2018 Sierra clutch slipping?

[Craw3773]

I need help from the TBH braintrust.

I bought a used 2018 sierra with 6.2l in February. It had 33k miles on it, and I've pushed it up to 39k now. Once in a while, I felt like the transmission would slip, but its minimal, and I chalked it up to getting used to a new truck. My last truck was a 2014 silverado, so very similar.

Twice now, I have put the truck in park, and let my foot off the brake. Obviously, the truck shifts as the transmission takes up the slack. Here's the funny part: after turning it off, it has slid forward another couple inches.

In general, it doesn't move a whole lot, but it feels like something in the transmission is starting to let loose. I'm under warranty until 43k, so I'll take it in this week, but I'd prefer to have an idea of what's going on before I get there. Any ideas?

[Philip-TX]

Not a transmission guy, but.....

(Edit: this is an auto not manual right?....you said "clutch")

I may be off but I thought (on the last transmission I had trouble with, '04 F350) the tranny was driven/operated by solenoid valves, controlling the fluid pressure/release. There was a bank of about 6 solenoid valves when I dropped the pan (6sp tranny). And that is how the programmers/tuners change shift pressures, etc.

A sticking solenoid valve can cause the symptoms you described. I tried to get the shop to give me the resistance reading (ohms) for the valves so I could replace it, but no dice. Can't blame him he was trying to run a business. Didn't dig the ohm values up, but I'm sure they are available somewhere on the interwebs. I let his shop fix it.

Do some googling on it with make/model.

[SmTx]

It's normal for the truck to roll forward. But you're saying it's rolling forward after brake is let go and then once you turn it off it rolls forward even more?

[Big Lee]

1, get in the habit of using a parking brake. Parking brake should be set on all vehicles, before letting the vehicle roll up on the parking Pawl. 2, what you are describing in park would not be any kind of clutch slipping. I encourage you to demonstrate this for the technician before leaving the shop. This will save you and him/her a lot of frustration.

[Big Lee]

As far as the transmission slip feeling. Be specific for the tech. Driving condition, towing, uphill, downhill, what speed, how long after driving. Any specific detail so they can duplicate the issue. Since 2018, there has been several programming updates to the transmission control module that address some anomalies in shift quality.

[91cavgt]

If the transmission is an automatic, it is not a clutch that is slipping after you put it in park and kill the engine. That would be the parking pawl not holding correctly. Had this happen on an old Jeep Grand Cherokee of mine many years ago, but it would slowly move a few feet. Got the parking pawl replaced and it stopped moving when it was parked.


You really should start using the parking break if it is moving like this.

[Craw3773]

Sorry, yes this is an automatic. It's definitely a lurch after the normal slack is taken out when in park. I'll start using the parking brake until I figure it out.

Otherwise the sloppiness I feel is when accelerating from a stop. It doesn't feel like the engine RPMs are corresponding to the acceleration. Once I get 50 feet down the road, I'll get a jerk as the truck shifts, and it feels normal. Could these be separate issues, or are they related?

[Big Lee]

Separate issues.

[RifleBowPistol]

Should be two completely separate problems. The piece that holds the truck, while off, and in park, is usually some notches cut in a disk attached to the output shaft or notches in a ring, that is on the outside of a drum, that is splined to the output shaft. Then there is a spring loaded parking paw that is supposed to drop into one of those notches, once you put the vehicle in park. It basically locks the output shaft of the transmission to the transmission case, to keep the vehicle from rolling. To make the ring strong enough to hold the weight of the vehicle on a incline, fully loaded, possibly with a fully loaded trailer. There is usually a good bit of metal between each notch in the ring. So the paw, is on a shaft and has a spring on the back side of it. So you can put the shifter in park, then if the paw is not lined up with a notch, the vehicle will roll some till the output shaft turn and lines up a notch in the ring or disk with the parking paw. Then the spring, if it is working correctly, pops the paw into the notch in the ring or disk.

Slipping while driving, more so accelerating, could be the convertor, or clutch disks in the transmission. Without experiencing that situation, I could not say if you really have a problem, then what type or degree of problem. It could be a case of the new truck has a torque convertor, that was made looser to allow the engine to rev higher, to help get the truck moving, or it could be the newer truck has a different transmission, with different gear ratios, again, to help get the truck moving up to speed. The way you describe the problem, it does kind of sound like the convertor may be looser than you are used to, then once it gets up to a certain mph, the lock up clutch in the convertor locks up, from that point on up, there is no more convertor slippage, unless you give it a lot of throttle, to accelerate. At that time, the convertor would likely unlock again, to help the truck accelerate. This could be what's going on, but there may be a problem with the transmission also.

I would check the fluid, make sure it's red, not brown or black. Then make sure it does not smell burnt. The newer fluids, have a bit of a burnt smell to them right out of the bottle and have a darker red color to them. Not a fan of whatever it is they have done to auto transmission fluids in the past 10 to 15 years. It makes it harder to tell if you are starting to have problems, such as just starting to slip or over heat for some reason. In years past, the fluid would be bright red and not have any burnt smell to it at all when it was new.

You may have to take it to a dealer and have them test drive the truck. They will know how your convertor and transmission should work, when working properly.

I know most of the manufactures were using six speed autos, up until a few years ago. Then Ford and GM switched to ten speed autos. I think I have heard of Dodge having eight speed autos, but not sure of that at all. Basically, I know in more recent times, the manufactures have change the transmissions, which would result in a very noticeable difference in how the transmission feels and shifts and how high the engine revs on acceleration. Often before making a switch to a new component such as a new ten speed transmission. It would not surprise me if they put a looser convertors in trucks made a year or two before they actually started using the ten speed transmission. The reason being, they would be trying to improve acceleration, to compete with some other company. For example, if I am correct and Dodge put in a eight speed transmission, in their trucks, a few years before GM started putting the ten speed in their trucks. The Dodge trucks would likely accelerate better than the GM trucks, when similarly equipped. As a temporary fix, GM could just put higher stall speed convertors in the trucks made before the new ten speed came out, to help them accelerate better. That could be what is going on with your truck. I think Ford came out with the ten speeds in 2019 or 2020. GM may have come out with them in 2018, but I would bet they started using them the same year that Ford started using them, since GM and Ford worked on that transmission together.

[Craw3773]

This is a huge help folks! Thanks for the info