How do I align headlights on 91 GMC Sierra Truck

Yes the headlight are left on auto all the time. Don't forget you have daytime running lights (DLR) which are a function of the headlights but not a s bright. So the headlights would appear to be on during the day, but it is the DRL system that is doing that.

You might look for a loose or corroded ground. Any problems with the horn or wiper wash motor? I assumed a 5.0L engine. Info I see says there is a ground splice point and a couple of common grounds. If those systems are affected, it would be even more likely. Check fuses as well.

I have a 2003 GMC Sierra Z71 Extended Cab and the headlight switch won't turn off the headlights while the truck is running. When I got the truck the Photocell Sensor which is the black bubble that sits on the dash that tells the daytime running lights to turn off and on was down in the dash so I fished it out with my fingers and clipped it back in and it made the daytime running lights work. I still can't turn my headlights off while my truck is running though. The owner's manual says that I am able to do so. There's 2 light's on the switch by AUTO and the O for off that should light up? Is it my whole switch?

Possible battery Voltage issue or more likely bad ground wire connection on light relays / chassis

there is to tabs right behinde the headlamp you pull up an can remoce headlight to replace bulb easier. then just twist bulb out an make sure not to touch the new bulb with your fingers. but if your have what they call a sealed beam then there is 4 screws in front of headlight that hold it in.

Heres the cheap old school way of re-aligning the headlights:

Find someone with a similar size height and weight GMC or Chevy truck and have them park 20 feet away from a brick wall or any wall when it is dusk and you can still see somewhat good. Have them turn on their headlights and get you some duct tape and tape 2 x's where you see the left and right beams hit the wall. Then thank your friend and have them drive off.

Now you park your truck in the exact same spot they did and turn your headlights on. Then run out in front of your truck and you will see some screws that you have to turn above and to the side of your headlights. These screws control the vertical and horizontal aiming. Turn the screws clockwise or counterclockwise so that the left and right headlights are pointing to the duct tape x's you got on the wall.

There you go old chap. Cheap Cheap fix for ya without paying the mechanics fee.

Cheers

I would start by checking/cleaning the connections in the headlight circuit. Poor connections will create resistance in the circuit, causing heat which can melt wiring and connectors. Hope this helps!

Look up under dash where brake pedal pivots, you should see a switch that is activated by pedal arm.

This is a pretty strait forward process but if you have never done it take it somewhere and ask them to show you how If you turn the adjustment screws it will be out of alignment or they will strip and your headlight alignment will never be right. You could also purchase a chilton or haynes manual for about $20 and it should be covered. You can get these at any parts store

i would try replacing headlight switch....