PREAMBLE: I've decided to aggregate all the information I found from the forum and web searches for my recent ATF fluid change. I'll try to be as descriptive as possible while organizing this post in an understandable format in hopes it helps people in the future from piecing dozens of forum posts together.
ADDRESSING THE MISNOMER: There is no drain plug. It is not called a drain plug. GM refers to it as the oil level check plug or the FULL plug.
6L90 Fluid Capacity: 6.7 qts
Source: Owner's Manual
6L90 Fluid Type: Dexron-VI (6)
6L90 Fluid Check Temperature: 86-122 F (30-50 C)
Source: Sticker on transmission
6L90 Pan bolt torque: 80 inch lbs
Source: Service Manual
6L90 Oil level check bolt torque: 18 ft lbs
Source: Service Manual
3/8 Ratchet or Impact Driver
Respective torque wrenches
14mm socket (for the oil level check plug aka FULL plug)
10mm socket (for the Pan bolts)
Trim tool (for the FILL plug)
Needle nose pliers (for the FILL plug)
Flathead screwdriver (optional for filter neck gasket)
Rubber mallet (optional for filter neck gasket)
Fluid transfer pump
I made my own transfer pump out of a garden sprayer for $25. I can provide instructions for your own if you'd like
Fluid extraction pump
I used a good ole turkey baster with a 3/8 ID hose zip tied to it.
ACDelco GM OEM Dexron-VI ATF (P/N 88865549)
I bought 8 quarts, and it was just enough. Recommend a total of 9 to be safe.
ACDelco GM ATF filter (P/N 24258269)
This is the larger filter for the trucks.
GM Transmission Pan Gasket (P/N 24226850)
Pan gasket CAN be reused if in good condition.
Jack up the car and place it on four jack stands. Ensure that it is LEVEL.
I had the vehicle about 18” off the ground and was able to comfortably roll and move underneath the vehicle as needed
Remove the oil level check plug (14mm bolt)
Be advised: With the engine off and transmission cold you should expect to drain a good ~6 quarts through the hole on a standard fill. The remaining quart will be sitting in the pan.
Remove the fill plug on the passenger side transmission at the very top.
It’s a black, rubber push pin style plug. For reference, in this picture it is the white plug Circle D uses in the red square.
I used a trim tool to pop the top and wiggled the plug out with needle nose pliers and my fingers, working it side to side.
Snake in your extraction hose through the fill port until it hits the pan’s bottom and pump out the remaining ATF.
I was unable to do this through check hole at the bottom because my hose would hit the transmission components inside, preventing it from snaking down.
Conversely, you can also go to the next step and bring the pan down, tilting it towards a drain pan. Bigger chances of a mess but efficient.
Loosen the 18 (10mm) pan bolts and remove the pan.
Be mindful, some oil may still be sloshing around inside.
Remove the filter.
The metal/rubber gasket that the filter neck presses into will most likely remain in the transmission. If it’s in good shape, you can reuse it.
If you want to replace it with the one in the filter kit, you’ll need to use a flat head screwdriver and pry enough to crush/collapse part of the metal ring in, then pull it out with pliers. It is a very tight tolerance fit and the best way to get the new gasket in is to tap it with an equal size socket on an extension with a rubber mallet. Be gentle as the gasket's lip nears the mating surface. Striking it too hard can crack or damage the metal base.
Slide in the new filter.
It’ll seat twice. The first friction point is misleading and feels like it’s set. It isn’t if you didn't push firm enough. Don't dragon punch it in, just give it another firm push and you’ll feel it fully bottom out.
The filter orientation should match that of the old one and sit just barely below the transmission internal components exposed by the pan removal
Clean the mating surface of the transmission and pan.
I shouldn't have to say it, but you never know your audience: Don’t use metal to scrape anything. A rag and degreaser works fine.
Clean the pan out and wipe off the metal friction debris from the magnet.
Inspect your pan gasket and reuse if desired. Otherwise install your new gasket on the pan.
Re-attach the transmission pan. Hand thread the bolts and then snug them up
Torque them down in this order. This isn't our exact pan, but same shape and bolt layout. Forgot to grab a full picture of mine.
Fill the transmission from the FILL port on the passenger side, top (where the rubber plug was) until it drips from the oil level check port (at the bottom)
I pumped in about 6 qts before the oil level check port started to drip.
This was my homemade doo-hicky.
Reinstall the oil level check plug.
GM FLUID LEVEL CHECK:
You're ready to ensure the fluid is at the correct capacity. It's a very fickle procedure and being within the below specified temperature ranges is important. Please make sure your car is level once more.
Notice: Transmission fluid temp (TFT) required during check is 86-122 F (30-50 C). Setting the fluid level with a TFT outside this range will result in either an under or over-filled transmission. TFT>50°C=under-filled, TFT<30°C=over-filled.
My first attempt took too long and the temp got up to 135; it will keep pissing fluid out until you plug it. The range is indeed very important. I let the car cool for a few hours until it got to 90* F and wrapped it up in that attempt.
Start your car.
Let transmission get to 86 F.
Engage emergency brake/depress brake.
Shift through every gear, pausing for 3 seconds each gear.
Let engine idle (500-800 RPM) for 1 min. Ensure TFT is still within range.
Remove the oil level check plug (the bottom plug)
If the fluid is flowing as a steady stream, wait until the fluid begins to drip. Reinstall the plug.
If no fluid comes out, add fluid until fluid drips out. Reinstall the plug.
I had none dripping, so I added approximately 0.5-0.7 qts before it streamed out and then dripped.
Torque the plug to spec.
Turn off car.
CAUTION: NEVER turn the car off with this plug removed. It will evacuate just about everything you put in.
Overfill 1-1.5 qt(s) if desired.
It is recommended for spirited days/driving and racing events to encourage smoother shifts and less/to no shift flare in the 1-2 or 2-3 range.
After having pumped in about 6.7-7 qts, I chose to overfill it with an additional qt. for a total of 8 qts.
Replace the rubber Fill plug on top of the transmission
This is an extra step I added to this procedure. With the transmission and fluid warm, go through and re-torque your transmission pan bolts. Half of them needed another squanch after things warmed up.
You can start the car once more, cycle the gears, and check for leaks.
Once you're satisfied, lower the car and you're done.
FLUID INSPECTION NOTES (From the GM procedure)
Inspect the fluid color. The fluid should be red or dark brown. If the fluid color is very dark or black and has a burnt odor, inspect the fluid and inside of the bottom pan for excessive metal particles or other debris. A small amount of "friction" material in the bottom pan is a "normal" condition.
If large pieces and/or metal particles are noted in the fluid or bottom pan, flush the oil cooler and cooler lines and overhaul the transmission. If there are no signs of transmission internal damage noted, replace the fluid filter assembly, repair the oil cooler, and flush the cooler lines.
Fluid that is cloudy or milky or appears to be contaminated with water indicates engine coolant or water contamination.
I think I included everything I could recall. I'll try to take more pictures next time, but it's a pretty straight forward procedure with the right notes and steps, which I hope this will provide.