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Dealer removed aux belt and thump was eliminated. GM service bulletin authorizes no repair. So, Friday I will match the OEM aux belt to a Gates Greenstripe at NAPA. If no result, will replace tensioner. Til then.
 

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Dealer removed aux belt and thump was eliminated. GM service bulletin authorizes no repair. So, Friday I will match the OEM aux belt to a Gates Greenstripe at NAPA. If no result, will replace tensioner. Til then.
excellent, same result as mine
please if you can post part number for gates green stripe when available
sincere thank you
 

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Take your belt and match it. Don't know if thats the solution yet. I don't even know what that aux belt drives yet, haven't looked yet.
 

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Apparently the fan has it's own drive belt. Page 91 of the 2020 diesel supplement manual.
 

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you haven't looked either? That book means nothing to me, has no specifications measurements, what components are made of, torque values, bolt sizes, its worthless.
 

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There may not be a belt to replace the aux belt ( 6 groove) other than the OEM part number. I'll find out this weekend.
 

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Normally what I’ve seen any aftermarket belt made for our trucks there not the same length as OEM belts. Usually there **mm longer.
ACDELCO 98035628 OEM for my truck 3036 MM
GATES K061195HD for my truck 3051 MM
CONTINENTAL 4061195 for my truck 3071.2 MM
 

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Still, nobody has a clue. I will not be able to prove belts until this weekend.
 

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It may just be horny, park it next to an Impala over night and see if I t goes away.....
 

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On my truck, it is either the aux belt, the tensioner, or both making the thumping sound at idle.

I thought you knew that. Apologies if I didn't make that clear.
or the geometry of the belt path -- multiple short, tight runs and a few runs that in comparison are quite long

One would have hoped that the gm engineers would have modeled this before creating the spec for the tensioner force, allowable belt stretch, etc.

we really need to know if the 2020 dual-alternator duramax models have the thumping sound --> different belt path, different opportunity for resonance
 

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I had a 1980 Monte Carlo that would eat alternators or alternator belts about every 3 months. I got real good at changing diodes. Many visits to dealer, but the never could figure it out. One day I was helping a friend rebuild his Chevy 350 and noticed his pulleys on crankshaft were all the same size. A light bulb went off and I went and looked at the Monte engine. Sure enough, from the factory it had the wrong set of pulleys on the crank. The one to the alternator, and the one to the emissions air pump, both went over the water pump. The dual pulleys on the water pump were both the same size. So the belts were trying to travel over the water pump at 2 different speeds.

Cut off the belt to the air pump and never lost another belt or alternator.
 

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Discussion Starter #100
If GM knows what the problem is and it is as simple as the fan clutch (what they told me) or the belt/tensioner you would think that first the diagnosis would be consistent and that a repair would be available.
 
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