My 96 1500 5.9 has an issue where the blower motor turns on *sometimes*. I don't know what the issue is, it is intermittent on all settings whether it's the a/c or heater im trying to turn on. It used to turn on pretty well when it was cold and after warmup it would be stuck off till I hit a bump, or got the the freeway. These days, it's mostly just broken. I've been searching high and low but the search results get mixed with a common issue of the 05 ign switch and lots of stuff for Cummins. I did actually replace the ignition switch today after my lock cylinder broke. I figured an extra $40 and maybe that was it since I was in there anyway what's 3 extra screws and 2 clips. I know the switch is vacuum controlled but I replaced the check valve already as well. Any other troubleshooting tips? Or a manual on taking apart the dash/ how to get back behind the switch? I think it's just the selector/ vacuum line broken

 

The heater blower motor? Just uses 12v from battery. Either need a new resistor or motor or both. Easy to replace either.

 

Although it's also possible the motor itself is bad. A third possibility is that you have leaves/debris jamming up the blades on the fan. These trucks don't have a cabin air filter, so lots of "external" stuff seems to find its way down into the plenum box. I found all sorts of goodies inside my fan when I replaced it several years ago. In fact, before I replaced it, I once even had a leaf fire inside my plenum box. That was fun!

If you check my profile, you should find a link to my YouTube channel. I'm FAR from an expert, so take my content for what it's worth...but I have a video showing the blower motor and blower motor resistor replacement if you are interested.

Good luck...whatever you find, it should be a pretty straightforward fix.