5 ton truck transfer case question

[Olly]

Long question ; I have an XM818 it has the T-138 transfer case with the sprag. Now I know from what I've researched it has forward and reverse and I've also read that if the truck rolls backwards while in forwards or vice versa it will bind up and could break the transfer case or sprag or something like that. I was thinking about putting in the T-1138 from the M900 series I know that in low gear it automatically engages the front axle, and I also know you can disengage that and put it on it's own switch. My question is if I put the T-1138 in my truck and engage it with a switch would I have to worry about it rolling backwards in forwards or vice versa? I read that the gearing in the T-1138 is different from the gearing in the T-138. Thought about putting my transfer case on a hand shifter but the difference in gearing from what I read would make a mess quickly. Have a lotta snow covered roads in the winter and even more hills. Need to be able to engage 6 wheel drive when I need it and I read that the T-1138 acts more like a 4 wheel drive transfer case on the road with a 1 to 1 gear ratio. I'm not sure on that though so if I'm wrong would appreciate any information. Also on the T-1138 does it have a forward and reverse engagement or is it just engaged or not engaged? There is one other thing if the T-1138 is left in high gear can you reverse I know in low it's a big no-no? I know it was a whole paragraph but I figured ask once instead of a hundred small questions. Any information would appreciated. Thanks.

 

[simp5782]

Long question ; I have an XM818 it has the T-138 transfer case with the sprag. Now I know from what I've researched it has forward and reverse and I've also read that if the truck rolls backwards while in forwards or vice versa it will bind up and could break the transfer case or sprag or something like that. I was thinking about putting in the T-1138 from the M900 series I know that in low gear it automatically engages the front axle, and I also know you can disengage that and put it on it's own switch. My question is if I put the T-1138 in my truck and engage it with a switch would I have to worry about it rolling backwards in forwards or vice versa? I read that the gearing in the T-1138 is different from the gearing in the T-138. Thought about putting my transfer case on a hand shifter but the difference in gearing from what I read would make a mess quickly. Have a lotta snow covered roads in the winter and even more hills. Need to be able to engage 6 wheel drive when I need it and I read that the T-1138 acts more like a 4 wheel drive transfer case on the road with a 1 to 1 gear ratio. I'm not sure on that though so if I'm wrong would appreciate any information. Also on the T-1138 does it have a forward and reverse engagement or is it just engaged or not engaged? There is one other thing if the T-1138 is left in high gear can you reverse I know in low it's a big no-no? I know it was a whole paragraph but I figured ask once instead of a hundred small questions. Any information would appreciated. Thanks.

T1138 is an overdrive unit. The T138 is not. You can roll backwards in a T1138 no issue. You would want it to engage the front axle with a TR1 switch. Meaning air to it its locked. The auto lock in low range is done by a modification washer attached to the shift linkage that was part of an MWO. Using low range without 6x6 is handy as well. Reverse on your 5 spd transmission would be fine in low range on the transfer case since it is not as low as the 10.3:1 of the allison with the torque converter multiplication.

You would have a double overdrive truck with the t1138 so a higher top speed

 

[Olly]

Okay, thanks again for the quick response. Sounds like I got all the answers I needed on that. Was wondering about the splicer 9 speed transmission and would it bolt up another words is the housing the same as the 5 speed? I know old Autocars use to have them and they use to have the 250s-super 250s-270s-290s so on. Just figured 9 is better then 5...Thanks for any opinions. And does anyone have a t-1138 for sale that can ship it to residential by chance?

 

[simp5782]

T1138s are getting pricey. In the 1500 range or more. I've got a source for several 7 ton Oshkosh 30000 single speed transfer cases. It's an under drive 1.36:1 unit . if you were planning on a 9speed with a double OD.

Bellhousing in the engine is an SAE 2 so most common medium truck transmissions will work. You just need the correct input spline setup to work with your clutch. You can go to a heavier Spicer since all Spicer parts are interchangeable it would make it easier


Shipping to a terminal near you would be $200 or so cheaper than to your house

 

[Olly]

How much slower would that 7 ton transfer case make my top speed? Also how much bigger is it from the 5 ton transfer case? Is it like the one you placed in your truck in the videos I've seen?

 

[simp5782]

It is the same as the Oshkosh 55000 and I have the brackets to bolt it into a 5 ton.... However even on 1600R20s with your .78 5 spd transmission in the truck now you would be turning 2150rpms at 50mph. So basically as fast as you are now on the 11R20s. But you could climb a mountain with big tires. a transmission swap with a .65 OD would be better around 2150rpms and 60mph.

 

[Olly]

How much would that transfer case cost with the brackets? Also what else would I have to change like in the driveshafts and stuff?

 

[98G]

The sprag works just fine.... just use the clutch competently and don't roll backwards on takeoff.

If you're contemplating a t-case change to change gearing that's one thing. But if the primary motivator is to get away from the sprag, then it's a solution in search of a problem.

 

[Olly]

I need the ability to have my truck in 6 wheel drive when I need it to be. Sprag transfer case only allows that when the back tires are slipping which does not help, also since there are pieces of gear teeth all over in the fluid and in the bottom of the transfer case and out of the side of the hole in the transfer case I had to replace it anyway and if you're going to fix something might as well make it better. Haven't been able to use the clutch to make sure it doesn't roll backwards on take off because I haven't been able to drive it yet and since there are no functioning brakes on the truck and I mean no brakes and the transfer case is dead..... I won't even be able to test that theory for awhile but I do appreciate your input and will keep that in mind once she is up and running. She's a survivor but no one took care of her until this point. And the gearing, and the double overdrive between the manual and transfer case make it a double win win.

 

[98G]

I need the ability to have my truck in 6 wheel drive when I need it to be. Sprag transfer case only allows that when the back tires are slipping which does not help, also since there are pieces of gear teeth all over in the fluid and in the bottom of the transfer case and out of the side of the hole in the transfer case I had to replace it anyway and if you're going to fix something might as well make it better. Haven't been able to use the clutch to make sure it doesn't roll backwards on take off because I haven't been able to drive it yet and since there are no functioning brakes on the truck and I mean no brakes and the transfer case is dead..... I won't even be able to test that theory for awhile but I do appreciate your input and will keep that in mind once she is up and running. She's a survivor but no one took care of her until this point. And the gearing, and the double overdrive between the manual and transfer case make it a double win win.

I somehow missed the fact that it was broken and you had to replace it anyway.

If you're replacing it anyway, I'd go with something a bit more robust than the one out of the 939s. And may as well try to get something that will increase top speed.

XM818 on 11.00s in stock form tops out right around 53mph. Pics of mine from this past week...

 

[Olly]

I somehow missed the fact that it was broken and you had to replace it anyway.

If you're replacing it anyway, I'd go with something a bit more robust than the one out of the 939s. And may as well try to get something that will increase top speed.

XM818 on 11.00s in stock form tops out right around 53mph. Pics of mine from this past week...

View attachment 763959

View attachment 763960

Cool Truck.... mine's a little camera shy at the moment. Do you have any ideas on what would be better? I was planning on in the future moving to a 9 speed manual.... but right now plan to keep the 5 speed as long as it works because there's a lotta other things that require my money ie brakes..... transfer case.... lines.... I'm open to any suggestion or ideas any help is appreciated.

 

[98G]

Its hard to go wrong following Wes's advice.

Things to think about -

You don't want to lose any bottom end acceleration. You want to make sure that effective first gear combination remains as low as it is.

At the point I'm contemplating a transmission and tcase swap , I'd be contemplating an engine swap at the same time. 400 Cummins comes to mind.

I've got another M818 on 16.00's. This takes the top end to 70ish. This one has a turbo to compensate so it didn't lose bottom end acceleration.

 

[Olly]

Its hard to go wrong following Wes's advice.

Things to think about -

You don't want to lose any bottom end acceleration. You want to make sure that effective first gear combination remains as low as it is.

At the point I'm contemplating a transmission and tcase swap , I'd be contemplating an engine swap at the same time. 400 Cummins comes to mind.

I've got another M818 on 16.00's. This takes the top end to 70ish. This one has a turbo to compensate so it didn't lose bottom end acceleration.

View attachment 763983

Already planned on doing a turbo with an intercooler rerouting the air intake, putting dual fuellines, new filters and fluids a must, and swapping in a disc brake system, and then also gonna put the big tires on.... changing the fuel tanks.... Like that truck with the big tires looks pretty badazz.... Was thinking about doing an engine swap but I'd rather see what I can get outta this engine. I'll post some pictures when she starts to come together... she was a pipeline truck so there's alotta bad repairs and looks like they found her in a mud puddle or dug her outta one anyway.

 

[74M35A2]

I have a super clean non-EGR ISX Signature 600hp 15L engine available for any that want to really make theirs go. Sounds amazing at idle, and would pull like a freight train. You'd only shift gears if you wanted to, not because you have to.

 

[Jakelc15]

If your replacing the transfercase, move it back a few inches. The 9 speed transmission is longer than the stock 5 speed.
I've been looking for a 13 speed, double over for mine. I've about wore out my transfercase. Pops out of high every once in a while.

 

[74M35A2]

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[simp5782]

The 13 speed RoadRanger I have measured shorter than Allison MT654CR. In addition to that, I think the engine side flywheel housing used for the Allison is an additional 1” deep beyond standard. Tape measure says so anyway.

the spicer in an 818 is well shorter than a Allison.

 

[74M35A2]

My fault. One day I’ll learn how to read.