i've searched and saw nothing that looked like it was similare to my problem.

2001 2500HD 4x4 crew cab short box auto.
100% completely stock. 125K on it.
i tow a 12Klb enclosed tag car hauler. i make a bout 6 or so 3-4 hour trips a year w/ it for the last 3 years.
i changed the trans fluid last at about 50K and at this point it still looks good.

problem:
only while towing and only after about 30 minutes of towing (points to it being heat related) the trans starts to basically slam into gear on upshifts. i always notice it first while driving along when the trans shifts in and out of 4th. but once that starts happening, i'll notice its doing the same thing when you come to a complete stop and it shifts thru all gears. its never done this when not towing. could this be a simple trans fluid chnage and its fixed kinda thing? new filter? low fluid? possibly a failing TQ converter? i checked the fluid level a few days ago and it looks fine along w/ the color.
at the least i'm looking for enough info on what the problem is to keep from getting screwed by a trans shop.
what are the normal problems we see w/ these transmissions? it has the factory trans temp gauge and it almost never gets above 200 while towing.

help.........

 

The hard shifts are a by-product of the real issue. The PCM will command maximum line pressure when slippage is detected. If you have data logging software you might try logging Engine RPM, Trans input RPM (4L80-E has seperate sensor for this that 60's do not) and VSS (output) speed sensor.
The slippage is probably in the converter clutch. You might get a generic slippage code P1870 if this continues. Bumping up the duty cycle for TCC lockup should help. The converter probably isn't damaged yet because the PCM detects the slippage and compensates.
Also, next time you service the trans fluid use Dexron-VI and new filter. The new recipe really does work better.

 

havent had the PCM scanned for cades yet. but i did notice that the hard shifts only start once the converter locks for the first time. its fine till the converter locks, then every shift after that is hard.... very hard. it will stay that way untill i shut it off. starts over once the converter locks.
are the TQ converters known to go bad? is it just the lock-up clutch's failing? since this only does this when towing, i've been waiting till i have a reason to tow and then i'll stop by a local trans shop i have a hook-up w/ and have them ride along and scan it.
any thoughts untill then?

 

got it scanned today. 1870 code. its dead.

 

PO1870 means the converter clutch is slipping this could be for several reasons The converter regulator valve in valve body may be worn , The PWM solenoid screen may be clogged. Or the converter clutch is just giving up.

 

so why has the trans shop told me the whole trans needs a rebuild?
what converters do you guys offer for towing?
i was also told by a friend that the valve bodies on the 80's doesnt wear out like the ones in the 60's. its common for the 60's he said.
i'm pretty machanically savy,just not experianced w/ auto's. whats it take to address all the items you listed above?
whats the PWM soleniod?
and thanks for helping me here.

 

The PWM in the 80e is the lockup solenoid, You could try changing it first but its likley your converter clutch is gone, I would suggest replacing the PWM/TCC solenoid and the converter and TCC oring on the inout shaft if the fluid is clean .

PWM solenoid REMOVE PAN
CONVERTER -REMOVE AND INSTALL TRANS.

 

feel free to send me a quote for a converter for towing if you like.