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vansite89

mack 18 speed makes loud scratching sound

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often times when shifting from underdrive to direct or vise versa, the 18 speed in my truck makes a loud scratching sound. is this normal for a mack? the only other mack 18 speed i had driven prior to mine was in a 98 rd with a 427 and i remember it doing the same thing but it also had 500k plus miles on it. is this normal? also sometimes when splitting from direct to underdrive to downshift, the transmission will neutralize and just grind. whats that about?

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Range syncro may be going bad.  Mack has updated the syncro specs several times because of issues.

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Couldn't say what the current price might be.

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3-4K. Depends on what updates have been done. I just did one for under $500, guy stopped driving immediately and did not ruin anything. 

Need only one specialty tool. Trans can stay in chassis and just pull down rear case. 

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what do you mean by just pulling down the rear case? would it be a time consuming job just to inspect what it needs?

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Takes 6/8 hours if ya got parts on hand it also can be a loose pinch bolt on the split fork or range fork can cause the same symtoms  Bit less costly how ever!!!!!!!!!!!!!!!

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Posted (edited)
9 hours ago, vansite89 said:

what do you mean by just pulling down the rear case? would it be a time consuming job just to inspect what it needs?

What’s your level of self motivation, mechanical competence and willingness to learn procedure? If you pop the rear trans inspection covers are you going to be able to point out the synchro? Do you have a book? Manual on trans? General firmiliarity with trans? If your range is grinding it is almost always synchro.

Edited by Mack Technician
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if i had a manual then i would inspect it myself but seeing that i dont i wouldnt be able to identify the synchro and if it were even worn because im not sure what its supposed to look like and or how far gone it could be.

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how quite should gear splitting actually be with this transmission anyway?

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Should sound like the first break stroke of a pool game.

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How are you splitting? I flip the switch while my foot is on the gas, then let off and it shifts. If I shift while not under load, it will do what you describe.

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We are talking about two separate things and I think your in the right by use of proper terminology. Your range is not grinding, but your gear split is because it is not synchronized like range is. Since split is not synchronized you have to gear match speeds.

Sounds like you are also shifting properly. Does it seem the splitter function (and/or possibly range) is popping out of engagement lock-up under heavy torque........but not popping out when unloaded?

 

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switching ranges from low to high and high to low is smooth. the only time the trans really grinds is when splitting from direct to underdrive...downshifting a half gear...and it doesnt even happen all of the time. sometimes all i hear is a slight smooth click and it pops right in. eighth gear direct to underdrive is the only gear that will grind most often and the only gear where the transmission will neutralize then i scramble to totally put the transmission in neutral and reset.

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Your Transmission is fine..... you need to switch to neutral to drop down half a gear or it will grind. Also only shift up under load and it will click in when you let off.

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Posted (edited)

You drop out of the split engagement when you relax the torque and allow the fork to pull the slide clutch out of engagement. Same scenarios when going up or going down. Disengagement occurs the moment the slider hits coasting (0 torque) lash. When you want to slide to underdrive you have to do a simultaneous motion of releasing the throttle, thumbing your switch, popping slider out in coast lash and immediately getting back into the throttle while slider clutch is fighting to find a home. You have to bring engine RPM immediately up to let slider get back in or it will sit and grind. No different then if you were going from a high gear at 55 mph to a lower gear while maintaining 55mph...you have to raise the RPM or you will not find your gear. Never pre select the splitter, it is not like the range shift. You have to make all thumb play instant relative to your foot. When your foot comes off the throttle your thumbs moves in unison. The amount of time your splitter clutch floats in a grind can eventually determine the life of your rear case.

IMO- You can tell a good driver by front fenders and his slider clutch wear. 

Edited by Mack Technician
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i do all of those things. i understand how to shift through the gears and the only gear that gives me trouble is usally only eighth while splitting. the other gears only make a real quick scratching type noise in between splits but it doesn’t always happen.

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Posted (edited)

Sounds to me your doing it right the only thing I have to contribute and you probably already know and has been mentioned  can't say enough times is  SELECT SPLIT switch it and break torque right away ! Don't switch and wait! it wears the hell out of the shift fork if you pre select and wait! that little grind you hear once and awhile  is acceptable ! But  worth experimenting with different techniques to correct IT !  IE: engine brake on /engine brake off / usr the clutch don't use the clutch on the offending gear! experiment !! The 12 speed suffered from this same issue back in the day  and there were guys out there that have it down pat!

I AM NOT ONE of them thou :rolleyes:!

Teck  said it Here

IMO- You can tell a good driver by front fenders and his slider clutch wear.

Experiment!

Edited by fjh
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