sprag/overrun testing

[rizzo]

I'm ready to hook up the air lines from the valve on the trans cover to the t-case. I want to test them afterwards to make sure they are hooked up correctly. when I have the trans in 1rst gear and jack up the front wheel which way should it turn?

 

[DDoyle]

Like you are going down the highway. It should only turn a minimal distance the other way.

Regards,
David Doyle
West Tennessee

 

[rizzo]

thanks for the confirmation. I spent some time going over it in my head and wanted to make sure I was right.

 

[Junkyard Wars Crusher]

Have you considered adding a valve to enable you to "disengage" the front drive?

It is a fairly simple and inexpensive modification. I have made the change to my M51, and it seems to work well. ( However, I'm not sure if there are any long-term ill-effects. If anyone knows of any, please speak up!)

I can eloborate on the procedure if you like.

 

[rizzo]

Have you considered adding a valve to enable you to "disengage" the front drive?

It is a fairly simple and inexpensive modification. I have made the change to my M51, and it seems to work well. ( However, I'm not sure if there are any long-term ill-effects. If anyone knows of any, please speak up!)

I can eloborate on the procedure if you like.

I havn't thought of that. I would put a valve on the supply side and shut that off. is that how you did it? did yo get any better mileage or anything? what were your reasons?

 

[DaveP]

DDoyle is correct. just had my XM818 hanging in the air. had to replace a wheel cyl and while adjusting the brakes the wheels only turned forward.

 

[Junkyard Wars Crusher]

I havn't thought of that. I would put a valve on the supply side and shut that off. is that how you did it? did yo get any better mileage or anything? what were your reasons?

Correct, the valve goes in the supply side. Additionally, you will need a method to bleed residual pressure from the other side of the valve. I used a brass ball valve with a Vented Exhaust from Fastenal.
http://www.fastenal.com/web/products/detail.ex?sku=0427041
You could also use two ball valves and a "tee", or a three way valve.

The main benefit (albeit minor) is to prevent damage to the roller clutch in the sprag - should you accidently roll foward while in reverse, or roll backwards while in a forward gear. . . .I know, we should know better, but when you let someone else drive your truck . . . .

I haven't had a chance to compare mileage - in theory it should improve, although I would expect it to be negligible. Mostly just there for novelty.

 

[Ajax MD]

Question: When performing this test, the air system should be pressurized either by running the onboard compressor or connecting a gladhand, correct?
The engine doesn't need to be running, just get air into the system?

Does the system make a noticeable noise or feeling when the front axle engages?

 

[eric67camino]

When going from neutral to 1st or neutral to reverse on my m51 I can hear the transfer case shift the sprag in the appropriate direction. Makes a thunk when it does.

I've never done it with the air system pressurized without the engine running, but don't know why it wouldn't work.

 

[Ajax MD]

When going from neutral to 1st or neutral to reverse on my m51 I can hear the transfer case shift the sprag in the appropriate direction. Makes a thunk when it does.

I've never done it with the air system pressurized without the engine running, but don't know why it wouldn't work.

Sorry, let me rephrase the question:

I have the normal "sshht/thunk" when shifting between 1st and Reverse. What I'm asking is, during a loss of traction when the rear wheels actually slip and the front wheels engage, is there a noise or feeling to it?

I installed a beautifully rebuilt transfer case a year or so ago. I'm hoping it works right. We've got some snow coming this week... I'm kind of curious to try it out.

 

[G744]

None of mine make any sound when engaging, it just happens. You'll never know when, but it can start to get stuck and then just walk out of trouble like magic.

DDG

 

[Floridianson]

None of mine make any sound when engaging, it just happens. You'll never know when, but it can start to get stuck and then just walk out of trouble like magic.

Most of the time there is a release of air from one side or the other of the transfer case cylinder that can be heard. Might not be much but most of the time heard mine when going from reverse to neutral or neutral to reverse. Getting old can not remember and been a long time since I had a 5 ton.

 

[tobyS]

The rear turns 17 rpm for every 16 that the front turns on the 809 series. The sprag is a one way clutch meaning that in one direction it is locked with a cam motion but in the other direction it allows the inner drum to spin one way. The back, while in forward motion, is always trying to overrun the front. Sprag relieves the difference in one way only. Thus the sprag only sees 1/16 of the wear from turning when driven, compared to not turning one member. Hope that makes sense.

Locking that sprag in the wrong direction with an air valve seems like a real risk also. Maybe I'm not clear about what a manual air valve would do. On the T1138 it brings in the front 1/1 but on T138, it operates the direction the sprag operates to relieve the front/back difference.

 

[tobyS]

Sorry, let me rephrase the question:

I have the normal "sshht/thunk" when shifting between 1st and Reverse. What I'm asking is, during a loss of traction when the rear wheels actually slip and the front wheels engage, is there a noise or feeling to it?

I installed a beautifully rebuilt transfer case a year or so ago. I'm hoping it works right. We've got some snow coming this week... I'm kind of curious to try it out.

Under normal operation there is no noise at all. The inner part rotating is an egg shaped cam that wedges an inner drum and an outer drum with change of direction. The back gear in the TC turns 17 rpm while the front does16. As soon as the back turns free, the cams lock and drive the front. Should never hear it or actively manage it, in my opinion.

 

[Ajax MD]

Under normal operation there is no noise at all. The inner part rotating is an egg shaped cam that wedges an inner drum and an outer drum with change of direction. The back gear in the TC turns 17 rpm while the front does16. As soon as the back turns free, the cams lock and drive the front. Should never hear it or actively manage it, in my opinion.

Ok, gotcha. I tried to slip the rear wheels in some mud last weekend and had no indication that the front axle engaged. I'm not even certain that I was able to cause the rear wheels to slip. In any case, it sounds like there is no noticeable indication.

I guess I'll find out if I get stuck!

 

[Floridianson]

Locking that sprag in the wrong direction with an air valve seems like a real risk also. Maybe I'm not clear about what a manual air valve would do. On the T1138 it brings in the front 1/1 but on T138, it operates the direction the sprag operates to relieve the front/back difference.

What we would be doing is cutting off the air with the valve to the poppets on the transmission. Yes if you had the air system aired up and then to be safe you would shut off the air and move the shifter from reverse to neutral a couple of times to release the air pressure from both sides of the transfer case air cylinder / lines. Also might be a good idea if you truck holds good air even if the truck has not been running to shut off air and drain the sprag cylinder. The transfer case air cylinder has a metal piston with preformed packing seals that hold the air back depending on which way the air is coming from. Then the same piston has two good size springs that mount on both ends of it. With full air pressure it collapses the spring in what ever direction you choose and engages the sprag. With out air the spring is not collapsed and the sprag is in a type of neutral.

 

[jasonjc]

Is there any adjustment for the air cylinder?? Mine seem to be engaged with no air. The case is out of the truck on a stand and I just put it all back together. It's on it back so maybe the weight of the sprag assy is over coming the spring????? Or I screwed something else up .

 

[Floridianson]

If the rod coming out of the transfer case is pushed all the way in or all the way out the sprag is engaged. All the way in reverse all the way out forward sprag. No adjustment. With the cylinder and no air it should be in neutral. Did you rebuild the cylinder piston and see the caps and retainer that holds the spring inside of the piston on both ends?

 

[tobyS]

I think you would have to block up all wheels so the back can turn free to see if the front is engaging. Or some mud, which comes with the risk of having to be pulled out since making the rear turn and not move the truck is the goal (to see if front is engaging).

 

[Floridianson]

Does the system make a noticeable noise or feeling when the front axle engages?

If I remember going from neutral to reverse is made a swish or heard air. Going to Reverse still could hear air release but a thung noise too as the piston / spring hits the transfer case. All my 5 tons you did not have to go into first from reverse to make the sprag switch. Reverse to neutral or neutral to reverse switches the sprag. Air up the truck and shut it down and switch between reverse and neutral or neutral to reverse and you should hear or feel the switch. Any forward gear slected after coming out of reverse the sprag should be in gaged for forward movement.