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Mack 12 speed


Jocko
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Hey guys my Mack 12 speed will not shift into high on the splitter I’ve gone through the air lines and and everything is good but when I put it in high on the splitter air just shoots out of the exhaust port. Any help is appreciated 

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On the very back of trans on back box, should be a couple of air cylinders, believe one is direct-overdrive and the other is reverse, don't know what model of mack your working on for ease of getting to the cylinders.    terry:MackLogo:

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Another issue with the cylinder is there is a snap ring that can break inside the cylinder and cause this symptom  you can do the oring job with out removing the lid from the back of the trans with the exception of the rail oring pay attention to the assembly as it is easy to get your head messed around to the way things are assembled!at least for me! :)Remove the four 3/8 by 4 bolts and remove things in order the end cap first  the outside barrel being second then the outer piston theres two as you can see by Robs post!An inner and an outer! install the new orings on the pistons  and if the snapring is broke replace it and reassemble!

Edited by fjh
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Just so I understand this correctly 

 

Air is going down the exhaust line on the stick when you shift it into high ??

Have you checked the O rings in  the splitter knob ????

If a regulator stopped working or someone had the pressure to high it push the O rings and seals out of their seats and then the air circuit is mucked up

 

Paul 

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You can buy a NEW Fuller from LKQ for $6600.00, that hardly covers the bills we get to repair stupid problems with the Mack 18, plus the Mack shift like shit and are noisy, and don't give me poo about the drivers don't know how to drive them, great for dump trucks but in real heavy haul I will take the Fuller all day long. just spent 12 hours to do 57 miles in the CH with a allready fixed once 18, been a more pleasant day than having to worry that my shift was perfect every time, the Fuller is way more forgiving, I'd rather be able to focus on the bucket trucks and cops than the stupid trans. 

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3 hours ago, PD84 said:

Hate to say....but unless there are two sticks through the floor . Mack transmissions are over complicated junk.

Heavy built , but just not reliable.....forgive me if I have sinned

 

Nope, not gunna forgive yah 😉

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If you ever had long distances of backing up to do you mack 12sp would run circles around the fuller or Mack 18 sp  and get twice the work done in a day! Its a shame they discontinued it ! Dispite its sometimes ornery last gear split it was a good trany for many vocational  job Applications  !

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The 4 reverse  gears in the 18 is great, use 3 of them all the time, never the 4th, we back up a lot, many jobs require to back into the crane, so yep kudos  for the reverse gears, you could do same thing with 4 speed aux. and since we run aux box in everything no big gain, I speak from our veiw and not that of just everday trucking, when the time comes for a repair or rebuild again of the 18 in the CH it will get a Fuller 18,  Bought C500 last week with steel hood, 20 front on slipper springs and 52 lockers in back with Chalmers, hope to get it in soon and see how it works, would have been nice to have  it had on Fri. but the Mack did her job and never complained.     

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According to owners manual; your not supposed to range shift a 18sp trans. unless you come to a complete stop. Not saying that’s the problem but just what I read. I think the Mack trans is very good but not as unforgiving as the Eaton. According to what I’ve read on how you shift the Mack trans; makes a lot of difference. I know I learned & it helped. Everybody I’ve ever asked about that have Eaton drove them differently; you can shift an Eaton about anyway you want so it seems. Main failure I hear about is range shift problems & splits. & Mack is different. Also if you run in hotter climate, I think they need 80 90 wt oil. They’ll be a lot quieter & still shift good. I’m talking about the T318, haven’t read about T218

Edited by 1965
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27 minutes ago, 1965 said:

According to owners manual; your not supposed to range shift a 18sp trans. unless you come to a complete stop. Not saying that’s the problem but just what I read. I think the Mack trans is very good but not as unforgiving as the Eaton. According to what I’ve read on how you shift the Mack trans; makes a lot of difference. I know I learned & it helped. Everybody I’ve ever asked about that have Eaton drove them differently; you can shift an Eaton about anyway you want so it seems. Main failure I hear about is range shift problems & splits. & Mack is different. Also if you run in hotter climate, I think they need 80 90 wt oil. They’ll be a lot quieter & still shift good. I’m talking about the T318, haven’t read about T218

When your talking range, do mean from low range to high range, how could you come to a complete stop?    terry:MackLogo:

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30 minutes ago, 1965 said:

According to owners manual; your not supposed to split shift a 18sp trans. unless you come to a complete stop. Not saying that’s the problem but just what I read. I think the Mack trans is very good but not as unforgiving as the Eaton. According to what I’ve read on how you shift the Mack trans; makes a lot of difference. I know I learned & it helped. Everybody I’ve ever asked about that have Eaton drove them differently; you can shift an Eaton about anyway you want so it seems

I was; that’s just what I read in manual. 

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On 6/29/2019 at 7:31 PM, terry said:

Around this area, they work on fullers twice to the mack once, maybe.   terry:MackLogo: I had a Dm dump with a mack 12 spd. overdrive and 4.17 rears, the best combo I ever drove!

Maybe that is because there are 100 times more Fullers on the road than Mack, you just lost your own argument.   ?????????

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My 2 cents worth sez Tax free)

Fuller 18 parts available @ all good cnr Stores n U aint spose to shift on the run in reverse (But we do)

Mack 12 or 10 parts scarce n pricey . ideal for hoon n along in reverse as U can proggressivly shift them cogs just like go n 4 ward

cya

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9 hours ago, david wild said:

Maybe that is because there are 100 times more Fullers on the road than Mack, you just lost your own argument.   ?????????

Don't believe I lost anything! I'll still take a triple countershaft over a double anytime.    terry:MackLogo:

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5 hours ago, Swishy said:

My 2 cents worth sez Tax free)

Fuller 18 parts available @ all good cnr Stores n U aint spose to shift on the run in reverse (But we do)

Mack 12 or 10 parts scarce n pricey . ideal for hoon n along in reverse as U can proggressivly shift them cogs just like go n 4 ward

cya

I don't see much disparity in pricing between the two makers myself. We have PAI here as one aftermarket supplier and I've not had problems getting parts from them, or my favorite truck parts supplier very quickly.

Dog.jpg.487f03da076af0150d2376dbd16843ed.jpgPlodding along with no job nor practical application for my existence, but still trying to fix what's broke.

 

 

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16 hours ago, Mack Technician said:

T300 series got to be quite a bit better than the T200 through development........but anyone who has rebuilt Fuller immediately recognizes Mack stole the T300 slider-clutch design from Fuller. No crime, it’s a better design.

 

To bad they hadn't stole the syncro design as well ! How ever problems seem to be less prevalent on these its still there!  

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Like I said they are great for dump trucks and lowbeds, that is what they were most likely intended for, no trans. was built for super heavy weights or severe abuse, some of the special autos are but normally not found in over the road trucks, the Fuller is easier to shift and when you are in the middle of a shit storm the last thing you really are thinking of is the next gear being perfect because if is not then your Mack trans will have you at a stand still, no big deal with a dump but different story with a big load on. like I said before our feelings toward the Mack trans is only based on what we deal with, we don't run dump trucks we run multi axle, dual lane and either super wide and high or we are running home empty, I don't think they were ever intended for what we do, neither was the Fuller but they seem to hold up better and are kinder to drive.  (if Mack trans is so great how come the 8LL is the most popular trans in the coal feilds)

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