This is from fellow QSClub member Butch 02 Sierra Denali. He runs AWDGM.com, a great site that caters to the awd GM trucks and suv's, and they even have a spot for our Quadrasteers . What a bunch of nice guys !!! You might even spot me over there from time to time !!

This has to be about the best tranny flush write-up I've ever read. I also know Butch personally, so I know it's on the money. (Nope, haven't done it myself, yet, but will here in a week or two. I'll post a follow-up here )

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I'll pass along a great trick I learned from my drag racing days to change all the tranny fluid in the transmissiom, converter and cooler. I used to change the fluid in my race tranny every 20 runs, and this is a great time saver.

1) If you have over 10000 miles since the filter was last changed, drop the pan, clean it
completly, change the filter and bolt back up with a new gasket. (If you jump ahead and get the fitting in and pump out the first 3-4 qts before you drop the pan it will be empty and much cleaner to do the filter change) If you have less
than 10000 you can skip ahead to #3, although its always safer to change the filter
and clean and check the pan for particles or clutch pieces (not a good sign!)
2) Add 3 or 4 qts new fluid to bring it to the full mark on dipstick.
(DON'T START IT)
3) Remove the upper transmission line from the radiator, only very little fluid will drip,
if any at all, because its higher than the transmission.
4) Screw a short fitting into the radiator that you can attach a piece of tubing to, I
think the fitting size is ¼". You will need about 2 or 3 feet of 3/8 clear plastic
tubing. Clear gas line works great, but you can use almost any plastic or vinyl
tubing.
5) Clamp the tubing onto the fitting you put in the radiator and run the tubing over the
radiator support and out the front of the truck and into a gallon or larger jug.
I tape a big nut as a weight on the end of the tube to keep it in the jug, if you have a
helper they can just hold it in.
6) Start the truck. Fluid will start pumping out the tube into the jug. Watch the tube
untill you see bubbles or hear a bubbling sound and immediatly shut the truck off. It
will take about 30 seconds and will pump out 3-4 quarts of fluid.
7) Add 3-4 quarts of new fluid, to the full line on the dipstick, empty the jug (don't
forget that) and start it up again as before and watch for bubbles, and shut it off
again.
Repeat again, adding 3-4 quarts, empty the jug, start, watch for bubbles and shut
off.. If you really want to be assured that you have 100% change, add just 2 more
quarts and repeat. If the fluid was dirty, or the new fluid is a different color, you will
see the color change in the clear tube, this is when you know you have changed all
the fluid. I usually watch for the fluid color change rather than the exact number of
quarts.
9) Remove hose, replace fitting with transmissiom line, fill transmission to full line,
start the truck and warm up, when hot check and fill to line on dipstick.

You've just changed and flushed every drop of old fluid from the transmission, converter and cooler system. If your changing to synthetic, you will be 100% synthetic, none of the oil will intermix with this flushing procedure because the fluid follows one path from the filter to the pump, through all the transmission passages, through the converter and then out the cooler line and back to the pan. As my Denali only has 7000 miles on it I haven't changed it yet, but in my last truck, a 2000 Sierra 5.3, it took 14 quarts as i remember to flush it. Be safe and get 15 quarts. This is not a back-flush and will not dislodge any particles in the tranny. It is completly safe and easy to do. It may sound difficult but once you get the fitting made, you can do a complete flush in less than an hour for only the cost of the fluid.
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Thanks, Butch !!! You da man .