mack88 Posted October 17, 2009 Share Posted October 17, 2009 any advice on why my air shifting is slow on my truck. It is a 1988 4th and 5th splits were always abit slow but now first and high to second and low is slow if i dont have the rpm over 1500 it will grind away. So i have to come to a complete stop start over.So i was wondering if the air pressuer can be turned up.I even found it is abit slow coming out of reverse to. Quote Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/ Share on other sites More sharing options...
HK Trucking Posted October 17, 2009 Share Posted October 17, 2009 The weather is getting colder, you didn't by any chance change your trans oil and refill it with a heavier grade, did you?Other than that, have you changed that spin on air filter on the back of the transmission?Sometimes cold weather will shrink the O rings in the shift cylinders causing air to bypass the piston and cause slow or no shifting. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42067 Share on other sites More sharing options...
mack88 Posted October 17, 2009 Author Share Posted October 17, 2009 The weather is getting colder, you didn't by any chance change your trans oil and refill it with a heavier grade, did you?Other than that, have you changed that spin on air filter on the back of the transmission?Sometimes cold weather will shrink the O rings in the shift cylinders causing air to bypass the piston and cause slow or no shifting.should i maybe put new orings in it is worse when it is cold the oil in the trans is 80/90 Quote Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42068 Share on other sites More sharing options...
HK Trucking Posted October 17, 2009 Share Posted October 17, 2009 should i maybe put new orings in it is worse when it is cold the oil in the trans is 80/9080w90 is OK for a 12 speed. Change that air filter if you haven't already, if that doesn't help, then it could be O rings in the shift cylinders, or maybe a problem with the selectair valve on the shifter. Maybe Rob will comment here, as I know he had to rebuid one of his selectair valves . Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42069 Share on other sites More sharing options...
mack88 Posted October 17, 2009 Author Share Posted October 17, 2009 thanks for the information Quote Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42070 Share on other sites More sharing options...
Rob Posted October 17, 2009 Share Posted October 17, 2009 As a general matter of pricipals I would go ahead and rebuild both the "Selectair" valve, and the shift cylinders in on the transmission top cover. I don't know what year of truck yours' is but the shift cylinders rebuild the same. The earlier transmissions had to have the main fork adjusted for the neutral position, the latter ones did not. If your truck is losing air pressure quite rapidly when the engine is not running, spray the shifter valve, and all fittings, regulators, cylinders etc. looking for a leak. Most times when the shift cylinders are leaking internally they dump air into the transmission case. If you remove one bolt that retains the top cover of the transmission and listen closely, you can hear it. Spray soap on the missing bolt hole and look for bubbles.There should be three regulators on the top of the transmission also. I think their output pressures are about 60-65 psi and this is adequate to operate the trans in any weather. Both Mack, and PAI have rebuild kits put together. The labor only takes a couple of hours to accomplish once the center floorpan is removed from the truck. There are too many small parts to monkey with to not go with a pre packaged kit. I think mine were about 70.00 for both kits and a transmission top cover gasket. When you've got the trans apart, look at the shifting fork for wear/damage. That SOB is expensive but it's not a high wear item from what I've seen.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/11089-12-speed/#findComment-42077 Share on other sites More sharing options...
Lmackattack Posted October 17, 2009 Share Posted October 17, 2009 as the guys mentioned those air cyl get gunk in them or the oil thickness can cause issues when cold. A worn orings let oil in and then the cold weather slows the piston movement if dirt got in with the oil. if it has an exhaust screen clean that out also. that would be my first things to look at. Quote Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42080 Share on other sites More sharing options...
mack88 Posted October 20, 2009 Author Share Posted October 20, 2009 I was gone for a few days so i just. read your info.Thanks again.I know when my air tanks are empty it does lose air by the shifter till the air builds up to a point and then it quits.I will doe that rebuild in the next couple of weeks.the truck is a 1988 RDS688 Quote Link to comment https://www.bigmacktrucks.com/topic/11089-12-speed/#findComment-42142 Share on other sites More sharing options...
Rob Posted October 20, 2009 Share Posted October 20, 2009 I was gone for a few days so i just. read your info.Thanks again.I know when my air tanks are empty it does lose air by the shifter till the air builds up to a point and then it quits.I will doe that rebuild in the next couple of weeks.the truck is a 1988 RDS688If it passes air at any time it needs a rebuild.I can send you the proceedure if needed. I think I've got it scanned already.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/11089-12-speed/#findComment-42143 Share on other sites More sharing options...
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