M-817 4WD Operation

[PeteNora]

The sprague unit works mechanically. The reason for air is to set the direction. When one shifts the transmission into first gear it sets the sprague into forward. When the transmission is placed into reverse the sprague is then in reverse. That is why one can’t coast backward when the unit is in forward. Same thing applies for reverse. Unlike the M35s, the five-ton sprague units are usually quite durable.

Ok that is beginning to make sense. So on the transmission cover there must be air control valves?

 

[Scrounger]

There is a supply line to the transmission and two lines going from the transmission to the transfer case. On the top right side of the transmission is the poppet valve assembly. Once the air tanks are charged, one will hear the poppet valves cycling when shifting into first or reverse. It doesn’t happen often, however sometimes the O-rings in the poppets start to leak. You will know because you will hear an air leak. If it should ever happen it is a simple repair. Remove the floor panel, remove the poppet assembly, disassemble and clean unit, replace O-rings and reinstall.

 

[PeteNora]

Thanks. My thick head has a pretty good grasp of the systems operation. I think for now I will leave it alone. I hope it woks ok as is. I will be bringing it to home or camp once the snow goes. I went up to look it over again yesterday. I sat in the seat and made notes. I was able to answer most of my questions. The help given here has been great, I have other questions but will address them separately. Thanks again.

 

[fasttruck]

Reference post 20: Scrounger is correct about the operation of the 809 series transfer case. The deuce has a rod going from the transmission cover to the transfer case to do this. The air operated device is to engage and disengage the Sprague. Only the newer trucks have this feature.

 

[Floridianson]

The sprague unit works mechanically. The reason for air is to set the direction. When one shifts the transmission into first gear it sets the sprague into forward. When the transmission is placed into reverse the sprague is then in reverse. That is why one can’t coast backward when the unit is in forward. Same thing applies for reverse. Unlike the M35s, the five-ton sprague units are usually quite durable.

When the transmission is shifted from reverse to neutral the reverse sprag switches to forward sprag and any forward gear you pick. That is why you can not use neutral to back / coast down a hill because it is in forward sprag.
Air leaking can also be the transfer case air cylinder preformed packing and is the problem most of the time I have found.

 

[msgjd]

When one shifts the transmission into first gear it sets the sprague into forward....

you really should edit your post, this statement in your post is incorrect ... Just listen to your poppet valve next time you are in the truck.. Reverse and neutral on the tranny shifter are the rev/fwd shift points on the transfer case .. This method applies to the M809-series and M39-series 5-tons equally the same

 

[BKubu]

you really should edit your post, this statement in your post is incorrect ... Just listen to your poppet valve next time you are in the truck.. Reverse and neutral on the tranny shifter are the rev/fwd shift points on the transfer case .. This method applies to the M809-series and M39-series 5-tons equally the same

Unfortunately, he can't edit the post as he passed away last August.