Mack 18 speed transmission when on high range on the throttle it pulls fine then I let off the throttle it goes in to neutral and make a terrible grinding noise any idea what wrong?

[Kirby Hawk]

I just bought a 1991 rd 688 with a Mack 18 speed transmission when in high range and I let off the throttle in goes in to neutral and makes a terrible grinding noise is there clutches for the high range ? It shifts good through the gesrs

[doubleclutchinweasel]

I do not know the Mack version.  But, I have been into a RoadRanger or two.  The hi-lo synchro unit has teeth in it that engage the mainshaft.  If these get worn too badly, they can try to disengage when you let off the throttle.  Not sure how the Mack units work.  But, it sure sounds like something in that area, especially if all is well in lo range.

Of course, you did not specify if the hi-lo appeared to be going into neutral or if the actual shift lever is going into neutral.  But, from your wording, I assumed you meant the shift lever did not move.

Edited October 16, 2020 by doubleclutchinweasel

[Kirby Hawk]

Thanks for the answer the shift lever stays in gear 

[Mark T]

$ynchro 

[doubleclutchinweasel]

Sounds like the hi-lo synchro is disengaging...

Could be the synchro ring or the gear it engages on the mainshaft.

[Joey Mack]

If the "high range" gears disengage regardless of splitter position,,,,,  the clutching teeth in the synchro are junk...... see: Mark T, and D.C Weasel above...   Merry Christmas..:)  Jojo.. The synchro in a mack tranny. is a huge version of a motorcycle clutch with a basket on both sides... there are special holding tools to use to do this repair in-chassis...  if you dont have the tools, then pull the tranny and stand it on its nose on 2'.... 6x6 blocks..  Remove the yoke, and the rear shift cover, and range fork cylinder,  then you can unbolt the back box and lift it straight up.....  when you rebuild the synchro, pay attention to the "R" stamped in the basket...(REAR)  Then alternate your discs,, friction disc, then fiber disc, then friction disc...... on both sides...  this gets tricky... when you install the hub.. .. timing the countershafts is easy.... there are clear marks or you can just use the keyways... it all divides by '3'..... let me know if you need more info Jojo

Edited December 9, 2020 by Joey Mack
More content needed

[Joey Mack]

Bad synchro........

[fjh]

12 hours ago, Joey Mack said:

If the "high range" gears disengage regardless of splitter position,,,,,  the clutching teeth in the synchro are junk...... see: Mark T, and D.C Weasel above...   Merry Christmas..:)  Jojo.. The synchro in a mack tranny. is a huge version of a motorcycle clutch with a basket on both sides... there are special holding tools to use to do this repair in-chassis...  if you dont have the tools, then pull the tranny and stand it on its nose on 2'.... 6x6 blocks..  Remove the yoke, and the rear shift cover, and range fork cylinder,  then you can unbolt the back box and lift it straight up.....  when you rebuild the synchro, pay attention to the "R" stamped in the basket...(REAR)  Then alternate your discs,, friction disc, then fiber disc, then friction disc...... on both sides...  this gets tricky... when you install the hub.. .. timing the countershafts is easy.... there are clear marks or you can just use the keyways... it all divides by '3'..... let me know if you need more info Jojo

This is an RD remember!!!!! just popping the rear lid off is not easy in the truck ! Trans removed is Much easier than inframe  and for someone has never done this and hasn't got the special tools! Remove the trans and get your wallet out! Volvo Has made this a crazy expensive repair! They would rather you buy a new truck! If You were to do this same repair to a fuller it would be half the cost and yup it happens to eaton  fuller as well just A more reasonable repair!

[Joey Mack]

"F" is right.... I have done these "in-chassis" on RD's.. DM's.... and 1 MR concrete pumper with dual steering... It's hard work....  Of course i've done plenty in CH'S and CL's... dual exhaust adds to the challenge.... anyway,,, listen to "F" from Canada... He/She  has a boatload of knowledge..... Merry christmas.. Jojo