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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)
I agree - a cool running transmission is a good thing - so that raises the question - if the point of the bypass valve is to heat up the fluid to achieve optimal temp sooner, but the temp doesn’t reach that range during normal driving, then why is the valve there at all? It doesn’t seem to have any noticeable effect.
I also see it as a potentially catastrophic failure point. I’m sure many of us have seen a thermostat get stuck closed, and if these valves work on the same principle, it’s possible it could get stuck closed, too. Then, when the fluid is in need of cooling, it’s not receiving it, and the transmission begins overheating.
it might also have to do with pressure. The cooler might add too much resistance to flow, before the fluid has gotten up to temp, and raise the pressure too much.
Some aftermarket coolers are advertised as "low pressure drop" , because they have a built-in bypasses for colder, thicker fluid.
Originally Posted by Gumby22
many of us have seen a thermostat get stuck closed,
never in my life 🤷♂️ But I supposed it's possible. So in that case, the fluid would just recirculate, and still never really get very hot according to you observations, right?
Eventually, the transmission goes into mitigation mode. Shows a message in the DIC, disables overdrive, etc.
At that point you probably want to replace the fluid. I've gotten my fluid pretty hot in the past a few times. (got up to around at least 250F) It wasn't catastrophic. The transmission was still working perfectly at around 200k
(Just wanted to clarify I’m only continuing the discussion - not trying to prove who’s right or wrong. )
Originally Posted by mountainmanjoe
it might also have to do with pressure. The cooler might add too much resistance to flow, before the fluid has gotten up to temp, and raise the pressure too much.
Some aftermarket coolers are advertised as "low pressure drop" , because they have a built-in bypasses for colder, thicker fluid.
never in my life 🤷♂️ But I supposed it's possible. So in that case, the fluid would just recirculate, and still never really get very hot according to you observations, right?
Eventually, the transmission goes into mitigation mode. Shows a message in the DIC, disables overdrive, etc.
At that point you probably want to replace the fluid. I've gotten my fluid pretty hot in the past a few times. (got up to around at least 250F) It wasn't catastrophic. The transmission was still working perfectly at around 200k
It’s possible the pressure needed for cold fluid was a concern, but then you have to look at other vehicles that have the “HD cooling” option. The ones I’ve seen have the 6L90 (which is essentially the same transmission), use the same the same fluid and don’t have a bypass valve.
I also lean towards fuel efficiency as being the reason. There’s a lot of effort put into raising CAFE numbers for the sake of not only tax breaks but increased sales. I don’t know how much fuel the valve could actually save, but I do know if it helps, they’re going to utilize it where they can.
As for thermostats sticking closed, maybe it’s not as much of a problem these days, but it used to be. That’s why the “fail safe” thermostats (when they fail, they fail in the open position) came to be - to prevent overheating in the case of a stuck thermostat.
I’d have to do some research to refresh my memory on the details of how they get stuck shut.
Yup fuel efficiency is definitely a factor. We've seen GM do the same thing with their water pumps. They changed them to a new recirculating design that gets the engine warmed up quicker. Maybe the HD trucks (with HD cooling) are already in a different CAFE category so they didn't bother with the fuel saving.
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.
[[b]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….
did it fix the Humming issue?? I have The same issue
did it fix the Humming issue?? I have The same issue
No. I’ve since traced the noise to a worn right rear axle and have just been living with it since. I’ll deal with it when/if the axle seal starts leaking.
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