Mack Transmission stuck in third gear

[Mack Snow Fighter]

Hello, 

While driving today, suddenly the 1996 Mack RD688S would not come out of third gear. The shifter is stuck there. Any ideas what went wrong? 

[Geoff Weeks]

Plenty, but without being able to see and feel, not much help.

twisted shaft, bent or damaged fork, interlock fail, shift tower problems, bearing failure, the list goes on.

Your going to have to investigate more.

[fjh]

Yup trans make?

[Mack Snow Fighter]

Its a Mack 9 Speed, Haven't found a plate on it yet. 

[Mack Snow Fighter]

Removed the cab floor,  and all the bolts on the big cover. Cover seems somewhat stuck. Looks like we might have to drop the trans in order to pull the cab floor. Not enough clearance. 

[mowerman]

as geoff said im sure its a problem to be addressed from under the top cover,,,might be an easy fix just keep working that top cover,im sure that's all you will need to replace parts good luck,,bob 

[mowerman]

especially if there was nothing wrong with it pryer to the issue

[Mack Snow Fighter]

Thank you gentlemen for your reply's.   We were able to remove the cover only partially, since the transmission is still in the truck.  Looked inside and all seems well with the transmission, clutches slide back and forth fine.  Put cover back on, added a quart of oil, drove on the road, and she's fine like nothing happened.   

[Joey Mack]

when you had the shifter out, did the slots for the shifter pawl look rounded out as well as the pawl ? It can get stuck in gear or even put it in 2 gears at once.  If there is too much wear.  Going back to what trans it is..  its a T-2090..

[Mack Snow Fighter]

I didn't notice that they were, but I was taking a good look at the forks  and the clutches, and not sure if I got a good look.  My son who drives it as much as I, says he revved it pretty high coming down a hill when he downshifted, then when it was stuck, it was almost like it was jammed in the wrong gear, in other words it was in third gear, but he wanted  first gear.  The thought of a worn shifter paw crossed my mind. That wouldn't be hard to change out. Can't get the cover off with transmission in the truck, unless I cut some of the floor out.  Thanks for the model on the trans. Looked it up, and yup, that's the one. 

 

 

 

[Joey Mack]

You will need to remove the passenger seat, then you can pull the floor pan . The bolts are 13 mm metric, and theres about 12 of them. Pull the floor mat back at the edges to see the bolts. The throttle pedal will come with that floor panel. It can stay mounted, just disconnect the linkage or wire harness,  not sure what you have for an injection pump.

[mrsmackpaul]

It sounds like a rod all selector fork is loose on a rod

On any other brand of box, we're the stick enters the top of the box becomes worn and when you race the gear change the stick comes out of the selector rod dog and leaves the transmission stuck in gear

But more detail is needed 

 

Paul

 

[Mack Snow Fighter]

Well that's very good to know.  I  better do it sooner rather than later, or I'll  again be driving the last couple miles home at 12 mph LOL! 

[mrsmackpaul]

A Mack box is fair bastard to pull out compared to the Road Ranger, however if it has to come out then it has to come out

However if the cab is a air ride cab, which I think this is, you can the floor sections out, disconnect the air bag and ride height valve, and probably air cleaners and a few other things and jack the cab up a fairway to get the top off the box, you'll be surprised 

Getting the top off the box is the easy part, getting it back on is a fiddly job but can be done 

Don't cake it's mating surfaces in RTV (I think you blokes call it that) or form a gasket stuff, it oozes out inside the box, drops off, gets sucked throught oil pump and plugs all the oil galleries feeding the bearings, then you'll notice you start jumping out of gear as the RTV stops the sliding dogs going all the way in, some ass clown mechanic had done this yo my old bucket of bolts before I owned it, I had to pull the box out amd fully disassemble it and change bearings and a couple of gears and sludimg dogs, theres a few huge days work right there 

Once you put the lid back on the box, drop the tail shaft off try it through every gear before calling the job done and lowering the cab back down

These are things I have learnt the hard way, the very hard way

The book (work shop manual) won't tell you any of this, the reality is most of us aren't equipped or flushed with enough cash to pay someone to do this

For me, I have learnt this type of stuff on the side of a road or in the shed at home 

Most mechanics will just fit a change over box and call it done were most times it doesn't cost a fortune

I doubt most mechanics would even know how to drop a box apart these days

 

Paul

 

 

 

[Mack Snow Fighter]

On 6/18/2025 at 5:45 PM, mrsmackpaul said:

A Mack box is fair bastard to pull out compared to the Road Ranger, however if it has to come out then it has to come out

However if the cab is a air ride cab, which I think this is, you can the floor sections out, disconnect the air bag and ride height valve, and probably air cleaners and a few other things and jack the cab up a fairway to get the top off the box, you'll be surprised 

Getting the top off the box is the easy part, getting it back on is a fiddly job but can be done 

Don't cake it's mating surfaces in RTV (I think you blokes call it that) or form a gasket stuff, it oozes out inside the box, drops off, gets sucked throught oil pump and plugs all the oil galleries feeding the bearings, then you'll notice you start jumping out of gear as the RTV stops the sliding dogs going all the way in, some ass clown mechanic had done this yo my old bucket of bolts before I owned it, I had to pull the box out amd fully disassemble it and change bearings and a couple of gears and sludimg dogs, theres a few huge days work right there 

Once you put the lid back on the box, drop the tail shaft off try it through every gear before calling the job done and lowering the cab back down

These are things I have learnt the hard way, the very hard way

The book (work shop manual) won't tell you any of this, the reality is most of us aren't equipped or flushed with enough cash to pay someone to do this

For me, I have learnt this type of stuff on the side of a road or in the shed at home 

Most mechanics will just fit a change over box and call it done were most times it doesn't cost a fortune

I doubt most mechanics would even know how to drop a box apart these days

 

Paul

 

 

 

Just notices now I got a message from down under, that's pretty cool and I guess you got Mack trucks there too. This just happened again, this time on the job. Never happens when I drive it, but guess who drives it the most!  Now we know what we have to do when it happens, just pull the 4 bolts of the shifter housing off, pull the think up and put it back in correctly.  Good point about the RTV. Mack has good stuff they call Silastic, never had anything better.  I had a Mack box out once to do the synchronizers on the splitter. Not too bad of a job, but had trouble getting it back together till I figured out what I was doing wrong with the splitter. 

[Brocky]

31 minutes ago, Mack Snow Fighter said:

Just notices now I got a message from down under, that's pretty cool and I guess you got Mack trucks there too.

YEAH!!!! This is a worldwide site.. Several Canadians, and Aussies. Paul from Ireland and Vlad from Moscow Russia!!!!

Sounds like you need to pull the right seat and floor, jack the cab, and take the top cover off and inspect it close for wear and put a few new parts in it?????