4l80e Rattles While Coasting

[Alex_M]

@NickTransmissions thought about messaging you directly to see if you had any input, but I'll post this publicly to improve the public information pool.

Hey all! I've just picked up another gmt800 3/4 ton with a 6.0/4l80e. 216k miles on it. Seems to have lived a hard life, but this is just a backup/secondary truck for me. It drives good, but has an issue that when coasting it starts to rattle. Give it some gas and you can feel it take up slop in the drivetrain - maybe trans? - and the rattle goes away. As long as I'm on the throttle even just a little it's all good.

I've got a 4l80e core left from the nv4500 swap in my primary truck, so it wouldn't be too big a deal to throw a kit in that trans and chuck it in the truck. I've still got to check and make sure rear end and transfer case are happy, but I'm thinking it's the tranny because it's also running a bit hot. Trans is staying around the 200*-215* mark.

Thanks all!

 

[strutaeng]

Check the exhaust mount/bracket that mounts to the transmission case on the passenger side. Actually, check the entire exhaust system. I once had an exhaust on my 99 Silverado that sounded like a rod knock.

These types issues it's best to safely lift the truck off the ground and let it run to replicate the issue. Start by removing the wheels and then driveshaft to make sure those are not the problems. In your case, it's very easy to remove the rear axles (10.5" full float 14 bolt.)

Is it a 2wd or 4x4? Check your usual things like u joints for slop too.

 

[NickTransmissions]

@NickTransmissions thought about messaging you directly to see if you had any input, but I'll post this publicly to improve the public information pool.

Hey all! I've just picked up another gmt800 3/4 ton with a 6.0/4l80e. 216k miles on it. Seems to have lived a hard life, but this is just a backup/secondary truck for me. It drives good, but has an issue that when coasting it starts to rattle. Give it some gas and you can feel it take up slop in the drivetrain - maybe trans? - and the rattle goes away. As long as I'm on the throttle even just a little it's all good.

I've got a 4l80e core left from the nv4500 swap in my primary truck, so it wouldn't be too big a deal to throw a kit in that trans and chuck it in the truck. I've still got to check and make sure rear end and transfer case are happy, but I'm thinking it's the tranny because it's also running a bit hot. Trans is staying around the 200*-215* mark.

Thanks all!

Hi Alex,

@strutaeng took the words right out of my mouth - rattling is not something you typically hear from 4L80Es as there's no hubs or shells like you would find in a 4L60E or 6L80/90...If you had mentioned feeling a very high frequency vibration in the seat of your pants, I'd recommend checking the diff as that usually indicates a worn out ring gear on the coast side.

Are there any drivability symptoms like delayed engagements, slipping, flaring, etc?

Is the fluid at the proper level?

How does it look and smell?

4L80Es are very robust transmissions (and in my humble opinion, the best transmission GM has ever produced) so they can take a lot of punishment and wear/tear before they begin to break down...I've seen and heard of units going 300+ thousand miles before overhaul (those units usually need reworked pumps and valve bodies when they come in).

 

[strutaeng]

Ya, I agree with @NickTransmissions I absolutely love the 4L80e's!

Here's the mount I was talking about. The nut on there is kinda hard to remove. I believe I used a 15 mm ratcheting wrench.

And the exhaust/cats removed out of the truck. This is from my 06 Suburban 2500, but I think it's the same for the 2500HD trucks. I removed mine to fix a rear main seal leak last winter.

 

[Alex_M]

Thanks gentlemen! I took a quick video. Truth be told it sounds like a parking pawl trying to engage when you're rolling. Intermittent engagement issue after shifting to drive and starting to pull out. Had an old dodge with a tired 727 that you'd have to rev up after it was in drive to get it to build pressure - similar symptom. There is a tranny fluid leak, but it's full currently. Haven't crawled under to see where the leak is coming from yet. No slipping other than that delayed engagement in drive, and no flaring when it shifts.

I'm leaning away from differential as it isn't immediate when coasting - there's a slight delay as though it happens after pressure drops in the tranny. More coincides with the time it takes the converter to unlock when you come off the throttle in these trucks.

It's louder the lower gear you have it held in, but I assume that's due to increased load holding the truck back.

Video link - https://photos.app.goo.gl/cXAti4Stt3fCo3Qn6

 

[NickTransmissions]

Thanks gentlemen! I took a quick video. Truth be told it sounds like a parking pawl trying to engage when you're rolling. Intermittent engagement issue after shifting to drive and starting to pull out. Had an old dodge with a tired 727 that you'd have to rev up after it was in drive to get it to build pressure - similar symptom. There is a tranny fluid leak, but it's full currently. Haven't crawled under to see where the leak is coming from yet. No slipping other than that delayed engagement in drive, and no flaring when it shifts.

I'm leaning away from differential as it isn't immediate when coasting - there's a slight delay as though it happens after pressure drops in the tranny. More coincides with the time it takes the converter to unlock when you come off the throttle in these trucks.

It's louder the lower gear you have it held in, but I assume that's due to increased load holding the truck back.

Video link - https://photos.app.goo.gl/cXAti4Stt3fCo3Qn6

If the parking pawl is making the noise then the parking pawl guide plate may have come loose or the little pawl retainer spring has broken or come dislodged...The guide plate is held on by two 13mm bolts and is directly accessible once the pan is off. The little spring secures to the pawl and the other end loops onto a small embossment on the casting.

Your dodge w/the 727 probably had a tired converter where the stator one-way clutch was beginning to roll over hence no/minimal power at take off.

Delayed engagements can be any number of things...at this point, I'd drop the pan so you can see what the fluid looks like as well as what may be collected at the bottom of the pan and go from there. You'll have an unobstructed view of the parking pawl guide plate as well.

 

[strutaeng]

I saw the video and it's kinda hard to tell. I did see a little increase in RPM in there, but wasn't sure why it did that... Or if that's related to anything? Did you give it throttle?

 

[Alex_M]

I will drop the pan and go from there. Hopefully figure out where the leak is coming from and check the rear end while I've got it up as well. I certainly wouldn't mind if it was the rear end - easier install and I have another 14 bolt. I appreciate it.

I did, after letting it make the sound for a moment I gave it some throttle to make it quit again. The sound does not come thru great in the video unfortunately. Oh, and yo answer your prior question - yes, it's 4wd. Pushbutton t-case w/o the auto 4wd.

Thanks gentlemen, I'll update when I get to it.

 

[Alex_M]

Minor update - couldnt help myself, crawled under the truck after I got home this evening to poke around. One hell of a bad universal joint - no needles left in two caps. I've had a bunch of bad u-joints - a few this bad - but none this bad this drove smooth down the interstate at 80.

I'll grab a new one and pop it in in the next day or two and hopefully that resolves the noise.

Found the leak while I was under there as well. Part of the gasket is hanging out and missing between the transmission and transfer case. Some oil in the front of the trans too, but looks more like it's rear main on the engine. I'll probably do filter and fluid on the tranny anyway, and while I'm at it I'll slide the t-case back and reseal between.

 

[Alex_M]

And here's the resolution! So there was in fact a very bad universal joint - half the cap on one side was missing. While pulling the driveshaft I realized - wow, transfer case sure does move a lot for something that isn't supposed to move at all. Turns out the extension housing on the back of the tranny had turned into two pieces. The t-case was being held on by one loose nut and the stamped steel bracket that holds the fuel lines. I pulled the t-case and swapped the extension housing with one off of my spare 4l80e and got it all back together with new universal in the driveshaft. Saw a few other things under the truck could use being addressed, but nothing immediately concerning. Tranny leak appears to be solved and all drivability issues are gone, including the drive engagement delay which I would guess was something binding up because of the t-case flopping around whilly nilly. Luckily the transmission output and t-case input, as well as all mating surfaces, appear undamaged.

I also rotated tires, put in a fresh air filter, and changed the oil, which if the sticker in the window is anything to go by was 17k miles over due. Gained 15psi of oil pressure at hot idle and driving around. Thinking it needs a ball joint on the driver side and a front end alignment and she should be in pretty good shape.