appears this is just a junction box for numerous circuits. The circuit that feeds the power lock switches also feeds the power seat and power mirror circuits. If you have either of those options and they work, that circuit breaker has to be okay, and the circuit to the Convenience center has to be okay. That just leaves the wire between the door hinges.

If you have to tear the dash apart, lets look for an easier route because I'm betting there's a 50 percent chance the problem is still a broken wire between the door hinges. Where I would start is by removing the trim panel from the left front, and look in there for a connector where you can back-probe the wires. Look for the orange / black wire, and if you have 12 volts there but not at the switch, you gotta have a bad wire.

I'm really not a fan of piercing a wire's insulation to take a reading, and I never allowed my students to do that, but in this case where the Convenience Center is hard to reach, I'll let you do that if there is no connector. Just be sure to seal the poke up with silicone gasket sealer when you're done to prevent corrosion. Don't use electrical tape because that will unravel into a gooey mess on a hot day.