Transmission issues... lots of questions

[Geotrash]

So last year, my original transmission failed at 165k while towing (in tow mode) with a custom tune. Tow mode was not part of the tune so it tore up my transmission. Brand new GM trans installed in July 2024. Lots of clunking after the new one was installed so the custom tune was checked and reflashed. Everything seemed ok with the 'normal' clunking every now and then. The clunk occurs around 30mph to a dead stop, like the downshift can't keep up with the braking. Transfer case was checked by my mechanic and it is fine.

Now that it's hot again here in MI, I've noticed a few things:
1. When I use the AC, the clunking seems to be worse.
2. Yesterday, in normal city/highway driving and on a hot day, the transmission temp reached 200 and the clunking was bad when stopping.

QUESTIONS:
1. Is my transmission cooler not working properly? How would I check that?
2. What is the relationship between these 3 things? AC/Air Temp/Transmission Temp?
3. Should I invest in a new, heavy duty trans cooler? If so, what do you recommend?
4. Is the custom tune bad, meaning, it's not properly tuned for the downshift?

I have two towing trips planned this summer, one at the end of the month. Scared to death the transmission is going to blow again.

2011 Yukon Denali XL, deleted

Other sources of clunking on these is play in the transfer case chain, worn front differential mounts, and also motor mounts can cause a clunk.

Given the age and mileage, you are due for motor mounts, and front differential amounts, if they’ve never been done, but in this case I suspect the front differential mounts.

 

[tkdgirl]

Other sources of clunking on these is play in the transfer case chain, worn front differential mounts, and also motor mounts can cause a clunk.

Given the age and mileage, you are due for motor mounts, and front differential amounts, if they’ve never been done, but in this case I suspect the front differential mounts.

Good points. I'll have the diff mounts checked. Motor mounts were replaced not long after I purchased it in 2021.

Also ordered a TruCool 40k and will invest in deep trans pan and a @BlackBearPerf tune when I can afford it.

 

[Geotrash]

Good points. I'll have the diff mounts checked. Motor mounts were replaced not long after I purchased it in 2021.

Also ordered a TruCool 40k and will invest in deep trans pan and a @BlackBearPerf tune when I can afford it.

Caution on the 40K. The 6.2 puts out a little more heat than the 5.3, and when I had a 40K on my 6.2 towing rig, it started heating up my engine temperatures. 190-200°F is pretty normal after maybe 30-45 minutes in the summer with the factory cooler.

 

[tkdgirl]

Caution on the 40K. The 6.2 puts out a little more heat than the 5.3, and when I had a 40K on my 6.2 towing rig, it started heating up my engine temperatures. 190-200°F is pretty normal after maybe 30-45 minutes in the summer with the factory cooler.

Well crap. Is there a better one? Not convinced the factory cooler is that great but the 200 trans temp is normal in the summer, non towing? I'm just trying to mitigate any further issues with the transmission.

 

[Geotrash]

Well crap. Is there a better one? Not convinced the factory cooler is that great but the 200 trans temp is normal in the summer, non towing? I'm just trying to mitigate any further issues with the transmission.

One thing that we’ve seen from some transmission shops is that they tend to bypass the oil to water cooler in the radiators end tank. That needs to be in the circuit to take a cut of heat out of the transmission fluid before it gets to the external cooler. You may want to take a look at how the cooler lines that run from the transmission are currently plumbed through the coolers.

But yes, 190 to 200 was pretty normal for mine before I made modifications, without towing.

Edit after some more thought: yes, there is a better option that mounts below the radiator and behind the lower grille on the Denali. It’s a Derale 13960.

 

[j91z28d1]

The stock tune was way worse and won't work with the DOD delete.

I alway say I'm no tranmission guy, I only have a very basic understanding of how these magic boxes work. but..

this is what strikes me as weird. if it's a gm reman tranny with a new converter, I would guess that it would drive like oem with the stock tune for the tranny modual flashed in. while a tcm tune can help make it last longer because of less programmed in slip, it should still go 150k miles without issue just like the original oem tranny would?


my worry is there's something funny going on in the tune. I feel like going to gm tech line and flashing a oem program into the tcm would be a good place to start trouble shooting why it's clunking around during shifts.

and yes, I've seen the transmission cooler inside the radiator bypassed when installing a nre tranny to avoid having to flush it or replace the radiator because of contamination from the blown up tranny. but as said above, a add on aux cooler is hard to handle all the heat by itself. air to oil is way less efficient than oil to water heat transfer.

 

[jfoj]

When the fluid temp gets high, the fluid thins out and once the fluid gets thin any valve body bore wear, spool valve wear or valve body internal leakage gets worse and leads to delayed and/or harsh up or down shifting.

The GM supplied transmissions are NOT NEW. They are rebuilds and are usually inferior to what a good independent shop can put together with higher quality aftermarket parts. The OE valve bodies are bad enough, the rebuild valve bodies are probably way worse.

I would guess the valve body is more like the main problem, possibly along with the TCM (Transmission Control Module) programming. But would expect the valve body to be more problematic.

You might look at this as a possible solution to your current problem.

Project Carbon® 6L80-E High Performance Valve Body (Lifetime Warranty)

The Project Carbon® 6L80-E High Performance Valve Body is designed to encompass the best of what we have to offer; equally sufficing both the daily driving crowd, and the high performance crowd. Whether your priorities are longevity, performance, towing or otherwise, the Project Carbon® valve...

nextgendiesel.com

But understand transmission programming is also key and if the torque converter is not a billet aftermarket unit, good luck.

Might also check this video out as well assuming your truck has a 6 speed

I understand why some people have the dealers swap transmissions, but GM uses some rebuilders that do not put the best aftermarket solutions in their transmissions, they repeat the same foolishness that they put in the the vehicle originally.

 

[ckeister]

If your towing with a 6l80 big cooler is a must IMHO. I know the earlier trucks (I have a 2000 Silverado) had a driveline clunk that was quite common. Mine has it have not done anything to it 215K and I abuse the shit out of it.
2007 Yukon Denali the D/s front diff mount went out. I felt it by vowing slow downhill and applying the brakes. That’s when it could be heard/felt

 

[tkdgirl]

UPDATE: After having a heavy duty transmission cooler install, we never made it out of the shop lot. Check engine light for a bad crankshaft position sensor. Soooo... after having that replaced plus the HD cooler, the clunking has almost disappeared. Yesterday when towing, the trans temp was fluctuating between 200-220. Should it be that high with the HD cooler? Would a HD pan help? I just don't know what else to do to get this thing so I can tow my camper. My mechanic is stumped (admittedly not a transmission guy).

I called the GM dealer and their master mechanic suggested replacing the transmission thermostat. Again, I have no clue about deep engine/trans stuff, but where is this part location?

I've also the updated thermal bypass for the 6L80. Good idea to replace it?

@NickTransmissions any advice?

 

[tkdgirl]

When the fluid temp gets high, the fluid thins out and once the fluid gets thin any valve body bore wear, spool valve wear or valve body internal leakage gets worse and leads to delayed and/or harsh up or down shifting.

The GM supplied transmissions are NOT NEW. They are rebuilds and are usually inferior to what a good independent shop can put together with higher quality aftermarket parts. The OE valve bodies are bad enough, the rebuild valve bodies are probably way worse.

I would guess the valve body is more like the main problem, possibly along with the TCM (Transmission Control Module) programming. But would expect the valve body to be more problematic.

You might look at this as a possible solution to your current problem.

Project Carbon® 6L80-E High Performance Valve Body (Lifetime Warranty)

The Project Carbon® 6L80-E High Performance Valve Body is designed to encompass the best of what we have to offer; equally sufficing both the daily driving crowd, and the high performance crowd. Whether your priorities are longevity, performance, towing or otherwise, the Project Carbon® valve...

nextgendiesel.com

But understand transmission programming is also key and if the torque converter is not a billet aftermarket unit, good luck.

Might also check this video out as well assuming your truck has a 6 speed

I understand why some people have the dealers swap transmissions, but GM uses some rebuilders that do not put the best aftermarket solutions in their transmissions, they repeat the same foolishness that they put in the the vehicle originally.

This is great information. Assuming a local transmission shop would be able to help with this?