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Tranny hi low issue

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I have a 07 mack cl700 dump I've been having issues shifting into low range it will grind and not move I have put over 10,000 dollars into this truck over last week or so new Tranny new shift selinoid and new shift handle and still having same issue has anyone else had this problem and if so what was the cause it's an 18 speed Tranny 

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Please clarify is the trans NEW/Reman? Or did you replace the range valve on the top of the Trans?  Does the Truck Have A cummins or Mack Engine Eaton or Mack 18sp ! Mack !!!!!  Check to see that the out put yoke is tight!!! If it has a cummins and a mack trans  the loose  yoke issue becomes more complex!

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It's a Cummins not to sure about the details of truck as I am the driver not a diesel mechanic in any way I do know it's a remanded transmission it's a mack maxitourqe 18 speed everything should be tight as a tranny shop did all the work the range valve is new on top of tranny as is the shifter knob I think it's an air issue it shifts fine in morning until tranny temp reaches 150 degree or the air pressure drops below 110 

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Recommend checking air pressure on both sides of the shift cylinder and shift between Hi and Lo, and do this with a warm transmission if possible.  Its possibly you just need some seals 

I've generally heard that the shifting gets better on a warm trans, but in your case the opposite could possibly be true.

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