6l90e Problems?

[Sarge]

My 09 i swear has trans issues. Sometimes downshift causes a clunk in the drivetrain and kinda feels like its jerking, 2-3 shift sometimes flares up and the shift is horrible. But 80%+/- of the time its 100% fine. Ive gone and added a little bit of line pressure to it from my handheld but doesn't really make a difference. I have 145k and my first step is going to be fluid and a filter. Are the trans mounts an issue on these? Anyone else have similar issues?

 

[Erik G]

What handheld?

Cold or hot? I notice in the morning, first mile or two, my transmission feels different than when warmed up

 

[Sarge]

Bullydog. But it did it before i even put it on the truck.

Id say its a little more noticeable cold. Still does it warm randomly, and today it shuttered when slowing down. Hopefully new fluid and filter will help

 

[OneTwo]

You're on the right track with the fluid and filter, be sure to update us with results.

Also, just one thing to check, often a broken motor mount can feel like a driveline clunk and broken motor mounts are VERY common on these rigs. Put it in first gear, hold firmly on the brake, step on the gas and have a buddy/mechanic watch the engine to see if it moves excessively. If it moves upward a few inches, bad motor mount.

I have a broken motor mount on my '07 Denali I just bought. I thought it was a driveline clunk or imminent transmission failure, so I started searching the Internet for common 6L80 problems or when they typically fail... I found multiple threads titled something along of the lines of: "Seriously, nobody is having issues with the 6L80?" with replies like "Yeah, lots of people are running TONS of power through them with no issues."

The only 6L80 failure thread I found was some guy who had catastrophic failure at 30k miles, which was obviously an anomaly. I'm sure, like every moving part, they are bound to have issues eventually, but a lot people have a lot of faith in these transmissions.

 

[63Blackseries]

68l0 is the 6speed right? because I think I am having the same problem as yours. I can hear a metal clanking noise coming to a stop, happens when down shifting. Also on cold starts, I get a loud thump thud noise right when leaving my driveway on to the street gaining speed. Thud happens around 20 mph

 

[kctyler]

68l0 is the 6speed right? because I think I am having the same problem as yours. I can hear a metal clanking noise coming to a stop, happens when down shifting. Also on cold starts, I get a loud thump thud noise right when leaving my driveway on to the street gaining speed. Thud happens around 20 mph

Yes, that is a 6speed.

You're on the right track with the fluid and filter, be sure to update us with results.

Also, just one thing to check, often a broken motor mount can feel like a driveline clunk and broken motor mounts are VERY common on these rigs. Put it in first gear, hold firmly on the brake, step on the gas and have a buddy/mechanic watch the engine to see if it moves excessively. If it moves upward a few inches, bad motor mount.

I have a broken motor mount on my '07 Denali I just bought. I thought it was a driveline clunk or imminent transmission failure, so I started searching the Internet for common 6L80 problems or when they typically fail... I found multiple threads titled something along of the lines of: "Seriously, nobody is having issues with the 6L80?" with replies like "Yeah, lots of people are running TONS of power through them with no issues."

The only 6L80 failure thread I found was some guy who had catastrophic failure at 30k miles, which was obviously an anomaly. I'm sure, like every moving part, they are bound to have issues eventually, but a lot people have a lot of faith in these transmissions.

I have an 07 Denali with that damn "thunk" or what ever you wanna call it. It has no consistency with the noise, other than at low speeds. Sometimes it will happen a few times a week, sometimes I can go weeks w/o it making any noises. I have mine custom tuned and it still shifts hard sometimes at low speeds. I thought about the fluid and filter change, but the "old timers" say not to do it if you don't know if it has been done. I bought mine with around 85K and now has 135K. Mine has done this since the day I bought it.

 

[vinnyray]

Is it the infamous clunk? There was a service bulletin about the slip yoke needing to be greased or replaced.

see post #2

 

[OneTwo]

68l0 is the 6speed right? because I think I am having the same problem as yours. I can hear a metal clanking noise coming to a stop, happens when down shifting. Also on cold starts, I get a loud thump thud noise right when leaving my driveway on to the street gaining speed. Thud happens around 20 mph

As vinnyray mentioned above me, that sounds like it could be the slip yoke. Greasing them might fix your clunk.

I have an 07 Denali with that damn "thunk" or what ever you wanna call it. It has no consistency with the noise, other than at low speeds. Sometimes it will happen a few times a week, sometimes I can go weeks w/o it making any noises. I have mine custom tuned and it still shifts hard sometimes at low speeds. I thought about the fluid and filter change, but the "old timers" say not to do it if you don't know if it has been done. I bought mine with around 85K and now has 135K. Mine has done this since the day I bought it.

You should be fine to do a drain and fill even if it's been done before. Most people don't even think about doing a transmission service until it starts acting up and those who are maintenance-conscious tend to follow the 100k service interval. If you bought yours at 85k, I would guess it's never been done.

Doing a transmission flush is what should be avoided. Just drain it, change the filter, replace pan gasket, fill it.

Either way, that clunk could be slip yokes. Check the link in vinnyray's post.

 

[Sarge]

Thanks for the input. Looking it over tonight

 

[Erik G]

whats the reasoning behind not doing a flush? just asking, I've never had this 6spd. Just draining and filling you're only getting about 1/5th of the fluid changed (if this is a normal automatic...)

 

[OneTwo]

whats the reasoning behind not doing a flush? just asking, I've never had this 6spd. Just draining and filling you're only getting about 1/5th of the fluid changed (if this is a normal automatic...)

Most people say that it dislodges materials that can end up blocking narrow passages or one-way valves and can actually harm your transmission. Some transmissions have a "lifetime" fluid that must retain at least 50% of it's original fluid or it will fail (my old BMW E34 auto was this way - or at least that's what I was told - I have no idea why).

To be honest, I'm not entirely sure why not to flush one of these transmissions. When I was researching the 6L80E, I came across multiple "DO NOT FLUSH THE TRANSMISSION" comments and exactly zero comments saying that you should.

 

[63Blackseries]

I brought my truck to the dealer. Hoping they would replace the slipyoke since I am still under power train warranty but the crappy dealer says they did not hear or notice anything wrong. In this case what can I do to fix this annoying clunk?. The TSB says 4wd Trucks cannot be greased?

 

[Denalievv]

Have you figured this out yet?

 

[drakon543]

Felt like chiming in regarding this transmission flush side topic. Most people i know that swear a dealership "flush" service is bad for your transmission are under the impression that a foreign cleaner is being sent through your transmission. Most dealerships that i managed to actually talk to a tech have informed me that no cleaner is actually being introduced to your transmission. As stated above doing a diy pan drop and filter change your only getting a portion of the fluid changed. As the process was explained to me all thats happening is new fluid is being pumped through the transmission in the same method as when your driving. It continues to collects the old fluid in a separate container until only clean fluid is coming through. In my opinion the discussion of having a flush done is being misinterpreted with the old method of actually adding an extra harsh cleaner that would indeed free up gunk and possibly lodge it where it shouldn't be.

 

[drakon543]

As a side note not all shops have the machine capable of doing this new method of changing your transmission fluid.

 

[gtp1500]

On my way home after buying mine I floored it to pass someone and felt the clunk... now I'm sure its the motor mount, I did the brake test and the motor flexed pretty good.

 

[OneTwo]

On my way home after buying mine I floored it to pass someone and felt the clunk... now I'm sure its the motor mount, I did the brake test and the motor flexed pretty good.

Mine did the same thing and it is all gone now that I've replaced the driver's side motor mount. What a relief!