M818 4x4 Problems

[CapHap]

Hi Guys. 23 years old and Brand New to the world of Army trucks so any help is great,
I recently bought a 1968 M818 from the local Forestry commission, I don't understand how the 4X4 is supposed to work. the truck has the high/low shifter and it works fine. when I put the transmission into reverse I can hear air pressure going somewhere and it sounds as if it releases that pressure when I go back to neutral. the front wheels will spin but they don't all the time. Have been in several situations where the back were spinning, and the front would not. and when you have 84,000 lbs of soybeans behind it in the middle of a muddy field I would like to know it is working......lol
I have heard the term Sprag and I have read a few pages on here trying to understand how this is supposed to work but no luck yet, can someone set the record straight for me? Should the front end engage any time the rear tires start to slip or do you have to have it in a certain gear setup??

 

[212sparky]

You do have a sprag in the truck. There is a linkage that controls air to the forward and reverse sprag. You do not want to roll backward while in a forward gear, because it will break the sprag clutches. Also I believe you have to shift into first after being in reverse. For the front end to engage the rears have to be turning faster than the rear is. Good luck draging that much weight around.

 

[CapHap]

Ok so the front end will engauge in High and Low and reverse? And what is the air doing? Theres a control box ontop of the transmission that the air feeds into and then 2 lines run out of it to the transfer case. One supplies air to the front side of the cylinder the other the rear, what exactly is that changing? And if this is a Sprag setup that means theres no way to keep the front end from pulling when i dont want it too?

 

[73m819]

pm sent

 

[CapHap]

Anybody know where to get new o rings for the air shift piston in the xcase? And how about new poppet valves?? Thanks

 

[seano11]

Boyce equipment was the lowest for poppet valves Memphis was the lowest for air cylinder kits. Boyce was the lowest for new sprags.