Now I know the first response to this is going to be "USE SEARCH" lol But I have been searching through this forum as well as the web and im about to break down and bring it to ford, but I hate doing that when it something that I could fix. So let me tell you what the problem is. I have a 2001 f150 4x4 super crew with shift on the fly 4x4. Now 4 high works just fine, when I flip my switch to 4 low from 4 high when in neutral and foot on the brake no clicking from relays, no response at all. I checked the relays with multi meter they test good, I checked the voltage on the switch in all three positions and it reads exactly what it says it should be in the manual. Only one of the solidoids in the engine bay on the passenger side fire wall is clicking(guessing its the on for 4 high).. Checked for vacume leaks NONE Checked all around the transfer case for loose conections and found none! I am really starting to go crazy I have to get this fixed because leave for Colorado the begining of Febuary and it would be nice to have it in the snow. If you guys can tell me anything that I have missed or anything else that I can check or check again another way I would really appreciate it...
Thanks
Alex

 

Check for a bad brake switch or brake lights are out. Anything that would tell the system that you are not in neutral and do not have your foot on the brakes. It would not be the front vacuum system as you have 4hi. It has to be something that says you are not following the procedure.
Could also be the switch.

 

Ya I just checked out the brakes( fuse/switch) and all seems to be well. Im going to see if I can find somebody to lend me there 4x4 switch to see if that works. Im not about to start buying stuff untill I know exactly what the problem is.

 

That's funny, same exact thing happened to mine this past Saturday doing a little off-roading. It would go into 4 hi just fine but wouldn't go into 4 lo. By the time I tried it a few more times I finally got it to go into 4 lo but it wouldn't come out of it. I drove for probably 30 minutes in 4 lo then finally I tried it one last time and it came out of 4 lo and went through all 3 modes just fine. I found my problem to be that I wasn't hitting the brake pedal fast/hard enough which is a probable faulty brake switch. 2004 F150 SuperCrew 4x4 5.4 NBS