M818 Transfer Case Upgrade T-138 to T-1138

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OPCOM

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This thread will discuss modifying an M818 by the removal of a stock Rockwell T-138 T-case and installation of a Rockwell T-1138 T-case normally used in the M939 type truck. The reason for this is to gain a bit lower RPM on the highway and run 60MPH instead of 50MPH.

T-138:
High: 1:1 ?
Low: ?


T-1138:
High: 0.732:1
Low: 1.72:1

reference:
http://www.steelsoldiers.com/5-ton-up/58387-serious-discussion-5-ton-speed-increase-via-gearing-transmission-differential.html#post679068



The T-1138 case has been delivered to the shop that is helping me. I am waiting for them to tell me to bring the truck over. I am not sure how many pictures of the process I will be able to take because while they are working, I will most of the time also be working. I cannot afford to take off 5 or 10 days from work. I will do my best to document any issues that come up, such as driveshaft, drive flange, shifter, air shift "solenoid", what to do with the original transmission air lines, etc. The 1138 case is an air shift, instead of the sprag. (I'll be looking for a deuce type air switch later unless someone wants to donate one to the cause hint hint.)

One nice thing is the 1138 case has a rod-operated switch that functions when the case is put in LOW. Normally this connects to the automatic transmission and locks out reverse, so that the T-case does not explode when a lead foot driver puts a 60,000 lb loaded semi in the 11:1 reverse gear (the 939 type auto trans has a very very deep reverse) and punches it. I will connect this switch to an indicator light. I have no intention of ever using low in reverse and have nowhere near 11:1, and no torque converter.

I will connect up the air shift after the big job, and probably cap off the transmission air lines for now, possibly they can be connected to indicator lights, and the reverse one to a backup beeper and backup lights.

I am told the flanges are different. I do not know the exact length or difference but it ought to be close.

Any coments on this swap? Any help with dimensions or useful suggestions will be highly appreciated!


So be it, now the phone call is awaited.
 

OPCOM

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I got the case off ebay for $1850 which I think is a fair price. In the end it was $340 freight from Florida to Texas and I am told that was cheap for a 500+ lb pallet shipped on demand. The case had been on ebay for a while, and also listed in the past without being sold. The seller told me the case is working properly and is not damaged. In the auction it plainly stated this. In the meantime I was quoted by the usual rather costly vendors an average of $4000 rebuilt, $3000+ 'take out', and told that the casing is worth most of that because the hard military situations accidentally destroy the casings. I also believe that most vendors and big truck transmission shops do not know this is an overdrive case but see it as a usual replacement part. I am sorry the secret is out. I do not think 4x4 and mudders will have an interest because they have engines that turn more than 2100RPM. They might have an interest in my removed T-138 with a mere 8000 miles since rebuild. Those go for about the same $ at the usual vendors BTW.
 
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gimpyrobb

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Dang, wish I had known that as there were a bunch of them in Columbus about 4-5 months ago. All new in the box.
 

OPCOM

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Here are pictures.. The result is 1800RPM at 60MPH. which is perfect. The job is not the prettiest but it seems solid and as soon as I change the engine oil I am going to do some more driving.
 

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OPCOM

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more.

Considerations:

Originally, I put 16.00x20 tires on goodyear rims on the truck. Engine redline of 2100RPM occurred at 62MPH. The speed was satisafctory but the very large tires, in my sole opinion, made the truck unpleasant to operate on-road.

16.00x20 tires raised the vehicle 5 inches above stock height and greatly improved the already large ground clearance for off-road use. Most of the driving will be on-road to and from various functions that include display and may involve off road activity.

It was decided to sacrifice some of the off-road capacity in order to regain some ease-of-travel.

The stock-size 11.00x20 tires were installed, reducing the vehicle redline to 52MPH, and a transfer case from an M939 seties of truck was bought in order to provide a realistically usable overdrive for highway travel.

It should be said that there are other ways to increase the speed of this truck somewhat, such as slightly larger tires (anything bigger than stock usually means going single), replacing the transmission with an RTOO (with more overdrive than the stock transmission), and adding a brownie box, or auxiliary transmission just for the rear axles. In a short wheelbase M818, that would be an overwhelming engineering problem because the rear driveshaft is already very short. All of the options except the brownie box and the transfer case swap would provide a redline of about 57-64 MPH being satisfactory for many purposes.
reference ongoing discussion: http://www.steelsoldiers.com/5-ton-u...tml#post679068

No one wants to run at redline, or, I do not want to run at reline. The T-1138 transfer case provides a redline of 71MPH, and a comfortable 1800RPM at 60MPH.

Both cases are used in '5-ton' military trucks. 5 tons is the off-road rating of these trucks. The trucks are rated 10 tons on paved roads.

The T-1138 is used in the M939 series vehicles behind an Allison non-OD transmission and Cummins NHC-250.

The T-138 case is used in 800-series vehicles behind a Spicer 5-speed OD transmission and a Cummins NHC-250.

The swap is not straightforward because the T-1138 case is a bit smaller than the T-138 case.

Both driveshafts must be resized.

The transmission-transfer shaft must be made up or made from a combination of parts from both trucks. In the end, making the shaft up with the help of a machinist to make it all fit was the best way to go according to engineering.

The parking brake lever is in a different location on the 1138 transfer case

The speedometer cable adapter is too short and will need to be extended.

The transfer case shift linkage presents an issue due to operating rod throw, height of rod in-chassis, and cab linkage components. It can be made to work by using a long arm on the rear bellcrank. The lever will have a short throw and be hard to move but that was deemed OK. I do not intend to shift the transfer on the fly.

The 1138 case requires air to lock it in high or low. This is not absolutely necessary, but in the 939 truck with the automatic transmission, it is used to prevent the case from being shifted except when the transmission is in neutral - which is necessary. In a manual transmission truck, disengaging the clutch to shift would provide an equivalent state. The locking is desirable and a valve on the dash provides this function.

The 1138 case does not use a sprag to engage the front axle. it is an air-shift arrangement using a piston to engage the front axle. A valve mounted under the dash provides this function.

With this level of overdrive, it will become necessary to downshift to 4th gear on hills. 4th gear can run up to about 48MPH, so the new situation is normal to operation on highways, where 45 can be maintained as a minimum speed if desired.
 

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hemichallenger

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Glad to see that you got it togather and working. I sold you the transfer case and thought it would take some time and work to finish. Great job and good thread.
 

swbradley1

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Thank you for the thread Patrick.

Lots of room under my 813 but it probably isn't in my budget. I haven't had a custom drive shaft made in 43 years. :)
 

Jersey4x4

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I am interested in this maybe with some more details of final operation? I have an M816 and was thinking about doing this modd to gain some more road speed and/or lower rpm to get just a tad better fuel economy on long journeys. Is there room for the PTO and would it use the same PTO box? I know the shafts would need to be modified but the actual PTO box would be a concern.
 
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