Transmission struggling

blt2ski wrote:
If it is a 4l80e, I am surprised it has that many miles on it doing towing duty. I usually only get 30K with 5-8K mile fluid, and every other change a full flush of the TC.

While a lot of folks think that trans is a great towing trans, it is in reality, one of the worst ones out there. It has the tallest first gear of an auto trans, so it really has to work to get a load going.

Marty

I have the 4l80 in my 2500hd and i think its been a great trans.My truck also has the 4.10 rear gears too.I do tow quite often but i also drive with a softer foot i dont race off the lights jsut gradually get moving.On the grades i aint floorin it to get up quicker just set the foot to half throttle and let the engine do the work where it finds its sweet spot.If i hit a slight grade where the truck is defintiely not going to pull 4th i let it drop to 3rd then drop the shifter to 3rd do it doesnt hunt for OD.Trans temps before aux cooler would hit 230 on a hot day 100 plus climbing long grades.Now with an aux cooler most it has hit was 200 on the same grade on a 110 degree day and dropped my avg temp by 15 degrees on the flats to 180 towing boat with camper.

It aint the fastest but has done well.My old 3500 work truck with the 6.0 gas had 160k on the first trans but the converter stopped locking up so we replaced it the second has 175k on it and still feels good.It was loaded every day for work and got lighter as the day went on and since it was a company truck i did drive it hard.

My trans issues were caused by a corroding engine computer. No codes thrown just super high trans temps to full trans failure, tear down, new torque converter, and stage 3 shift kit to figure out the out the PCU failure.

Have you checked for any stored OBD2 codes? Apparently there are a few which can cause the transmission to default to high line pressure (i.e. hard shifts) to save itself.

One example is a slipping torque converter clutch. (like this guy)

If it is a 4l80e, I am surprised it has that many miles on it doing towing duty. I usually only get 30K with 5-8K mile fluid, and every other change a full flush of the TC.

While a lot of folks think that trans is a great towing trans, it is in reality, one of the worst ones out there. It has the tallest first gear of an auto trans, so it really has to work to get a load going.

Marty

Delayed upshift can often be attributed to engine performance but diagnosed as transmission issues. Doesn't really sound like your symptoms but delayed upshift can be much harsher than normal when they finally happen. Just throwing that out there.

It is a 2000 GMC 2500 Sierra with 119k miles. Only had it a year but original owner pulled pretty large Airstream- definitely bigger than our 27 footer
The hard shift is just that, it builds up RPM and then up shifts (not in cruise control) with a bang and jerk to next gear. Again, this is on flat land after quite a bit of down hill.

Engine is 6000 Vortec (364 CID)
4 speed Auto Electronic Trans w/Overdrive w trans cooling system

I had it into the Transmission shop recently and he could not find anything wrong (of course it wasn't towing anything) but he did perform the service.

Depending on what year it could be lots of things that are almost impossopible to troubleshoot over the Internet.
What is "older"?

Maybe.....throttle valve cable, pump pressure regulation too high, broken accumulator spring, check ball wore hole through valve body plate, valve body sticking, vac line to accumulator leaking, etc.

We had a sensor go out on our last truck (ford) and it did something similar. I forget the part but it sensed engine and transmission RPM and adjusted to make the shift smoother. Changed out the sensor and all was good.

Older? What year? What trans?
First order of business would be a fluid and filter change. I seriously doubt this will fix your problem, but for a hundred dollars probably worth it to try.
If this does not fix the problem, then it is a transmission rebuild.

Can you describe "Upshifting very hard"? Is it a jerking motion or something else?