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mackr1989

mack 18 speed problem

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2180 in 2000 rd688sx dump truck seems to be slipping when splitting gears from from underdrive to direct in high range while the truck is loaded. this doesnt happen in low range and doesnt happen when the truck is empty. if i shift from underdrive to direct while in high range and slowly lift my foot off of the fuel pedal while still keeping slight pressure, the transmission slides right into gear while splitting. prior to this happening, i could  split and fully and quickly take my foot off the throttle it shifted fine but now i have to baby it.

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Any audible air leaks when parked, engine off, working split?

How many miles on trans? 

Much oil coming out range-split exhaust port? 

If you split and drop RPM fast does it miss engagement and grind past? 

Does thumb and fork audibly react fast with full system pressure and key off? Or delay when operating in this manner?  

Can you pop out of split engagement with full load and full throttle/torque. 

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the truck does leak air but no where noticeable which tells me it could be coming from somewhere internally. air tank is always empty in the morning after being parked at night. air psi while driving is usually around 95-100. 157k miles on trans. not sure about the range split exhaust port? the trans does miss split engagements when shifted fast and grind. i guess the splitter and range selector does engage quickly when the truck is nit running and at full psi but maybe a slight delay..ill check on that. nit sure what you mean with your last question. several people are convinced its a synchro or snap ring but i think it is poor air pressure/dirty air lines or possibly synchro. by the way, the clutch is adjusted but doesnt mean it doesn’t have something to do wi5ht the issue.

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no the clutch was adjusted at around 152k and still has the right amount of play in the pedal. the trans as well as the truck has 157k miles.

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If it’s a synchro snap ring you would be driving it home in low range and no high available.

Last question is whether you are “slipping” under torque like Ttwo said or if your popping out of an engagement under torque? One of two is happening if your experiencing a slip. 

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could this problem be something like a synchro? im worried it could be. again, the trans does smoothly upshift when splitting very carefully by obviously preselecting direct, slowly taking presuure off throttle and keeping slight pressure and the shift slides right in.

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slipping under torque sounds more like it. quickly letting off of throttle after split then quickly getting back on it after rpms drop causes the shift to delay, grind and knock into gear or it just grinds and doesnt go into gear and i have to move the shifter to neutral to stop it.

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Pre select your range. Never pre select split. Your thumb should move 0.1 seconds before your foot is releasing the torque of the engine. Range has synchro plates and a position sensing mechanism, split has steel on steel. From the moment you thumb your either burning fork against an immovable engaged slider gear or rolling and rounding over your slider gear teeth while you drift and hunt for a RPM mesh. 

You need to get good at it or the splitter is going to be ruined. If your unfirmiliar with the truck avoid the down-split till you get comfortable with the rig and then pick it up again. Drop a full gear. 

IMO- i always dropped a full gear and split upward. The grind dwell time is too much on down split. 

Edited by Mack Technician

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ive been told to always preselect splits but only just before or after getting on or off of throttle. the only time splits smoothly engage is by preselecting and slowly getting off of the theottle.

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Imagine you have 15 minutes of lifetime total grind time in a splitter. Split fast any direction. Up is instant. Down split is a double motion on your pedal like “double clutch”. On thumb, off pedal, into grinding hunt, on pedal to find higher RPM. Upshift is off pedal and drop in. 

When you drop a gear on your stick you tease and click the tips of your teeth lightly then jam when you feel it mesh. A splitter is like putting your foot on the shift stick and kicking it to get it into gear, full pressure, no feel, no finesse. You have to minimize the damage with speedy delivery of meshing RPM.

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Your operator conscience is good and sound and telling you “this is not good, this is not smooth, I may kill my money maker, I gotta figure this out before I damage something”. Find a guy who has years on his truck and only used up 5 of the 15 minutes of total lifetime grind in 500K miles. Have a good pow-wow with him and see what he does, buy him lunch. 

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12 hours ago, Mack Technician said:

Your operator conscience is good and sound and telling you “this is not good, this is not smooth, I may kill my money maker, I gotta figure this out before I damage something”. Find a guy who has years on his truck and only used up 5 of the 15 minutes of total lifetime grind in 500K miles. Have a good pow-wow with him and see what he does, buy him lunch. 

Yup agree with Tech Get on the powwow There are occasions where the shift lid is at fault the shift fork pinch bolt has loosened off  how ever I would do as Teck has stated for now if possible!

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ok so how exactly am i splitting incorrectly. i always kept my foot on the throttle and preselecting to direct for only a split second before releasing throttle, then id let the rpms come down then get right back on the throttle again. total time with this was fast and only took about a second. if this is incorrect, how should splits be done? release throttle entirely, split to direct and engage, then rethrottle?

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turned out that the synchros in the trans were worn out. i was told there was a recall on these and mack updated the synchros with harder metal. i wound up putting a reman in seeing that it wasn’t that much more expensive than rebuilding what i had. i am noticing that it can be difficult sometimes to move the shifter from gear to gear, as in its getting stuck and wont move. ive shifted a lot of different mack trannys and the only other transmission ive used that does this is a six speed low hole..2 stick.

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A slight twist in mainshaft would do that, but not in all gears. When rebuilt the tech needs to sand and polish the galling out of the shift rails. If they didn’t polished the gall, and then assembled dry, it could get sticky in all shifts initially. 

Edited by Mack Technician

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so is the transmission going to stay this way unless it gets pulled apart again? or does it need to be broken in?

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Mackr, I don’t want to make predictions, cause you keep disproving & confusing me, like why was splitting the issue, but synchro the failure? I’m hoping someone did not try to take advantage of your dissatisfaction? The Synchro (singular) generally fail from a bad, misdesigned, mainshaft snap ring and it’s undercut land. Plates and clutches were not the issue, they were well constructed. I will recant if one of the other mechanics call me on it....there was never a modification to “harder synchro metal”.  If that guy told you Mack built an updated mainshaft with a double thick snap ring to retain the range synchro thrust bearing I’d say the guy was squared up, but sounds like a line from a guy who thinks there are multiple synchros in a non-gear synchro shift trans. Then again....if your driving the crazy thing without grinding.......something had to have been happening???

 If your reman transmission(after running a month) is undriveable your going to need to position yourself for a warranty claim. Part of the claim should include a credible witness to the issue....as in a repair order from a dealer that condemns the transmission as having an internal issue or driveability issue that may include a suggested repair for the transmission. 

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well wont a bad synchro cause a a splitting problem? mack didnt come out with an updated synchro design? i was told there were multiple synchros that were worn out. im not disagreeing with what youre telling me. im just repeating  what i was told and i dont know much about transmissions from a mechanical standpoint. also, the power divider was accidentally engaged while it was getting worked on. the switch label is worn and illegible and i couldn’t remember if it was supposed to be in the forward or backward position on the dash but did notice that i didnt think it was in the position it was in when i dropped the truck off. i switched the power divider from forward to backward as in “out” and now it shifts just fine. no problems with the transmission and luckily, the power divider still works.

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Automotive transmissions have synchronizers to bring individual gears up to speed before engaging them. That’s why you don’t have to think when shifting a car or pickup. Your transmission has one synchronizer for the range shift. Hence you have to RPM match gears manually or you get a lot of clash. No sychro(s) between gears.

If you try to shift a manual car without pushing in the clutch you would experience a Mack 18 speed in your car. Synchronizers can’t work without clutch depressed. I’ve tried it, it’s a bit tricky and not good for a synchronized trans, but experimentation and curiosity wins over common sense sometimes. 

One thing to remember is to never range shift to low when going at high speed. A Mack tranny repairman from UPS (wouldn’t guess they use Mack tranny?) told me 3 times shifted to low, at 55 or above, and synchro is junk. 

Edited by Mack Technician

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Mack syncrhos starting failing in 2003.  This is when I started doing inframe repairs. First they said it was range shifting in reverse that was breaking snaprings.  So they updated the aux mainshaft with thicker snap rings and grooves.  Synchros kept failing.  Then it was the synchro shift fork bolt loosen up and causing incomplete shifts and eventually failure.  So start checking torque on the bolt but  Synchros kept failing.  So they updated synchro, lo and hi gears.  Guess what , you still see failures.  Then you have the barrel trans cooler under the “cactus”  hose failing and pumping coolant into the trans destroying Synchros. 

Seems like the T2 trans where doing well, then they got cheap with the T3s, it’s never been the same. 

Have synchro failures actually ever been eliminated?   I would say no. 

Updated this, updated that, is something that dealers say to make you fell better.

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