If it doesn't kick on after EXT4ME's test, check the fuses
Borrowed from suncoreindustries.com:
There are three fuses used specifically for the suspension, and if one fails it could be the cause of your issues. The Real-Time Dampening fuse is located in the under hood fuse block, labeled RTD and is usually a 30 amp fuse, and also located in the under hood fuse block is the STUD #1 fuse, which is often a 40 amp fuse. One more fuse is located in the instrument panel fuse block, labeled IGN 3 and is often a 10 amp fuse. Checking these with a multimeter on continuity will let you know if any are blown, and your suspension problems could be solved by simply replacing one of these components.
Well, tried putting my 240 lbs. on the tailgate and that got no results to kicking on the compressor. So on to checking fuses, and under the hood. My 13 Owners manual shows under the hood #2 for Auto level Control Exhaust looks good, (my #1 shows right trailer stop/turn) # 54 which is labeled Automatic Level Control Compressor Relay, also looks good and