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I have an 03 SD CC 6.0 with the electric 4X4 switch. I have noticed lately that it seems to only go into 4 Low when it wants to. I have the truck in N and 4 hi then hit the switch for 4 low and sometimes it goes and sometimes it doesn't. Also a few times I had trouble getting it out of 4 low.
Trans in N. Foot on brake. Truck not moving. Flip the switch, wait a few seconds, and it should go in or out of 4Lo. If not, put it in gear, move the vehicle a foot, repeat the above.
If *that* doesn't work, something's broken. There are little motors on the transfer case to make the switch, and it's possible you have one that's on its way out. (But that's pretty rare; usually it's the hubs that have problems on the ESOF system!)
That was it. I didn't realize there was an interlock between the brake pedal and the 4Low switch.
I think the reasoning is that they don't want you moving when all those big straight-cut teeth mesh, at what could be fairly mismatched speeds even at slow ground speeds. (Anyone who's ever driven an ancient 4WD vehicle, like the 1960 Land Rover I used to have, knows exactly how awful it sounds when you do that!) Foot on brake = no moving!
I was having the intermittent engagement problem with 4 low. I got stuck once and couldn't get the truck into 4 low. Man was I mad. I felt like a huge moron when I realized I had to have my foot on the brake to make the 4 low engage. (of course this was after I had been pulled out, and 4 low probably wouldn't have done much for me anyway.) Duh.
Luckily I too figured it out before complaining to the dealer.