I have a '91 W250 and 4wd has stopped working. I had the hubs on "Free" all summer but now that the first snow has fallen I locked the hubs in and nothing happened. The 4wd indicator lights are working but the front axles aren't engaging. Could something be frozen? What actually happens when the hubs are in the "Lock" position to engage the 4wd? Be gentle because I am a complete novice at 4wd.

 

try greasing the front hubs, mine did the same thing to me and they were dry put some grease in and they work fine

 

When you lock in the hubs a set of external splines on the hub engage with a set of internal splines that are attached to the axleshaft. So locking the hubs completes the link. when the hubs are free the wheels are not connected to the axleshafts, so each spins independantly. BUt anyway, like SMOKE'EM said, grease em up good and they should work. One question, when you engage 4X4, does the front driveshaft turn? if so, then the problem is indeed in the hubs, but if not, somoething is not right ni the transfer case.

 

You should be able to isolate the problem by grabbing the front driveline and seeing if it spins with the T-case in 2wd and the hubs locked and vice versa.

The hubs do not physically force the spline engagement when you turn the lever. They apply a spring load to engage splines, and vice versa when you disengage, that way you can actually turn the hub when things are loaded and/or do not line up.

Hubs are REALLY simple, go ahead and take them apart, 6 small allen heads and a large internal snap ring and the whole hub can be pulled out. Pry the snap ring out with a screwdriver. you will figure out how they work really quickly and can diagnose what is wrong... also remember that you will NOT have 4wd if only ONE hub is bad, because of the open diff in the front.