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Question on 2000 mack RD 688 rears


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I had a stranf thing happen today. We were hauling gravel to a very sand soil area. I had just dumped and found an old woods road to turn around. Once i backed up my tires started to spin. I decided I better look for a better spot and as i started forward my tandems started jumping. I raised my bed and noticed the very back tires on passenger side were not turning. The other three sets were. I tried backing up and then going forward but the soft sand and that back passenger set of tires wasnt moving. I finally got back on the road but  that back passenger set never moved. Got out made sure nothing was broke, slack adjusters were good. Got back in released parking brake and all four tires started moving. Any idea what happened?

 

next question is how do the rears work. Do i have a power divider? Are the front set positive track and the rear set limited slip? I dont have a button inside to lock the axles. IMG_3293.thumb.jpeg.56dcbabc8809bad149611bc2d1c2cfa0.jpegIMG_3292.thumb.jpeg.391104016219f20e8178d836f0f3cbf6.jpeg

Edited by Ricky Beals
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  • Ricky Beals changed the title to Question on 2000 mack RD 688 rears

Ricky.  The front diff. Will have a longer power divider with an air line going to it. There is a shift fork housing there. The picture of the front diff. Doesn't show the whole thing, but looks like it does NOT have a locking power divider.  

The rear axle is the main drive axle, the front is supposed to come in when a rear wheel looses traction..  it doesn't work very well. 

Did you have a stick or a rock stuck in the right rear slak adjuster, that held the brake on.

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Mrsmarkpaul i dont really know the answer to your question. That rearend was bouncing. When that back tire was off the ground it seemed i was making little hopps forward.  When i got out on the road and could watch everything that tire was not turning, the other three were turning and that whole rearend section was bouncing. 

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I think i fgured out my bouncing problem. I was taking off in 1st in that loose sand was not getting traction. Dont know why the back passenger tire wasn’t turning. This happened again this morning and i immediately went to lo and problem solved. Hey Im a rookie.

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Im getting ready to change fluid in the rears. Thinking of going synthetic 75-90 and have a few questions. If i drain all the oil there still has to be some old oil in there. This is not synthetic so will that hurt.

now i cant get traditional 80-90 mobile for free. Is going synthetic worth it? 

 

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I'm just trying to understand what happened 

So if there's no skid mark and no wheel turning then the wheel must not of been touching the ground 

So for this to happen either something is getting caught on the lead axle stopping travelling back up or the trailing axle stopping it going back down 

There's not a huge amount of parts on a camel back that could make this happen 

So perhaps a good look over it all might be the go

Looking for any fresh paint missing etc

 

Paul

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Those rears were born with 75-90 syn. Also the rear has 2 places to drain oil, and the front has 3 places. I would pull the big side cover on each after you drain the top reduction and inspect the bevel gears...  When you're fill them, you start at the power divider on the front, then the top reduction, then the bowl. Rear , start at top reduction, then the bowl.  5 gallons should do both.

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22 hours ago, Ricky Beals said:

I think i fgured out my bouncing problem. I was taking off in 1st in that loose sand was not getting traction. Dont know why the back passenger tire wasn’t turning. This happened again this morning and i immediately went to lo and problem solved. Hey Im a rookie.

They will wheel hop in loose sand and do all kinds of things trying to get traction.    terry:MackLogo:

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If you don't have a dash switch up next to the ash tray then you have a peanut style automatic power divider. It requires at least one wheel to spin on each axle before you're "stuck". Not exactly sure how to interpret your particular situation you found yourself in, but I'm guessing your power divider was engaging and you had traction on your forward drive but not as much on the rear, so you had some weird tire actions.

If you have the dash switch up by the ash tray then your power divider is manually turned on/off. To use this style you have to stop all wheels, flip switch on, then try going.  One of my trucks has this style, I tend to reserve it to get myself unstuck rather than using it to get stuck. If that makes sense.

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