M818 Transfer Case Upgrade T-138 to T-1138

[77 AMG]

I have this same set up, other than 1400 20 tires in the XM818 that I acquired recently. Outstanding on the interstate- 70 MPH at 2100 RPM, 7- 7.5 mile/gal all day long. (until you hit a hill, and then, the truck falls FLAT on its' face. Turbo time!)

 

[77 AMG]

Also, late info.... the PTO box is NOT a problem with this swap. Mine is still installed and functional. The shifter linkage for the X-case itself, however, is still needing to be constructed. (hopefully, this weekend)

 

[unix84535]

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-...ing-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.

Like you ideas to improve the speed and lower rpm for M818 I have a 1970 and am very interested in your progress.
Will be following you progress looking froward to see the end results.