A little light duty for this site but my son in law started to having a slight "shutter" problem with both up, and downshifting, along with an intermittent transmission overtemperature annunciator, along with the check engine light. It is a 2017 GMC Sierra Z-71 with an eight speed automatic. After a little internet research I find these transmissions can be troublesome and the thermistor in the transmission pan, (which is also the solenoid control harness) can fail; rather routinely. There are several TSB's out of this transmission and GM has been hit with "class action" lawsuits over the problems this transmission has given in the past. Regardless, I ordered up a replacement harness and got the truck up onto a lift: Drained the "molasses" from the suspected drain port, (more on this later): Dropped the pan to reveal the trans filter, solenoids, and harness to be replaced: Wiring harness after filter removal; encircled area is the thermistor:
Put everything back together with new harness, new pan gasket, and then couldn't find a filler tube. It seems GM decided to save a buck on this truck and installed a plastic plug where a normal dipstick tube pushes into: Lots cheaper than a dipstick and tube I suppose: Yankee ingenuity at work using a section of "PEX" tubing snaked into the hole from above once the shield is removed: Real "scenario" going on here; "PEX" tubing, heater hose, and a flex funnel coupled together to fill the transmission:
GM's pressure fill port with a special tool which is what I thought was to drain down the level with: You are supposed to purchase this special fitting that allows you to pressure fill the transmission until fluid starts escaping the tool for level. *** happened to looking at a dipstick for level!!! With the transmission filled and test driven, I then raised the truck again and checked the level with a borescope in the pan. All good with level and the truck is back to shifting very nice. We are going to run it a couple thousand miles and service it again as that fluid was very dark and the new replacement fluid is a upgraded type. These transmission are known for running very hot and darken the fluid quickly. I'm also told they need serviced often. It sure is nice but an eight speed certainly is not needed in a 1/2 ton truck.
That's a real nice complete write-up with photos. My son is a full-time wrench, and just replaced two of those in the last couple weeks. On the bright side, they are relatively easy to pull, and easy to find remans. He says GM isn't the only one eliminating dipsticks recently. I wonder how much the stealership would have charged to replace that tranny? I'm loving our old 4R100 trannies more every day. 310k on the one in my 2003 7.3 dually, still shifts fine. Of course, we run Mishimoto tranny coolers.
How many miles on truck?
Just over 82K on the clock. It has a 60K powertrain warranty so no coverage for this.
Little more to add to this subject through additional research. The replacement fluid recommended for this transmission and the 6L80, (six speed) is Mobil 1 LV ATF HP and there is a black, and blue label bottles. The black label is part number 122210 in a six quart case, the blue label is part number 124715. The black label fluid absorbs water or moisture and is suspect to be cause of these transmission woes. The blue label is a later product and does not absorb water. Both are synthetic based fluids but GM has TSB's out to discontinue usage of the black label product, and only use the blue label product. I ordered two six quart boxes yesterday from amazon to change this out in a couple thousand miles again to ensure a complete and thorough flush of the former Dexron VI fluid. Here is a link to hopefully head off any problems at the pass: https://www.amazon.com/Mobil-124715-Synthetic-Quart-Bottles/dp/B07Q3H159N
I couldn't help but notice in your first picture of the truck on the lift that, even from the factory, they spelled out on the grille and highlighted the problem in red for you
Yup. "Gotta Mechanic Coming, Garage Man's Companion", and several other less than honorable namesakes come to mind. I figure General Motors trucks have become a transgender make any longer. Used to be a vehicle for work, not the luxury/comfort people and grocery transportation system it's become.
The 1241C, and 1241D were very good transmissions when a power tower was incorporated. They were stressed behind a 350 Cummins when a driver was disrespectful to equipment in general. They ran winch trucks very well and were plenty powerful. I have a 6231D in my 51 Mack dump truck since new and it works very well. With 150hp on tap, it needs the extra gears too..... I've had dirt and sand spilling over all three sides and piled upon the cab guard but no problem pulling the load. Turning with the manual steering is another matter altogether but not really too bad when rolling.
Returned the GMC to my son in law last evening and he tells me it now shifts as it used to. The transmission operation is normal with no detectable shudder, and after stabilization, temperature is steady in the mid 190's. He is on board with servicing the unit in 2K miles with the new style replacement fluid, and thereafter every 24 months for maintenance as I do on my vehicles. He really enjoys that truck and wants to retain it but reading on the internet and it's negative stories/harping, (which is often unfair) had him with doubts.
Had a 2015 Denali half ton that grenaded that 8 speed with about 40 thousand kilometers on it. Tried to accuse me of off roading it. Last GM I owned.
Personally, I'm old school and don't like to see tranny temps that high unless it's transient as it towing uphill in summer. Is 190F common with newer trannies now? I had a 2018 Super Duty with the 6R140 tranny and stock Ford cooler and yes it ran hot like that. But I sold that truck before adding a better tranny cooler. Maybe Mishimoto makes a cooler for your SIL's truck.
They regulate the trans fluid temperature between 190, and 205 degrees for input to the transmission control module. Seems high to me also but this one is right in the window now. This 8L90 trans does run the fluid very hot by nature.
The ten speed in my 21 Tahoe stays right around 155 even when toweling the 24 ft pontoon or hauling cars the 19 Durango we traded would hit 200 plus just driving
I brought this truck back in and serviced the trans about 2K ago. It was shifting and driving fine but the fluid was darker than I usually like. We will service it again in another 7K or so just to ensure all is good. It will then be placed on my biannual first week of October routine service schedule which includes transmission service on the vehicles.