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Hey guys, Looking at getting a 6.6 gasser. 2020-2023 offer the 6L90 and 2024-2025 have the "Allison" but not Allison 10 speed. I don't have to have the latest and greatest or the fastest, just want something reliable. I had a 1500 with the 6L80 and had the typical converter issues around 100k miles. Do not want to see anything like that in the 6L90. The 10 speeds seem to have their quirks, and aren't even a true Allison, which is a disappointment. Basically, a 6 speed that has been around for years (6L90) or a newer updated trans that has not been out long enough to show reliability (10L1000). Thoughts?

Hello all, I'm going to be adding the transmission oil cooler (GM Part Number 23409058) to my 2020 Trail Boss. I’m going to order the ACDelco Oil cooler lines (GM Part Number 86560008). Does anyone have any detailed photos of how the lines are run under the body, through the engine compartment, and out of the core support to their cooler? Any help is greatly appreciated. Thank you so much!

As everyone here already knows…”it shouldn’t take more than a few minutes” turned into me pondering how many fist sized dents I needed in my door panel for aerodynamic efficiency or the “golf ball” look. Anywho. I delved into the underside of my custom trail boss in an attempt to install a dipstick in my 6L80. Of course, I must mention that the 6L80 was present in the 4.3 models until at least the 2020 model year (RPO code MYC), this dipstick fits all 6L80/90 transmissions as far as I’m aware, in the 6/8 cylinder engines alike. Here is the order of operations. NOTE: TRANSMISSION CROSSMEMBER REMOVAL OR DROPPING THE REAR OF THE TRANSMISSION WAS NOT REQUIRED. FOR MY APPLICATION. Remove dipstick from tube to make installation easier. The dipstick gets in the way otherwise. Also, lube the seal and tube with white lithium grease to ease insertion. In order to access the hole where the plug is (that must be removed prior to installation), there is a heat shield that needs removal on the passenger side of the transmission. It is held on by two 10 mm bolts. Beware of the front one, it’s blocked by the bend in the shield, and too close to the catalytic converter to get a regular 3/8” ratchet and shallow well socket in there. After removing the heat shield, you can use a trim tool, your fingers, or a screwdriver to remove the plug where the dipstick enters the transmission (there is only one location it will go). Then, find the studs on the backside of the bellhousing, and locate the one below the starter hump. Using a 15 mm deep well, swivel, and breaker bar or 1/2” ratchet, remove the stud. Test fit and place it in its approximate location, but beware the ground strap on the back of the block, right next to the path of the tube, there is a chance it gets in the way like mine, and you’ll need to loosen it in order to route the tube unabated. 15mm bolt. Install your dipstick seal first, you can use a thin layer of white lithium grease for ease of insertion. Follow up by seating the tube in the grommet, and then reinstall your bell housing stud. Be mindful that torquing that stud could cause the dipstick to raise out of the hole, so it’s wise to keep light pressure downwards on the tube to ensure it stays in the transmission until you snug and tighten the stud. Once the tube is seated and the stud installed, make sure your O2 sensor wire holder/zip tie is still in its pocket on the transmission, and reinstall the heat shield. If you had to loosen or remove the ground strap on the block, reinstall/tighten the bolt. Use the procedure sent with the dipstick to check level and add fluid as needed. Mine needed 1 quart (this is typical, mine didn’t leak and hasn’t lost fluid.). This dipstick does work with aftermarket and deep pans. Lubelocker also makes a reusable gasket for the 6L80 and one for the 6L90 if anyone is interested.

Hello all. I am a semi-frequent user of GMTrucks.com. I’ve had several cosmetic and suspension modifications to my truck (Bilstein 5100’s, roadactive suspension, hellwig rear sway bar, interior mods), nothing major in the drivetrain. I just crossed 50,000 miles, and it looks like yet another TCC failure (first one at 24,500 to 25,000 miles). It’s a 2020 Custom Trail Boss, 4.3. When the first converter failed, they ended up putting 2 thermal bypass valves in it. After the last TBV change, transmission never sees over 150°. What do I need to do to have GM put a better converter in my truck to prevent future hangups (sonnax, maybe)? How should I approach this to ensure I don’t keep losing my truck for weeks at a time? I don’t overload it, I don’t dog it, and the fluid still looks clean (changed with last thermal bypass valve at 30,000 miles, total fluid exchange and filter replacement). It seems like every 25,000 miles, I’ll be back for another torque converter. I don’t haul or tow frequently. When I do, it’s usually no more than a single axle utility trailer with some light household refuse, or an enclosed single wheel with a flat track motorcycle in it. Bike weighs 600 pounds. Tongue weight below the 1,760 payload rating on the sticker in the door. I’ve never gone more than about 40 miles when towing. Usually interstate or rural rustic road (55 mph and no traffic) Any suggestions? I really like the truck and don’t want to trade it in. It suits me for what I need, and I’ve put a lot of time and love into this machine.

Just finished a Top end on my 6.0 at 191k and made a run to South Dakota dropping off a FWC camper. Loaded my 4K lb. truck camper, making a run to Oregon for a new client opportunity. 40 miles from home felt a slight resonance in the steering wheel while rolling onto an overpass. RPMs at 1800, in 5 gear noted a slight increase in rpm (50) when resonance cycled every 30-40 seconds. Switched manual gear selection to 4th showing 3000 rpms and resonance stopped. Knowing the climbs ahead, 81 miles out, turned around and headed home. Took the truck to the shop, gave a detail report. Shop confirmed my observations and pulled the pan. I’ll update the site and detail the repair. 15k miles on third transmission filter change. Pan showing metal bits and magnet covered as well. Shop suspects a torque converter fail.

I sell, fabricate, deliver, install and train workers how to use my packaging lines. In 2015 selected 2500HD WT 6.0 as the platform to use as my office (Truck Camper) and and tow vehicle. It provided great HD service and economical (per mile basis). On a trip to OR noted a resonance with my thumb on the steering wheel, when engine load changed going over an overpass. At first I thought it was roadway groves, but the rhythmic vibration stopped when manually dropping into a lower gear, increasing to 3000 RPM's. Not willing to chance break down north of Mount Shasta, returned home, switch vehicles and headed north. Upon return the rebuild search began. Deep dived into transmission repair cave and almost selected a remanufacture unit from Certified Transmission but learned shops are under no obligation to use their products and there's limit participation on the left coast. Most shops refuse to install a third party transmission citing warranty issues. Finally settled on a local shop in West Sacramento that rebuilds your transmission to the level you want. As a bonafide cave dude wanting to understand the how transmissions work, why they fail and how to repair, called (late Friday) Pro-Built Transmission and spoke to Don the owner. I outlined in detail what the problem was, and through a series of questions from Don about the trucks' service history and how I use it, he was confident what the problem was. Arrived two hours late from my Monday 8 am appointment, Don pulled me through (no signage stating for insurance purposes you can see what we're doing to your ride gate) his shop showing various components, their failure points and although the 6L90 is a simple transmission pointed out common failure points. He showed various 6L90's requiring rebuild, some under 60K miles, and could not believe mine made it to 194K. I relayed after the first transmission filter change, started pulling 5.5 quarts out and replacing every other oil change, in other words, a poor man transmission flush. My reasoning was pay 25 dollars instead of $250 for a complete flush. Don smiled and said that explains it, noting above all else, fluid change saves transmissions. Satisfied Don wasn't a hack, dropped off the key and within an hour, after a test drive and noting data stored in in codes sent me this. I knew it was bad and knowing a bottle of shudder stop wasn't gonna fix this, authorized repair. With high mileage, I asked about universal joint replacement, Don relayed he had a shop that could replace joint, balance the driveshaft and check the center bearing. Also since transmission was removed, and although showing no signs of leakage, had him replace the rear oil seal as well. Don pulled the transmission and confirmed what he suspected, torque converter friction plate failure. The metal on metal grind produced fine grains that worked its way pass the bell hosing bearing and through the pump. Damage continued through the valve body and solenoid pack (TCM). Solenoids contaminated Note the metal grit in the screen on the right. What was unexpected was the metal grinds didn't make to the clutch packs. After 194K miles, 90% were still serviceable with some showing wear on the outer ring. What's interesting with GM is, instead of applying full pressure to the clutch plates, they control the amount of pressure applied. The black rubber ring on the piston drum engages the plate outer section. This accounted for the most wear on the clutch plate. Don showed how the replacement piston drum engages the full surface of the plate; provides a sharper shift and lowering the wear. THE FIX. Drive shaft showed the center bearing was bad and was along with universal joints replaced. Don sent the pump face (forgive if I don't name the parts correctly) to be machined smooth (8-9 thousandths). He switched solenoid rods from alloy to steel, and spoke about the OME cost. Chevy spends perhaps 50 cents on alloy rod where as three steel one cost him $80.00. He installed a new pump and higher valve pressure components. Here's a picture of some parts replaced in my 6L90. Don sent out the torque converter to another shop. They split it open and here's the condition. Don then explained his vendor replaces the back half (part the friction ring presses against) with a thicker (1/4 inch instead of the 1/8 inch) material machined from a billet. This handles heat better and prevents the converter material from warping. They also flash braze vane tabs to prevent movement (picture of open converter is not mine) If you get a good shop and can wait four days, one can retain the original unit and get upgraded parts. Note the remanufacturing industry is designed for a quick turnaround for transmission shops. They cite a higher level of expertise, however the reality is they created a process based on volume allowing a price point favorable to the transmission shops. Nothing wrong with that and note I wanted to retain my original transmission. Programming Invoice. Yes its high, but keep in mind two other shops were involved, addition work was completed and Don took time to educate me on the transmissions, AND its a tax write off. In the final analysis and under heavy duty conditions, if you have a 6L90 strapped to a 6.0 and do fluid changes, your transmission will survive to 150K miles. If I had a better understanding of transmissions when observing the gray paste material surrounding the pan magnet at the last transmission filter change, I could minimize cost with just a torque converter change. As for this repair, (most expensive during ownership) it cost me .0229 per mile. This and the top end rebuild posted earlier, gives a total cost of $5944.85, (.03 cents per mile) returning my truck to service, hopefully getting another 150k miles. Test drove my truck and report shifts are crisper with the only change being when going over overpasses, and sensing the torque change, rpm will drop 25-50 rpms and return on completing the elevation change. Next week I'll go up the Grapevine with a 4k pound truck camper and report how she does.