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Cat 7155 tranny
- Thread starter gusto1
- Start date
StreetbikeDepot
Member
Why would you switch it out, those trannies are great, they have a deep low range and an overdrive. But it could be swapped out, I'd take your old one of your hands for ya!
CatMan
New member
Cat 7155 Transmission
Acouple of things to remember.
The 7155 operates with air shift and requires clean dry air to the shift valve pack on top of the transmission.
The transmission has two clutch packs one in front and one at the rear. When they shift, the forward cluch unlocks, the rear clutch unlocks, the air valve shifts the gears inbetween and then the two clutch packs lock up again. all in the blink of an eye.
The killer on these transmissions is operating them at a low speed ( I think the danger zone was 1100 to 1300 RPM but check the manuals and the warning plate in the cab. That is the speed range when the clutch packs are NOT Fully in lock up mode. They will slip the clutch plates, overheat the plates and they will warp and the transmission is toast.
If operating correctly, they are a fantastic piece of machinery. They were too expensive to produce for the commercial markets and that was what ended
production.
Back in about 1979, THE HOT ON HIGHWAY SETUP was a longnose Peterbilt with a 450 HP CAT 3408 with brakesaver retarder coupled with the Cat 7155 transmission.
Cat Man
Acouple of things to remember.
The 7155 operates with air shift and requires clean dry air to the shift valve pack on top of the transmission.
The transmission has two clutch packs one in front and one at the rear. When they shift, the forward cluch unlocks, the rear clutch unlocks, the air valve shifts the gears inbetween and then the two clutch packs lock up again. all in the blink of an eye.
The killer on these transmissions is operating them at a low speed ( I think the danger zone was 1100 to 1300 RPM but check the manuals and the warning plate in the cab. That is the speed range when the clutch packs are NOT Fully in lock up mode. They will slip the clutch plates, overheat the plates and they will warp and the transmission is toast.
If operating correctly, they are a fantastic piece of machinery. They were too expensive to produce for the commercial markets and that was what ended
production.
Back in about 1979, THE HOT ON HIGHWAY SETUP was a longnose Peterbilt with a 450 HP CAT 3408 with brakesaver retarder coupled with the Cat 7155 transmission.
Cat Man
NMC_EXP
New member
The transmission itself may be a fine piece of equipment but the pneumatic controller is a basket case.
I was an engineer at Caterpillar Inc. I was responsible for a bunch of rubber parts including o-rings. The transmission factory asked me for help on this controller. It had a lot of o-rings in it.
The controllers were assembled in a small room off the main shop floor, by one man, usually with a transmission engineer helping. These things were built by hand, one at a time. After assembly each controller was bench tested. The failure rate was high. If it failed the tech would tear it down and start swapping out o-rings, pistons and such until he found a combination of parts which would pass the test.
I suspect it would be close to impossible to rebuild one of these if it failed and the cost of a replacement would make other CAT parts look cheap.
I was an engineer at Caterpillar Inc. I was responsible for a bunch of rubber parts including o-rings. The transmission factory asked me for help on this controller. It had a lot of o-rings in it.
The controllers were assembled in a small room off the main shop floor, by one man, usually with a transmission engineer helping. These things were built by hand, one at a time. After assembly each controller was bench tested. The failure rate was high. If it failed the tech would tear it down and start swapping out o-rings, pistons and such until he found a combination of parts which would pass the test.
I suspect it would be close to impossible to rebuild one of these if it failed and the cost of a replacement would make other CAT parts look cheap.
wreckerman893
Possum Connoisseur
The biggest issue I had with the Cat tranny in the M916 I drove was when the air pressure in the system dropped below 80 PSI
(still high enough to keep the brakes from locking up) it would kick into neutral....bad news if you were going down hill with a heavy load on.
The military and the manufacturer came up with a fix that added extra air tanks to prevent this.
Once you learned how to "slap shift" the tranny you could move on out with a load on.
(still high enough to keep the brakes from locking up) it would kick into neutral....bad news if you were going down hill with a heavy load on.
The military and the manufacturer came up with a fix that added extra air tanks to prevent this.
Once you learned how to "slap shift" the tranny you could move on out with a load on.
dittle
Well-known member
You also have to remember to hit the button on the side of the shifter otherwise you won't move when you put it in gear on the shifter. We always get a kick when we need to move our M916 and someone who has never driven our M916 says "Hey I'll move it". They get in, start it up and wait for the air to build.....then nothing. They come back saying the transmission is broke but they didn't know they had to push the button on the side of the shifter to make it actually work.
You are indeed correct There are a ton of O rings, poppet valves, and springs in the contoller group ! But I'm 3 for 3 on rebuilding them myself. A good amount of hours work! but if its all flat and clean and assembled correctly they work as good as new after testing them
I Would not install 1 on a truck without a airfilter /regulator and oiler on the input, Which should be the outer passangerside air tank on the rear fitting. 
466Navastar
Member
R Racing- can you share what kind of air filter/regulator and oiler that you have found works well?.....Name brand - - model /part # and what kind of oil do you like for the oiler?...what pressure do you set the regulator at - and why ?
I used a Parker unit Primarly because of the temp range its good to 175 degrees ( although its not near anything hot ) Also there NEMA 14 weather proof and have removable tamper proof knobs. So in plain english they were darn expensive. I'll get the part #s posted. I went with a 3/4 pipe size unit . Althought I believe a 1/2 " unit would also work and save you money. I use a air tool oil since its thiner. MSC sells a lubraplate air tool oil thats great and not very expensive. I set the air pressure to 95 PSI. The Trans needs at least 85-90 PSI for R1 R2 1 2 3 4 gears for the rear clutch thru the clutch regulator . By using 95 PSI your not so hard on your O rings and shift pistons , And it does shift smoother.The regulator allows you to use less air so you now have more reserve. I installed 1 at the Cat dealer before the rebuilt manifold went on my m920. And the Service manager of 36 years thought it was a great Idea . I drove home the m920 Weds with the rebuilt Controller and man It shifted so darn nice I had a grin ear to ear. I imagined thats how they drove when they were new. 
NMC_EXP
New member
I never could blame the problem on the o-rings. Suspected it was the floating pistons or the bores they rode in.You are indeed correct There are a ton of O rings, poppet valves, and springs in the contoller group ! But I'm 3 for 3 on rebuilding them myself. A good amount of hours work! but if its all flat and clean and assembled correctly they work as good as new after testing them
If these can be field rebuilt that tells me all the parts swapping and hand fitting at the CAT transmission factory sorted out pistons and other components which function as intended (probably meaning the parts that worked actually met the print). I wanted to get a pile of pistons and bores inspected in detail but was told to mind my own business and stick to checking the o-rings.
All this ISO certification is a load of manure. Companies believe if they give a supplier an ISO certificate he can hang on his wall that they never have to inspect that suppliers parts to see if they meet the print....riiiight.
Side issue: (1) every time you tear something down with an o-ring in it, replace the o-ring with one of the right size and right material type. (2) Parker and a few other big companies make good o-rings but there is a lot of junk out there. Making o-rings is a relatively cheap business to get into (think China, India, etc). You cannot tell a good one from a junk one by eyeballing it.
Regards
Jim
I like my Cat trans. but like it better now I know a lucky Redneck who can fix them.
NMC_EXP
New member
Cat p/n 3P-1155 one of the o-rings for the rebuild? That was the one on the floating piston when I was working the problem. The 3p-1155 o-ring is a 75 durometer fluoroelastomer. Do not recall the dash size. Fluoroelastomers (Viton) are great for use with all petroleum based fluids and high temp. Just do not use it for engine coolant, hot water or steam.New O rings are used, Some are Cat most are Viton Fluoroelastomer with Durometer hardness of A75. and meet ASTM D2000/SAE J200 and MIL-R-83248C I guess I must be a lucky redneck
ASTM D2000/SAE J200 are just standard frameworks that show how to write a rubber material spec. The supplier has to fill in the values. So if a supplier says his stuff conforms to ASTM D2000, he is not telling you much. Did not work with MIL standards.
I dunno about luck. I think you rebuilt systems that CAT originally built the hard way. That is rather than inspecting the parts to verify they met the print before starting the build and rejecting parts out of print, they kept swapping out parts during the build until they found a combination that would pass the bench test. So only the good ones (finally) got shipped to the customer.
Not right bright, in my opinion.
Interchangable parts and mass production have been around for a long time - longer than me even. Some of my colleagues just forgot about that, I reckon.
Regards
Jim
Cat has it as a 5P-8942 , I actually use whats known as a double O ring that gives about double the sealing surface, and seem to work the best. The 1s on the last manifold I tore down were wore to the same diameter as the poppet and would fall thru the hole. It was a 1992 rebuilt manifold and very clean inside ( it was the manifold that came out of my m920). but the O rings were worn to a flat side.I truely believe O ring Tech has changed a bit since these were designed, And believe there may be some styles that are better suited.I don't 1 dought what you were seeing at Cat durring that time. I have a semi retired Cat GURU that is near by and has rebuilt a few of these for the military and he has some stories,lol.
NMC_EXP
New member
I worked this problem several years later than '92.
Your "double o-ring" sounds like what the o-ring suppliers called the "quad-ring". The cross section looks like a 4 leaf clover. If nothing else that should help prevent the piston from tipping in the bore and getting wedged. I recommended the quad-ring but these were military contracts and it took an act of God (actually the DoD) to change the slightest detail of the design.
I had never seen an o-ring design like this one. The transmission engineer told me this was called a "floating piston" design and until the piston was seated it was designed to let some air blow by the piston/o-ring. It could not be a tight seal or the piston/oring would not move. If the air pressure was high enough to move a tight piston, it would wear the snot out of the o-ring because the cylinder is dry.
I looked up floating piston design in the Parker O-ring Handbook. It said flat out that Parker did not recommend using a pneumatic floating piston design because unless everything was perfect, it was a big pile of trouble. From what I saw, Parker knew what they were talking about.
I suspect the problems they were having were related to parts (other than the o-rings ) being out of spec or not meeting the print. The transmission guys were unwilling or unable to correct those problems. The result was I had to prove the o-rings were not the problem. After that, I stopped getting calls and lost track of the situation.
Like I said, the units that finally made it out the door must have been good ones.
Regards
Jim
Yes that is indeed the ring . It seal very well on all sides. Its a bit stiff once assembled and you apply air. But I found a trick to get them all in line and they are so far flawless. I have ove 20 hrs of test on the first 1 and about 15 on the second unit and about the same on the 3rd. I use a oil mister to keep from having a dry o ring issue. Alot of thought went into these transmissions, And I tip my hat to the people involved. I have to say I love how they shift .I must for all I have went thru to repair mine LOL . Next is getting manifold 2 on the m917 and get it road worthy soon I Pray .
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.
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.