Cat 7155 tranny

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NDT

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I just got my M915 on the road and the Cat 7155 tranny sure is odd. Seems to shift just fine, though. If it develops a problem, does it only affect a single gear or does the whole tranny stop working? I mean, which is more typical? Are there any warning signs before it quits, or is the tranny designed to pick the most inconvenient spot on a highway to freeze up on me?:-?

I did notice something odd when backing up to hook onto a 5th wheel trailer. I put it in reverse-1 and try to nudge gently back to the trailer. The truck just sits there until I get the revs up and then it lurches back and bangs into the trailer. How can I prevent this? On a stick tranny, I can feather the clutch to get a light touch on the trailer.
The trans won't "freeze up", what starts to happen is gear changes won't happen when you move the lever. Most of the time a shot of oil in the air line will clear this up, that is until the o-rings discussed above are damaged.

I have a similar issue with backing up. Try applying the brakes at the same time as the accelerator, that way you can have delicate control.
 

reefmaker

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I have a problem with my M915. I heard on the forum it would be a good idea to install an air tool oiler in line with transmission shifter. I did that and now it won't go into high gear anf 12th doesn't work at all. I disconnected it, but still have the problem. Does anyone have any suggestions.
 

R Racing

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Unless you used some sort of thick oil. It didn't mess up your manifold. Most of these were made in 1980 ( 32 years ago) there all on borrowed time til they need a rebuild)
 

kb1122

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Thats what happend to my 7155 I had to rebiuld the manifold. I went to the local cat dealer and they printed me out a manual all the o rings are still available threw cat. With the help of Rracing on here i was able to rebuild it. Definetly leave the oiler on there.:driver:
 

Kalashnikov

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Bumping an old thread, what all is involved in rebuilding the manifold and what are common symptoms that the manifold needs rebuilding? There is a truck I am looking at that supposedly needs the radial selector plate for the shifter rebuilt, the transmission control group rebuilt, and the manifold rebuilt Does a rebuild consist of just replacing orings? I've rebuilt plenty of hydraulic pumps so I'm ust wondering how it differs and how much more indepth you have to go into the transmission.
 

NDT

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Bumping an old thread, what all is involved in rebuilding the manifold and what are common symptoms that the manifold needs rebuilding? There is a truck I am looking at that supposedly needs the radial selector plate for the shifter rebuilt, the transmission control group rebuilt, and the manifold rebuilt Does a rebuild consist of just replacing orings? I've rebuilt plenty of hydraulic pumps so I'm ust wondering how it differs and how much more indepth you have to go into the transmission.
There is a thread here on rebuilding the manifold written by RRacing. Search "Cat 7155"
 

Warden

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Hi everyone , I'm new here & I'm not sure wether to start a new thread ? If so tell me ,I purchased a m916 today and drove it for the first time , it has only 2000 miles since new , not rebuilt ,,,, my question is on the trans , it seems to slip after 11th , it is so disconnected that while going 55 on a slight down grade if I hit the engine brake , the engine just shuts off , on level it won't hit 30 mph , the oil has zero burning smell and in fact no heat what so ever , I just drove 20 miles , 10 of which were over quite a mountain , and the oil is maybe 70 degrees F the trans will not load the engine in any gear really but sometime in first or reverse will pull right thru the maxi brakes ! It actually feels quite normal up to 10 th gear
then it is like trying to drive my semi with a road ranger with the clutch 95% disengaged , and still no heat , not the oil not the trans case , nota. It's 20f here rite now and my dog feels warmer then the trans case , can anyone enlighten me ?
 

R Racing

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Sounds like your clutch regulator on your manifold is not set up right. There are 4 pressure ranges for 4 groups of gears. High gears use the least amount of pressure . Your allowed +2 psi or -2 psi or it doesn't work correctly . Also if you went down any step grades and the engine rpm exceeded more than 2275 to 2300 the trans will automatically disconnect the output clutch on the trans so you don't over rev the engine and hurt it. also you could have a cracked or damage O ring ( There are several hundred in the manifold) in the clutch circuit .
 

Warden

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interesting , not sure where to begin , I am a mechanic , but lack info on this , is it likely that the trans is healthy because of the lack of heat and burnt smell , and it's just one of what you mentioned ? Is there anyone that is near me that works on these ? I need to understand the functions better to begin myself , like if the trans is toast what would it act like ?
Is their any flow chart or trouble shooting guide ? The place I purchased from said the trans was fine and I'm inclined to believe them , cause they used the truck in warm weather and it seems to get worse as temp drops outside , I owned it only 12 hours so far so it's not like I know much at all about it , I just always wanted 1 since I was a kid and sat in 1 in 1979 ! .... Is there a way to lock up the trans in each gear to test things ? I assume a torque converter is in the mix ? What confuses me is it will stall @ 1200 rpm or sometimes not much at all @ 2200 rpm. This is in 1st , there is nothing her key and jersey about this trans like I read in other peoples posts , it acts like cat power shift but softer most of the time , very very strange for me to describe , I would say 2 soft all the time with no full engagement ever , you can drop engine rpm and match to trans and ground speed in 12th but it won't bite in , I'm not sure if 8th is all that much better or just has that much less load so it feels better , I'm looking for the magic override button that locks this thing up under full load in every gear so I know it's basically sound and worth learning about and finding the issues ! How the heck does the oil stay cool with. Such slippage ? Does this cool oil make sense ? I appreciate any and all input ! I may even be of use to some of yal on other trucks that I know well , but I'm lost on this one !
 

Warden

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Sounds like your clutch regulator on your manifold is not set up right. There are 4 pressure ranges for 4 groups of gears. High gears use the least amount of pressure . Your allowed +2 psi or -2 psi or it doesn't work correctly . Also if you went down any step grades and the engine rpm exceeded more than 2275 to 2300 the trans will automatically disconnect the output clutch on the trans so you don't over rev the engine and hurt it. also you could have a cracked or damage O ring ( There are several hundred in the manifold) in the clutch circuit .
where do I begin .? How do I know if the trans is healthy ? Any way to lock up 16th and prove it can be good? Does the cool oil make sense to u with all that slipping 16th on flat ground @ 2200 rpm may hold u at 25 mph , my brain will not allow my foot to do that for long and my foot keeps searching for a rpm/ground speed match , it's next to impossible to find , I'm a mechanic , and a commercial driver in. Both meanings of the phrase , I drive race cars and heavy trucks , so I'm sensitive to engine and ground speed not matching , this truck is tuff to tell if it's connected or not , extremely tuff ' I appreciate any and all Input , thank u
 

R Racing

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Yes you definitely don't want to drive it like that or you will burn up the clutches. Let's try the simple first. Go to your passanger air tank the 1 most out board. With the truck not running and all the air bled down. Remove the airline from the back of the tank and fill it with air tool oil, and reinstall it trying to keep the oil in the line (put as much as you can possibly fit) . If the regulator is stuck. This should unstuck it! Then shift the Trans thru the high gears as much as possible going between 10 and up. If that doesn't work your going to have to remove the floor panel in front of the passenger seat and I'll see if I can dig up pictures to show you which port you need to open to check your clutch presure.
 

Warden

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Yes you definitely don't want to drive it like that or you will burn up the clutches. Let's try the simple first. Go to your passanger air tank the 1 most out board. With the truck not running and all the air bled down. Remove the airline from the back of the tank and fill it with air tool oil, and reinstall it trying to keep the oil in the line (put as much as you can possibly fit) . If the regulator is stuck. This should unstuck it! Then shift the Trans thru the high gears as much as possible going between 10 and up. If that doesn't work your going to have to remove the floor panel in front of the passenger seat and I'll see if I can dig up pictures to show you which port you need to open to check your clutch presure.
Ok , Great I will get right on this in the morning , I still can't understand the lack of temp in the oil and on the outside of the trans case ? I guess I should ask what oil to use at this point and to check while running in neutral or in gear, it was a gal low running in N , I added Schaffer simplex 315 ( a torque conv oil or hydrostatic oil good for wet brakes and clutches but not cat T04 ) I use this in severe duty sundstrand hydro apps , with another additive that makes it a monster oil , very light but capable of more extreme pressure service then gear oil or grease , yet it's 20wt , I would like to know the recommended oil tho , I do work for several manufactures and race teams finding lube solutions , so what I added may be wrong but it mixes wit all and I doubt it can do damage to
anything ! FYI I seen and sat in my first m916 when I was a kid in 1979 , I wanted one since , I finally today was able to afford to purchase as a working toy ! I luv this thing , but I'm already starting to eye that eaton. 8LO.verdrive in the shop corner on the floor , my left knee is so bad tho that I really should make this cat work , and I love all the noises and newonces that others on this site complain about , I just need mine to lock in and go , then it's getting hooked to me home made poor mans landoll ! Thank u
 

R Racing

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Welcome to the forum !!! The trans uses 10 wt. I use 15-40 myself. The trans is part hydraulic, and mostly air actuated. Unfortunately most of the O rings in these old units get dried out, Crack or break. They can be finicky if there not set up right ! In the picture you can see the little gauge to the right. It says CL next to the 1 in the picture. It has a pipe plug in the hole now. You will need to remove the pipe plug and install a good quality air gauge that can handle up to 120 psi . once installed you need to write down the pressure in each gear and report back .
 

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Artisan

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Warden,


SS Member R Racing is our resident 7155 Officianiado' .
He is the 1st person I would go to if I had an issue.

I took R Racing's advice and added the pressure regulating, air filtering, oil misting
devices he dreamed up FWIW.

I am watching trying to learn hear myself.


REGARDING 7155 OIL.


Right or wrong, when I bought my M916, the tranny was DEFINATELY burned
up before I ever sat in the seat, and it was all still new. I grabbed a new
tranny and threw it in and when it came to oil I did not know what to do.
I have 2 spare new trannies so I am currently doing an experiment, again
right or wrong.


From day one I loaded my tranny w/ Orielly Auto Parts
"PREMIUM UNIVERSAL TRACTOR FLUID" P/N 74205

I REALLY LOOKED HARD AT THIS, I PRAY I MADE A GOOD DECISION.
But I took all things into consideration, including "I betya the military
simply wanted to make things easier" AND PRICE! Anywho. I have
run my truck from Los Angeles to Ft Huachua AZ and BACK w/o issues
and my tranny shifts wonderful...

I think what sold me is what the SPECS for that oil says, in part;

"...assuring smooth operation of clutch packs and wet brakes" of which
I believe the 7155 has TWO sets of clutches and TWO set of wet brakes...
 
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