I have an 05 f250 6.0 4x4 works if i manual lock the hubs if i put my defrost on high the hubs will auto lock if defrost is off hubs won't lock anyone know what's the cause or where to start looking. checked most of vacuum lines all look ok anyone got an idea thanks

 

need to follow my threads. i'm still tracing them down. also, shoot me pm if u have questions. i've made significant progress. i've been changing every main line and vacuum pump, pulse hub solenoid, and lines. with regards to vacuum lines to the hubs, u can get the steel braided ones. about $90. i think diesel site sells them. i've also regressed and replaced the o-ring sticker and bolts on the auto-lock-out hubs. good luck. i'm sure someone else will chime in later.