2015 Silverado LTZ 5.3
4x4
6L80 Transmission (RPO MYC)

I’ve been troubleshooting a humming noise on my truck and wanted to drop the transmission pan for inspection. While I was at it, I decided to add a drain plug, because dropping a pan without one can be quite messy. There’s several options for doing so, but this is how I decided to approach it as I was doing the job at home and didn’t have access to all the tools I would normally use for a job like this, including a welder.

Mileage on my truck at the time was about 180,500. I had no “external” reason for suspecting the transmission was at fault - fluid has always been clean (relatively speaking) with no burned smell and it’s never shifted harshly, slipped or presented any other concerns. Nonetheless, I wanted to check for excessive metal in the pan, just in case, and it would be a good time to service the filter as well since I was unsure of the last time it had been replaced.

Although this is on a 6L80, the general process applies to several different makes/models of transmissions, provided there’s a pan that can be removed for filter service.

TOOLS AND SUPPLIES

1) Approx. 2 gal ATF - exact quantity depends on model but most transmissions will require about 1/2 total capacity for filter service.

2) Trans filter & pan gasket - some gaskets are reusable if they’re in good condition. If you get a filter & gasket kit make sure to verify it’s correct for your trans - the filter may fit but the gasket could be incorrect.

**Don’t forget any additional parts you may need - to remove the pan on my truck, the exhaust Y pipe needs to be dropped to allow the pan to clear the valve body, so I also had to replace the manifold-to-pipe gaskets.**

3) Drain plug kit or equivalent - there are many options for this - I opted for an M14 x 1.5 plug from Dorman and picked up a metric jamb nut from a hardware store. What’s important is making sure the set up has a low profile inside the pan as there’s only about an inch of clearance below the valve body.

4) Drill & bits appropriately sized for the plug

5) Files/sandpaper/grinding stones/equivalent for smoothing and prepping the hole and surrounding area.

6) JB weld or equivalent epoxy (if not welding).

7) Basic hand tools for removal & installation of the pan and setting the plug. Torque wrench recommended.

8) Solvent based parts cleaner for cleaning the pan, gasket surfaces, and prepping for epoxy. I just used brake parts cleaner.

I wanted to minimize the mess as much as possible, so I started with drilling the hole while the pan was still installed. Using a stop collar on the drill bit ensured the bit would only punch through the pan far enough to let the fluid drain without damage to the valve body.



Stop collar is ESSENTIAL if drilling the pan while still installed.

I chose the position for the plug because I knew it would be clear of the filter and at a low point in the pan, providing the most drainage possible.

As soon as the bit pushed through the pan, I walked away and gave it time to drain, then removed the pan. It should be noted that the initial hole was barely half the size of the 5/16” drill bit I used as just the point had cut through. It took a while for all the fluid to drain. (Tip - removing the dipstick will assist with venting the pan and speed up the draining process.)

The first thing was to check was for metal debris in the pan or on the magnet. It’s important to note that some amount of metal is normal - what you’re looking for is large particles of metal - splinter size and larger. Thankfully, there was nothing of concern. The photos below show the magnet as it was in the pan and after wiping the particles off.





Here’s a pic of the valve body with the solenoids labeled.


Pan & filter removed

A couple of things you can’t see:

1) The transmission fluid temperature sensor is internal to the TCM, which is 1 assembly with the solenoids (technical name: Control Solenoid Body & TCM assembly). Service info states if the sensor fails, the TCM/solenoid body assembly must be replaced and as of now, I haven’t had the opportunity to dissect one to see if repair is possible or not.

[Note: If you read this shortly after I originally posted it, you may recall I stated the TFT was part of the wiring harness. A couple days after posting, while researching operation of the 6L80 a little further, I learned about the TCM/solenoid body design and the TFT being integral to that. I apologize for the mis-information. It was an honest mistake that was based on earlier model designs which did have an internal harness with the TFT sensor.)

2) The fluid pressure switches are on the top side of the solenoid assembly.

A few notes on transmission operation:

-All of the solenoids are controlled using pulse width modulation, which just means the control module is turning them on and off at an extremely fast rate. The average of on time vs off time results in the desired effect. It’s the same method used for controlling fuel injectors.

-Using various combinations of shift solenoids 1&2 and pressure control solenoids 1-5, fluid is then directed to hydraulic circuits in the mechanical components of the transmission. Combinations used vary depending on speed, engine load, and driver input.

-The internal mode switch tells the module what position the manual gear selector is in. It’s what used to be the manual range switch and/or the neutral safety switch which was mounted externally on the shift lever shaft.

With the filter removed, the filter neck seal should be replaced to ensure good suction from the new filter. The old seal may look OK but it could be damaged while installing the new filter - it’s also been stretching/forming to the old filter and may allow leakage past the new filter.

Removal depends on the type of seal. The following pictures show how to remove it with a screwdriver - all I did was place the screwdriver next to the seal lip and then twist/rotate it to deform the seal, then worked the screwdriver around the seal until it was loose enough to come out. Just be careful - any scratches to the filter bore may prevent adequate sealing.



Arrow marks seal lip






Repeat around perimeter until seal is loose enough to remove.

The new seal can be installed using a seal driver or socket of proper diameter. If using a socket, make sure it’s contacting the perimeter of the seal and doesn’t fit inside it. Make sure the seal lip fully contacts the bore all the way around.

Finally, apply a thin layer of lubricant to the filter neck to help it seat without damaging the seal. Trans assembly lube, ATF Or petroleum jelly is recommended.



Not all filters have stop tabs.



Part 2 coming soon….

 

Now to finish the drain.



Once the hole has been drilled to the proper size (no larger than 1/16” diameter of the plug, especially if you’re using epoxy), ream and clean the edges to remove burrs. An easy method to ream without enlarging the hole is to use a drill bit at least twice the size of the hole. If it’s sharp enough the burrs will be easily cut off and you may be able to do it by hand.




Prep the area surrounding the hole and one side of the nut with a grinding stone, reamer, sandpaper or equivalent. The surfaces need to be rough (relatively speaking) to give the epoxy something to grip.




Once the surfaces have been prepped, clean the nut and pan thoroughly to remove all debris. Aerosol brake cleaner followed by compressed air should do the job “well enough,” however to ensure it’s as clean as possible, follow up with a handheld torch once the cleaner has evaporated. Heat the pan and nut for +/- 10 seconds to burn off residual cleaner.

To prevent the epoxy from adhering to the bolt threads (I opted for a secondary bolt just to make sure I didn’t damage or ruin the plug), apply wax (aka chapstick) or another type of greasy lubricant to the threads.




The next step can be tricky. The pan needs to be flat to keep the epoxy in place but you also need access to both sides of the pan. I set the pan on a couple of scrap 4x4’s to accomplish this but there are several other ways to do it.

Mix epoxy according to manufacturer instructions. I’ve found plastic knives or popsicle sticks work well for mixing and application. Apply epoxy to the pan, then insert the bolt, thread on the nut, and tighten the bolt hand tight. Leave alone until the epoxy CURES (should state appropriate time on epoxy packaging). If you try to remove the bolt too soon, it may break the bond and the nut will not remain in place when removing the plug in the future.



Tighten bolt hand tight - leave until epoxy cures.

Once the epoxy cures, remove the bolt and clean the excess. Using a sharp utility knife, cut the epoxy around the nut, leaving no less than 1/8” around the circumference (sorry I didn’t get a picture of this), then scrape the excess off. Flip the pan over and cut the excess out of the area around the hole, then install the plug finger tight.







You can use a traditional drain plug gasket (copper, aluminum, plastic or rubber) if you want. The type of gasket depends on what kind of flange the drain plug has. I opted to apply RTV to the threads and not use a gasket, since I don’t intend to drain the transmission very often.

Install the pan. Bolts and drain plug should be torqued to 15-20 ft*lbs. Reinstall any parts that had to be removed to drop the pan.

Add approximately 6 qts fluid to pan for initial start. Cycle the gear selector to each range, pausing for 2-3 seconds in each position, then recheck fluid to ensure there’s enough to be in the cold fill range.

Verify there’s no leaks around the gasket or plug. Test drive to bring the transmission fluid up to temperature and recheck (for the 6L80, hot fill check is 160-200 degrees Fahrenheit, accessible via the driver info center).

It’s important to note that many 2015 & newer GM trucks utilize a thermal bypass valve for the transmission cooler. The valve works the same as a thermostat and doesn’t open until the fluid is at approximately 190*F to allow flow to the cooler. You will most likely have to “brake stand” the transmission (hold brake and rev engine to load trans) in order to get the fluid hot enough to open the cooler circuit.

You’ll know if your truck has this by the block style fitting where the cooler lines connect to the transmission - this is what contains the bypass valve.




I’m actually working on a separate write up on how to gut the valve and get consistent flow through the cooler at all times, but I’m not sure when I’ll have it ready.

Check for leaks and level again after a few drive cycles to make sure everything is sealed and fluid is at the correct level. Don’t forget to cycle the shifter through each manual gear position - the goal is to have the transmission as completely full of fluid as possible when checking the level.

 

Thanks for the write-up and photos.

That 6L80 valve body looks drastically different from a 4L60




I'm a little surprised you decided to add a drain to your pan. Judging from all scuff marks on the front on your pan and running length-wise, it looks like it gets dragged through stuff. I would rather have a smooth pan than have it catch on something that rips it out.
I don't mind dropping the pan to clean it out. If you lower one side slowly it's not too messy, especially if you have a large catch tray. It also allows you to inspect the magnet, as you did, and clean out any sludge in the pan. I always change the filter when I drain the the fluid. They're cheap enough.

Please post pics of the finished result after the epoxy cures.

 

I've heard this about modern transmissions. The more gears you have, the cooler it runs. It allows the tranny to use more aggressive TCC lockup to minimize heat buildup and maximize fuel economy. Toyota stopped putting external coolers on Tacomas which pissed off a lot of (probably clueless) people.

 

The scuff marks were there when I bought the truck, a little over a year ago. I drive primarily highway/pavement with only occasional “off roading” in the back yard to hook/drop my trailer or to dump brush in the burn pile at my family’s farm. That’s not to say I’ll never get it in a field or timber, but those occasions are few and far between.

I have a drain adapter at work for dropping pans. I agree it’s not too difficult, but I try to keep things as clean as possible at home, so that’s why I added the plug.

I actually did this about a month ago - the picture of the nut inside the pan was taken after the JB weld cured but before I trimmed and cleaned the excess. I’ll try to edit it to give a better idea of what it should look like.

I can’t speak to the specifics of gearing vs temperature. All I know is these 6 speeds with the bypass valve don’t get hot enough under normal driving conditions to open the valve. When I service the filters or replace the cooler lines at work, I always have to brake stand the transmission to get the fluid up to temp.

I gutted the valve on my truck because I just don’t agree with the operation. IMO, fluid nearing 200*F before it starts circulating through the cooler is just asking for trouble.

 

yeah post the pan exterior please.

well the engine operates near 200F right, so the coolant entering radiator is around that temp. Doesn't seem to make much sense sending the tranny fluid there below that temp unless you wanna warm it up.

 

At the moment, I’m in my PJ’s [aka gym shorts and no shirt] and not willing to give the neighbors a peep show just to get a picture, so I’ll post that tomorrow.

First, please go back to the notes in my first post regarding transmission operation - while researching the 6L80 I learned I made a mistake and edited the post accordingly. (Section after the valve body pic.)

Second, I found this webpage which looks like a service manual independent of GM. Lots of good info on the 6L80, including a complete rebuild guide….

GM 6L80 - https://atoc.ru/uploads/manual/5fb79aebb40df.pdf

I get what you’re saying about the operating temp of the engine/coolant, but purpose of the bypass valve is to allow the ATF to heat up faster - just like a thermostat for engine coolant.

Below 190* the valve is in the bypass (closed) state - fluid still flows through the valve, but it’s immediately returned to the transmission. At or above 190* the bypass is blocked off and the fluid is directed through the cooler.

I’m not sure what the exact characteristics of dex 6 are at higher temps, but I do know that most ATF begins breaking down around 215* - 220* and I think it’s a bit foolish to push that limit before cooling the fluid begins. The cooler circuit only holds approximately 1.5 - 2.0 quarts of fluid while the entire system takes 12 - 13 quarts.

IMO, with the bypass valve setup, the flow through the circuit is inadequate to properly cool the fluid and return it to the transmission to provide temperature regulation to the internal components. It creates a delay in fluid cooling that allows too much heat to build before the valve opens. If it opened at a lower temperature, it would be less of a concern.

These transmissions are also available with “HD cooling” in which the bypass valve is eliminated altogether and the lines connect directly to the cooler ports. It doesn’t make sense to me to eliminate the valve on some vehicles if the theory is that the fluid will be adequately cooled once the valve opens.

But that’s just my opinion.

 

I'm guessing that it doesn't just snap open or closed. It'll be more of a variable flow like a coolant thermostat?

the major source of heat in a transmission is the slushbox. Fluid shearing in the torque converter. TC is only working on acceleration when engine speed is significantly different from trans input speed. With more gears, the ratios are spaced closer together, and TC isn't needed as much. TCC is engaged very quickly. This is why you have to do brake stands to get it warm.
Dex 6 is a highly refined base oil. Service life and temperature stability is much better than the previous generation was.

 

Here’s a couple pics of the pan exterior.







I’m not sure if the valve snaps open and closed or not - it utilizes a wax pellet and spring just like a thermostat but I don’t know how much movement it takes to close the bypass and open the cooler, and vice versa.

When I first started seeing these valves at work, I’d attempt to get the trans up to temp by driving as far as 30 miles and it wasn’t enough, even if I manually changed between 5th & 6th on the highway. So, I finally started going on shorter drives to get the fluid temp up after a filter service, then brake stand to bring it up to temp for a hot check.

Since gutting the valve on my truck, I’ve only seen the fluid temp exceed 180* once. It was just a few weeks ago - it was almost 100* ambient temp and I had to drive through city traffic for nearly 2 hours (thanks road construction ) to run some errands after work. When I finally got back on the highway, the temp dropped from 183 to 160 after about 15 minutes. During my normal commute, it stabilizes at around 150.

 

Thanks. Looks pretty clean



well that's good right... A cool running transmission.


I always thought 160-190 was the optimal range for ATF because it needed to reach a certain viscosity


(Dexron III)