My 1986 k5 blazer is acting up. It was fine but has been sitting for a month. I jump started it today and it ran fine. The voltage gauge goes up and down with the blinkers so im guessing the alt. is fine. the moment i shut it off the car would not start. I got the battery checked at sears and they said it was fine but the machine is not always right. so i came home hooked up the battery and drove the truck for about 15 minutes and the car would not start again, I think its thee battery, got any ideas

 

If your battery isn't being charged by the alternator it will do exactly what you described. What does the voltage gauge read at idle? Most parts stores will check alternators for free so take it to one and have them check it.

BTW this forum is for f-bodys only so expect it to be moved.

 

so do you think the battery is junk, or the alternator?

 

Battery. if its your alternator, you'll notice your lights get dimmer as your driving. Happened to me last summer so I got a new alternator. Also my battery went dead over the winter while it was in storage, and was doing tha same thing as you. I got a new battery and the problem is now fixed.