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Trans issue in 2004 GMC 2500HD, 6.0L w/ 4L80E



the truck is a 2004 2500hd 4x4 crew cab with 205,000 miles. I had a proformance tune put on the truck a couple years ago, and had the shifts firmed up at the same time. A month or two ago I started having a issue with the transmission. The trans does not slip taking off from a stop or in any gear. But something seems to disengage when the trans goes into a coast mode ( letting off the throttle with no engine braking, with no load on the trans). When this happens the is a roaring coming from the transmission. If you press on the throttle it will re-engage and drive normally. At first it would do it going down long steep grades with out any throttle, now it will do it by just letting off the throttle and coasting to a stop. This is my only daily driver. I plan on rebuild it over a long weekend, but need a idea of what maybe broke inside so i can have the parts on-hand. Anybody have a clue what maybe wrong?


Thanks in advance Daniel

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sorry ,i built a million 4l80s and my crystal ball never worked to see what blew up ...


it has to come out and apart first,no one buys anything before that except a rebuilt kit full of seals ,gaskets,fiber and steels,,,


hard parts are expensive


price out a GM rebuilt tranny ,,,3 yr warranty ,no brainer ,you will be rolling by monday


flush the cooler while its out for warranty purposes...compressed air and brake cleaner will work

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Sorry I was refering to the hydraulic. I have the ATSG manual, but it dont have have the hydro flow. All the charts and diagrams I have been finding on-line has all been functions during acceleration, which doesn't help to diagnosis my problem. I am not looking for a crystal ball to tell me the exact probelm. but am needing help to narrow down where I should be looking.

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