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R686ST with hammering noise


dozerman1
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I have a R686St 300+ with 5 speed.I have a hammering noise ,sounds like a tooth missing from a gear.It only does it in 4th and 5th gear.Not noticeable as much while in a pull and vanishes if taken out of gear while rolling down the highway.Is noticeable when downshifting as well but only from 5th to 4th.After shifting from 4th to 3rd noise goes away.Anyone have any ideas?Have checked for metal in transmission and rears and found nothing.Also checked registration with infrared heat gun while driving truck during the work day and all components a simular in temp.Hate to start pulling stuff but not sure how to proceed.

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Sorry Glenn.I believe Mack dealer said it was a trl107.I think that's right.It's just what I call a straight 5 speed with no high or low switch.Seems like everyone calls it something different.Will be glad to look on transmission for more info.Any idea where info would be on it?

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Sound like it could be a tooth off a ring gear and pinion. Try driving it empty and roll down the window and leave it in 4th or 5th and let it coast down to a low speed and listen for clicking. That is a quick way we check out to see if its in the rear ends. If nothing shows up we put the rears up on stands and run the truck in high gear and listen for the noise. Be sure to remove all rocks from the rear tires if you do this.

good luck

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Drain the oil and remove the 8 hole pto cover, take a look up inside the transmission. Use a pry bar and try to move gears and counter shafts around. If the main shaft has excessive play near the front-where 4/5th gears are at, it could be that the spigot bearing has come apart. With the front 8 hole pto cover off then you will have a chance to look around. The tranny model and serial/part numbers are located above and slightly behind the front 8 hole pto cover, stamped into the case.

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are your drive tires all the same height? I had a similar problem years ago after putting on 4 new tires across the back,the difference in height made the power divider want to engage when I shifted into hi gear, sounded like someone beating on the transmission with a sledge hammer

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are your drive tires all the same height? I had a similar problem years ago after putting on 4 new tires across the back,the difference in height made the power divider want to engage when I shifted into hi gear, sounded like someone beating on the transmission with a sledge hammer

If the power divider is almost wore out it will bang in and out almost constantly when put under almost any load. I had the same issue when 1 of mine was on its last legs. The divider was engaging and disengaging without a whimper while I was using it daily. But with the wear, it became very noticeable within a few days of use.

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