I am trying to help my boss find out whats wrong with his Foreman 400. It is 4-5 years old and is used on a farm. He says it has full time 4 wheel drive but the problem is that only one rear tire turns when going up a muddy hill. Anyone have any idea what the problem can be???

 

If we are talking rear tire here, you have a problem with the hub and/or axle, and riding it is just going to do nothing but more damage!

These machines have solid rear axles, so the rear tires should always turn together at the same speed.....so this problem has nothing at all to do with the "4-wheel drive" mechanism as your title implies.

The hub is splined onto the axle and held in place by a nut. Most likely the nut worked loose and being slack has stripped the splines (???). He is most likely in for both a new hub and axle.

If it is the axle, it won't be a simple job like on a chain driven sport quad, because you have the rear gear case to mess with as well. First thing to do is pull that hub off and see what the actual damage is. Then get a service manual and see what all will be involved. Might want to just replace the bearings too while you are in there (?).