Hi. I have a 96 5.8l f250 2wd that runs ok as long as I don't have headlights or blower motor on. I have recently put ignition module & alternator on. I had this issue 1 other time before I replaced module or alternator. I stopped & looked for anything unplugged or vacuum line off. Found nothing. Started back up & all was good. When I replaced alternator & drove truck it missed after about 2000rpm & smelled rich. Checked voltage & had 13.7 with nothing on. When I turned on lights & blower motor voltage dropped to 13v. Took alternator back. New alternator i had 14.2 & 14 with lights & blower motor on & it still runs like crap! The alternator is from autozone. Any advice would be appreciated. Thx. Keith

 

From what you describe, you started on the right track by verifying power available to the electrical system..

What you need to do next is verify that power is available to the ign system. The problem could lie in any connection from the battery to fuse panel, ign sw on to coil, ign module etc. You can start start from coil, work your way back to ign sw. You should always have battery voltage at those places with key on and blower or headlamps on.
Let us know what you find,thanks.