Im at a loss so hopefully someone can give me some ideas. I have a 97f150s/c4x4, 5.4 with the floor shift transfer case. In the warm weather, all is fine. In the cold(from several degrees above freezing on down), I have no 4x4 light and no vacumn to engage the front cv's. The strange part is when the cab warms up, I hear a switching noise in the pass. side dash, the 4x4 light comes on, and the cv's engage. I know it's cab temp. because I've taken it out of the garage( approx. 35F) and driven 20+ min. with no heat, in 4high and nothing happens. Turn on the heat for a couple of min. and suddenly it's all systems go. When cold, I've checked the vac. connections on the pass. side firewall (under the hood) and there's no vac. Warm the cab and vac returns. Please help as it's winter here already (about 5in of snow and temp. near 0*F).

 

Sounds like moisture has gotton into the vac. line and freezes and thaws when truck is warming. I don't know the total answer but i would think you need to displace the moisture first. Maybe pull off the vac line and let hang when warm or blow it out with some dry air and this may help out.