2015 GMC 3500 Denali Towing problem

trhaviland

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I have a 2015 GMC Sierra HD 3500 Denali diesel that will not maintain RPM nor shift from 4th to 3rd gear when towing a heavy load up hills. Speed will decrease until about 40 mph before it will shift. I've had similiar GMC trucks for 18 years, towing basically the same type of load. Never had this happen before. Took to dealer they told me that's the new design. You have to change to manually shift and shift yourself. I call BS. Anyone with information on this?
Thanks,
Tracy
 
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