Simp's Cat 7155 Swap into a M923A1

I recently got to ride in Wes' 923 with 16-speed transmission...all I can say is WOW. This is the transmission that this truck series should have come with from the factory, it really harnesses the torque of the NHC250 very well and is a real eye-opener. With Wes running 395s instead of 14.00s, made it even more impressive. I just couldn't stop smiling the whole time I was in his truck. It really gets up and goes, and runs well at freeway speeds. Very impressive!

Wes has done a fine job, and really has a winner with his Cat 7155 swap. I hope his HEMTT transfer case swap goes just as well!

Wow is he really working on 977 now¿

99nouns said:

Wow is he really working on 977 now¿

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Im swapping a hemtt case in for more strength for an upcoming repower. No sense in blowing a t1138 if i dont have to. Power power plus bigger tires means ill still have good road speed

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Oh so you are putting an hemtt transfer case into a 939? Did I get it right?

99nouns said:

Oh so you are putting an hemtt transfer case into a 939? Did I get it right?

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Yea mo.

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simp5782 said:

Yea mo.

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Cool, cant wait to see it get done.

Welp, Just went past 1100hours since the install. Went ahead and changed the oil on the transmission. I went with synthetic 1540 Lucas Magnum oil per Soni when I first put it in. Around $30 a gallon. Decided to go back with regular 15w40 this time around and yes the transmission is normally pretty noisy and there is not any noticeable difference between the standard 15w40 and the synthetic. My 11th and 12th are particularly noisy at times. it is hit or miss.

I did install a remote filter unit for the system which will make it easier for a cooler install in the summer time. I never see temps more than 180 or so even in traffic. I did see up close to 200 when I was sitting on constant traffic at 90 degrees, with no cooler installed. but otherwise it never gets to 160 towing with 70 degree ambient temps.

I don't have any problems with it really. Every so often when I throw it in a gear and let it sit at a light or even at home it will dump air out of one of the vents. I change gears and go back to that one, typically 7th, and it wont do it that time around. A little weird but oh well. She runs well on the highway though. In traffic it kinda sucks. I am sure a turbo engine would make traffic conditions a little quicker from stoplight to stoplight. I did figure out that if i hit the 2200 mark I can shift up 3 and it doesn't bog below 1400 so it makes alot better traffic driving IMO.

Try this for your upshifts: Just let off the throttle enough to remove power, then shift. Seems better than coming completely of the throttle like a manual.

NDT said:

Try this for your upshifts: Just let off the throttle enough to remove power, then shift. Seems better than coming completely of the throttle like a manual.

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Yeah I do that alot especially when loaded to keep it from slowing down on the shifts. I dont let off the pedal till the 3rd air burst starts then back on it as soon as the 4th burst ends. I have a video to upload of a 0-60 upshift at 55k gross and its at 60 in under a mile. She hits 60 empty in under half a mile. i do have to shift 1 at a time 14,15,16 to get the top power band better though.

Thought I would give a little update as I just went thru 18,000 miles with the transmission so far. She seems to be holding together well. Not any problems on the cold mornings we have had. Or any valve body issues luckily. Heaviest I have towed with it was the Case 1155E track loaders giving be a 69,000 GVW. and a haul i just took down to Ft. Benning that had me around 71,000. She doesn't seem to miss a beat. Even with the loaders on the trailer I was still starting out in 5th on level ground with no serious jerk or take off bogging. Being that 4th is rated for 75k gvw. Averaging around 55 to 59mph at that weight with the same 5.5 to 6mpg loaded.

I let CSM Davis drive it the other day. He seemed to be impressed with how it pulls on hills especially at 40,000. He just hadn't driven a 7155 in awhile so a little rusty on shifting.

It takes some getting used to. Even I still hate the downshifting thru turns. Especially if you miss the shift point. It doesn't take much to bog it down to have to downshift again.

Reverse 1 is very nice though i will say for backing up a loaded or unloaded trailer. It's slow deliberate movements in that gear doesn't let you make much of a mistake on backing up.

Best Cat transmission I have ever driven and I have driven quite a few, need to work on Bill's 916's.

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Aight well I will give a demonstration of how nice the cat transmission is. This was a steep hill climb. Would not climb it in 4th gear in high range with the HEMTT case. Loaded at over 68k. Gravel road. And I do not have a front drive line installed until after my big cam conversion. The loose gravel kept slipping and you can see in the first few seconds of the video the spring brakes would not even hold it and it would slide back down.

So I left the spring brakes applied. This is the push/pull valve type rather than the standard emergency brake handle. Spring brakes being left applied in a open diff almost simulates a locker. being that the tires aren't slipping as much since the wheels are brakes are locked. Well here we go up the hill in 1st gear turning 2000rpms. Fan clutch and Air conditioning on as well.

The backhoe bucket arm has a loose chain on it to allow the flex on the trailer and truck up that steep axle with all the washouts and what not.

I can say from previous experience that a truck with an allison would have bogged down and not have climbed it, even in low range. It was a steep climb with that backhoe on there. It was a heavy JD 710D combined with the gravel. it was rough.
https://www.youtube.com/watch?v=6ijj1am0xv4&t=32s

simp5782 said:

..... Spring brakes being left applied in a limited slip axle almost simulates a locker.....

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So you have LS-diffs in both your rear axles?

That was one serious hill climb... Thanks for sharing!

gringeltaube said:

So you have LS-diffs in both your rear axles?

That was one serious hill climb... Thanks for sharing!

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Sorry they are open. basically simulating a locker. for some reason LS came to mind. I have some lockers to put in. Just trying to get my 14ft iso bed and do all that at one time.

ISO Bed? As far as I know only the USMC trucks had those. We only have about 5 or so left locally that have not gone to auction. I'll watch and if one goes I'll drag it home! None of them are runners so they will likely go low.

sandcobra164 said:

ISO Bed? As far as I know only the USMC trucks had those. We only have about 5 or so left locally that have not gone to auction. I'll watch and if one goes I'll drag it home! None of them are runners so they will likely go low.

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I located one in Pensacola a person has. I gotta install AC in their 931 to get it though. There were 4 trucks in a lot of 10 trucks that went on Gov deals a few weeks back that all had 14ft iso beds. Went too high tho.

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Welp. First problem out of the kitty cat. Well it was my own dumb fault. Had both rear bolts in the rear mount break off in the casing so I ended up swapping that out and the rear bearing retainer. Worked well for a bit. Had a slow shift rear clutch engagement from a o ring that had fell out of the brake section so I pulled it apart again to replace it and the other 4 in there. Went back together with it and the rear clutch housing must have been in a bind when pulling the bolts up cause I heard a pop and a ringing sound and then a the tail shaft was spinningmpretty freely. Apparently the bind broke the side of one of the ring gears on the rear clutch fork. Symptoms were a Locked up transmission in all Odd gears and neutral on all even gears. Weird. But pulled the cab and the valve body off to find the busted gear. Going to get the part ordered Monday and disassemble the transmission. I think I can do it with it still attached to the motor. The rear clutch is fun however being that it weighs 300lbs or so.

On another note. Another member was putting one of the 7155s back together and did the exact same thing I did and broke the same gear. Strange.

I ended up putting a leaf spring style mount on the rear of my transmission. As well as the clutches in it are dated 22nd April 1988

Fun filled day. Tore down the broke trans for rebuild. Swapped in the spare for now. And swapped my valve body on.

Oh man Simp sounds like all the fun I'm having getting ready for this move.

It just gets my goat how utterly fearless he is about tearing stuff down and rebuilding on these trucks. And fast. It's like he's working on Honda Civics or something.

My new transfer case arrived and I got it home today. I'm sweating just swapping out whole components, not even tearing anything apart!