sritchhart Posted February 28, 2012 Share Posted February 28, 2012 (edited) I have a 1987 Superliner with a 12 speed. It does not shift into high side when the transmission is cold. After I run it for an hour or so it will click into high side on all the gears, and run fine the rest of the day. What can I do to fix this issue, or should I take it to the Mack dealer? Any thoughts would be appreciated. Thanks! Edited February 28, 2012 by Farm Man Quote Link to comment https://www.bigmacktrucks.com/topic/23512-12-speed-high-side/ Share on other sites More sharing options...
Rob Posted February 28, 2012 Share Posted February 28, 2012 I have a 1987 Superliner with a 12 speed. It does not shift into high side when the transmission is cold. After I run it for an hour or so it will click into high side on all the gears, and run fine the rest of the day. What can I do to fix this issue, or should I take it to the Mack dealer? Any thoughts would be appreciated. Thanks!Rebuild the shift cylinders on the transmission top cover as they are leaking internally when cold. I would also rebuild the "selectair" valve, (shifter handle) while at it. Your problem is common as those transmissions age.l This is an easy repair but do consult a service manual before breaking things apart.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/23512-12-speed-high-side/#findComment-104463 Share on other sites More sharing options...
Lmackattack Posted February 28, 2012 Share Posted February 28, 2012 sounds like the shift Cyl is worn or you have gunk build up? worse case is you have bad shift forks. like rob said..an easy fix Quote Link to comment https://www.bigmacktrucks.com/topic/23512-12-speed-high-side/#findComment-104466 Share on other sites More sharing options...
Superdog Posted February 28, 2012 Share Posted February 28, 2012 A friend of mine replumbed the air lines from the splitter with larger lines after having similar trouble when his truck was new,said that help alot.Those transmissions also have a spin on air filter that is often overlooked. Quote Link to comment https://www.bigmacktrucks.com/topic/23512-12-speed-high-side/#findComment-104470 Share on other sites More sharing options...
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