The sprag is controlled by a valve on the trans tower and an air cylinder on the front of the transfer case. The sprag is shifted in the forward drive position in neutral and any forward gear. It is shifted to reverse drive only when in reverse in the trans. Build up air pressure in the truck, kill the motor, then shift the trans from neutral to reverse. You should hear a clunk sound under your feet each time you move the lever. If you do, the air shift system is working. If you do not, there is a problem in the air supply or in the cylinder. Check the air supply to the velve and beyond, and if you have it working right, then take the cylinder apart and check for set-up grease or crud in it that is hindering its operation, especially in cold weather.
Another possibility is that if the air tanks have water in them that has gotten into the rest of the system, it could be frozen in the lines or valves causing the problem. Drain the air tanks regularly to prevent this problem, but you have to do it when it is warm enough the water is not frozen. Warming up the whole truck will let the air tanks be drained and water in the lines be purged.
Regards Marti
