There isn't much about the modern electronic automatic transmission that won't induce a panic attack in most supposed "mechanics." Like the internals of a differential, computer or great white shark, you very rarely ever see them unless something goes terribly wrong. But, not everything in a transmission is a nightmare to replace, and the solenoids in this tranny are a few of those happy exceptions. Little more difficult than a standard oil and filter change, the worst part about this procedure may just be getting the Dextron III out of your hair.
Chock the rear wheels, lift the front of the vehicle and insert a pair of jackstands under the frame. You can use a set of ramps for this job, too, but chock the rear wheels anyway. The transmission is holding the vehicle in park, and you're working on the transmission. This is not the time to find out how to manually release the parking pawl.
Place a clean drain pan under the transmission, and begin removing the transmission pan bolts from the rear to the front. If you do this right, the pan will gradually open from the rear and cascade fluid into your drain pan in a neat waterfall. It's your decision whether or not to re-use the transmission fluid, but you might as well replace it if the existing fluid has more than 20,000 miles on it.
Remove any gasket material clinging to the transmission case or the oilpan, and clean the oilpan and case mating surfaces with carburetor cleaner or an equivalent solvent. Remove the filter and either save it or toss it, depending upon whether or not you want to replace it. Again, you might as well replace the filter if it has any more than 20,000 miles on it.
Look toward the rear of the valve-body. The valve body is the transmission's hydraulic "brain," and the shift solenoids are the electronic valves that open and close the valve body's hydraulic passages. The solenoids themselves, as installed, look like little black, plastic boxes hanging off of the rear of the valve body. You'll see a plastic-covered wiring harness bolted to the bottom of the valve body. The center "spine" of the plastic harness cover points directly at the 1-2 solenoid. The 1-2 solenoid is the one closest to the gear-selector-side; the 2-3 solenoid is the one almost right in the middle of the transmission pan opening, to the right of the 1-2 opening as you face the front of the vehicle from underneath.
Look at the solenoid where it plugs into the valve body, and you'll see the tip of a U-shaped, metal retainer poking downward out of a slot in the valve body. Slide a flathead screwdriver into the open area in the retainer (the bottom-inside of the "U"), and twist the screwdriver or use it as a lever to pull the retainer downward. Catch it as it comes out. After you get the retainer out, unplug the solenoid from the wiring harness, and pull the solenoid out from the back.
Lubricate the O-ring on your new solenoid with a bit of transmission fluid, and push the new solenoid in as the old one came out. You'll feel it pop in. Push the retainer up and into the slot, engaging the tip of the solenoid and locking the solenoid in place. Reinstall the filter or install a new one, and reinstall the oilpan with a new gasket. Hand-thread the bolts in and tighten them to 97 inch-pounds (NOT foot-pounds) of torque in a criss-cross pattern. Start with the middle bolts on each side, then work in a star pattern to ensure even torque distribution and a leak-free install.
Pour in either your new transmission fluid, or the old fluid if it's low mileage, clear, red and unburned. New fluid is cheap insurance, though, so you may want to consider it.
If you're stuck for help or your parts retailer doesn't list a solenoid for your vehicle, you may find it helpful to know that the 4L65-E saw use in the 6.0-liter Escalade, Silverado, Sierra, Hummer H2, Pontiac GTO and Isuzu Axiom.
