You have an air operated sprag. The switch is on top of the transmission and works when you shift from first to reverse and so forth... When you have air built up and shift the main tranny from first to reverse and back you should hear a ssssss clunk... that is the sprag being shifted from forward to reverse and so forth.. A second issue is whether your sprag is in fact working...once it is put in proper position by the air cylinder, it has to have a slip by the rear wheels by x percent inorder to start driving the front wheels.. It is similar to the deuce sprag, but air operated rather than mechanical linkage... Do not compare to air shift deuce as it is not the same type system... ACE... oh yeah, the 5 ton sprags can get lazy and may need to be flushed and driven with diesel or kerosene in em for a while to loosen em up....don't forget to put The G0 90 back in when it starts working... ACE