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there is a front air pump that locks the front differential in 4wd. but there is also a switch on top of transfer case that lets ecm know truck has been shifted into 4wd and it then turns on compressor. It is possible that you have a bad air pump,but I would lean toward this switch. it will be a lot cheaper to change.
Transfer 4WD Switch Removal and Installation 1. Remove console box and lift transmission control lever boots No. 2 and No. 3.2. Remove transfer control lever boot No. 2 (3), unclamp wiring on transfer case to free it and pull off coupler (1
this switch also controls the light in the dash
Have you checked this switch? the website says you have replied to this answer but i cannot see any new info. if you did please repost thanks