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Originally Posted by B_and_D
I wasn't driving, so I can't tell you what the RPM's were. I just felt the truck's transmission "slipping" out of 3rd whenever DH put his foot on the gas hard.
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True automatic transmission "slippage" is a sharp rise/fall in the RPM
during the shift. When an automatic transmission shifts gears, the set of bands and clutches that are used for the original gear are released, then another set of bands and clutches needed for the new gear are tightened. If the new set of bands/clutches don't engage promptly, there will be a very noticeable delay between the gears and the engine will rev up sharply until the new set of clutches finally take hold, then the engine RPM will drop and hold steady for the new gear. A transmission that has really bad clutches/bands will also have some slippage if the throttle is sharply opened in a gear...the RPMs will rise by as much as 1000 RPM...very similar to a manual transmission car with a really bad clutch. For a really good explanation of how an automatic transmission works,
go here.
My guess is what you were experiencing was a quick downshift (no noticeable delay or spiky RPMs) when DH stompled the throttle, followed by a rapid rise in engine RPM in the lower gear for a few seconds, then a quick, possibly jerky upshift back to the gear you were in when the throttle was stomped. I'd also guess that the period of time spent in the lower gear was longer and the engine was rising to higher RPMs before it upshifted when you used Tow/Haul mode. Given the 20-30 mph speeds, I'd also guess that the transmission was going from 2nd (maybe 3rd) all the way down to 1st, then back to 2nd when it hit its upshift point.
If these guesses are correct, then what you were experiencing was not actually "slippage" but rather the quirky "hunting" of a GM transmission when it is very close to its programmed upshift points under high load and wide open throttle. If this quirky behavior was occuring mostly when you were not far from Yosemite (IOW, at elevations around 4000 feet), you were also experiencing the deleterious effect that altitude has on engine performance. 4000 isn't very high but it is high enough to cause a 10 to 15 percent drop in performance which in turn is enough to produce effects you're not used to in near-sea level operations.
If my guesses are not correct, please advise with more details.