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Cat 7155 tranny

simp5782

simp5782

Feo, Fuerte y Formal
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Mason, TN
Adding this as Ponway shared this overdrive article from 1974 on the transmission.

Basically it cant burn up if you run it out of oil. I experienced this on Saturday as my heat exchanger failed and the end cap started leaking transmission oil. So it coasted in neutral. Bypassed the leak and refilled the transmission and it was fine. IMG_24371.jpgIMG_24191.jpgIMG_24341.jpgIMG_24311.jpg

Sent from my SM-G965U using Tapatalk
 
Floridianson

Floridianson

Well-known member
Steel Soldiers Supporter
7,409
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Interlachen Fl.
Yea when empty I would just lift up on the shifter to bypass any number of gears. Same with the down shifting if I had to do a quick stop and not do a lot of down shifting. Still like the T handle but love my M915A2 better just wish the dang Government would have installed factory air. Wish I had the job of ordering the trucks. Air conditioning in everything and lockers plus XM radio.
 
Last edited:
P

ponway

New member
26
13
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Location
Hartford City, IN
B42D4346-2D8C-4B3F-BD77-C2419D32A0E2.jpgA643EC56-1D69-4501-A9BA-7AB96CDC1E2D.jpgSoni had this on a shifter that I got from him. Looks like he made an adapter to shroud the sharp edge of the rectangle shift lever. Gives a little more to pull up on.
 
O

OUR MURPHY

New member
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Location
PORT CHARLOTTE FL.
thank you to R Racing for man things posted on the 7155 trans. a tip for those rebuilding the 9 valve housings on the air shift manifold.... i found it difficult to get the pistons back in the housings after putting on the new seal and ring. what finally worked .. i put some assembly lube on the ring then tightened a home made ring compressor made from a strip of thin aluminum and a hose clam minutesep,, with clamp fully tight i put the piston w/clamp in the center of some ice for about five minutes then quickly removed the clamp and put it to the housing and found that while still cold the piston seal and ring could be slid into the housing easily by hand.
i do have a question for any one who may know. i got a gasket kit from CAT for the manifold plates. all the gaskets in the kit are identical to the originals except for one..the gasket between the second and third plate has all the small holes for the 50 plus small valves , but then the original gasket has two large holes for the two large valves and the new gasket only has
two small holes for the two large valves, Does anyone know if this is a change that is meant did i get a reject gasket?
 
P

ponway

New member
26
13
3
Location
Hartford City, IN
OUR MURPHY said:
thank you to R Racing for man things posted on the 7155 trans. a tip for those rebuilding the 9 valve housings on the air shift manifold.... i found it difficult to get the pistons back in the housings after putting on the new seal and ring. what finally worked .. i put some assembly lube on the ring then tightened a home made ring compressor made from a strip of thin aluminum and a hose clam minutesep,, with clamp fully tight i put the piston w/clamp in the center of some ice for about five minutes then quickly removed the clamp and put it to the housing and found that while still cold the piston seal and ring could be slid into the housing easily by hand.
i do have a question for any one who may know. i got a gasket kit from CAT for the manifold plates. all the gaskets in the kit are identical to the originals except for one..the gasket between the second and third plate has all the small holes for the 50 plus small valves , but then the original gasket has two large holes for the two large valves and the new gasket only has
two small holes for the two large valves, Does anyone know if this is a change that is meant did i get a reject gasket?
Click to expand...
Did you ever figure out what was going on with the gasket?
 
M

M923A2

New member
2
3
3
Location
Fort Worth/TX
I've got a newly acquired M917 with a cat 7155 that does will not engage the rear clutch (no air pressure at the clutch port in any gear).

The shifter has been replaced recently, but I went ahead and pull it apart and went through it, everything looks good, no broken/cracked O-rings, all the valves are free and lubed, no metal shavings.
I checked the umbilical cable, all the tubes are clear.
No air leaks

I followed the 7155 service manual "ENGINE DOES NOT TURN DRIVESHAFT IN ANY SPEED FORWARD OR REVERSE"
Output clutch has no air pressure > Input clutch has air pressure > breaks have air pressure > Make a replacement of 9N1681 Manifold Group.

If I apply air pressure to the rear clutch port, air just dumps out the 2 rear exhaust vents.
If I plug the rear two exhaust vent ports and apply air pressure (starts moving at 25 psi) to the clutch pressure port, it holds air and the truck will move in any lower gear and both reverse gears. (I did not try starting off above 6th gear)
I've tried shifting to all forward gear ranges and reverse, no pressure at the rear clutch port (checking to see if the pressure regulator worked in any of the 4 ranges (1-4, 5-8, 9-12, 13-16). I got no pressure at the rear clutch port in any of those gear ranges. The transmission sounds like it's shifting with no hang-ups.

I pulled the rear clutch pressure regulator, it looks very clean, well lubricated and the valve (center of the hole on the valve body) looks good and moves freely when I push on it.

It's strange to me that I can't get any pressure at the rear clutch in any gear. If one of the protection circuits was blocking air flow because a shifter fork did not fully engage, I would think this would only happen in some or one gear, but not all...
If the clutch exhaust valves were stuck open, I would expect air pressure to show up when I block them off...

Any thought on next steps? Pull the valve body and rebuild?
 
R

R Racing

Active member
2,767
16
38
Location
St. Leonard, MD
M923A2 said:
I've got a newly acquired M917 with a cat 7155 that does will not engage the rear clutch (no air pressure at the clutch port in any gear).

The shifter has been replaced recently, but I went ahead and pull it apart and went through it, everything looks good, no broken/cracked O-rings, all the valves are free and lubed, no metal shavings.
I checked the umbilical cable, all the tubes are clear.
No air leaks

I followed the 7155 service manual "ENGINE DOES NOT TURN DRIVESHAFT IN ANY SPEED FORWARD OR REVERSE"
Output clutch has no air pressure > Input clutch has air pressure > breaks have air pressure > Make a replacement of 9N1681 Manifold Group.

If I apply air pressure to the rear clutch port, air just dumps out the 2 rear exhaust vents.
If I plug the rear two exhaust vent ports and apply air pressure (starts moving at 25 psi) to the clutch pressure port, it holds air and the truck will move in any lower gear and both reverse gears. (I did not try starting off above 6th gear)
I've tried shifting to all forward gear ranges and reverse, no pressure at the rear clutch port (checking to see if the pressure regulator worked in any of the 4 ranges (1-4, 5-8, 9-12, 13-16). I got no pressure at the rear clutch port in any of those gear ranges. The transmission sounds like it's shifting with no hang-ups.

I pulled the rear clutch pressure regulator, it looks very clean, well lubricated and the valve (center of the hole on the valve body) looks good and moves freely when I push on it.

It's strange to me that I can't get any pressure at the rear clutch in any gear. If one of the protection circuits was blocking air flow because a shifter fork did not fully engage, I would think this would only happen in some or one gear, but not all...
If the clutch exhaust valves were stuck open, I would expect air pressure to show up when I block them off...

Any thought on next steps? Pull the valve body and rebuild?
Click to expand...
Sounds like either a bad air manifold , or bad output clutch. but my money is on manifold.
 
J

JH1

Member
305
5
18
Location
Seattle, WA
My last post on this was 11 years ago or so. Thankfully, my transmission shifts perfectly. I do have a spare, factory rebuilt, transmission and a new shift tower in case I ever need one.
I learned from a retired Army truck driver that you have to be really careful going down a long grade with the Cat tranny because it will disconnect from the driveline over something like 2400 rpm, to save itself. Fine, so far. Then what happens is the nut behind the wheel panics, gets off the gas, slows down, and the transmission locks back up all of sudden and then the driveline explodes. I like the CAT transmission, but then I'm a cat-person. :)
 
M

M923A2

New member
2
3
3
Location
Fort Worth/TX
Thanks for the help. I pulled the manifold/valve body and found a broken valve. It broke off where the piston slides on the shaft of the valve (area of the shaft where the diameter transitions). Replaced the valve, cleaned it up and put it back together (new gaskets) bench tested the manifold and it works! Put it back in the truck, it now drives in all gears.

Broken Valve Top Left -.JPG
Sheard Piston Shaft Side View -.JPGSheard Piston Shaft Front View -.JPG
 
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