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Discussion Starter · #1 ·
I got my truck back on the 26th after the dealer had it since the 8th. Well, today I received the paperwork from them so I looked through their write up. I was surprised at what they found.

It all started a few months ago with the dreaded shudder. Over the months it got worse and then developed a hard shift. I was not ready to take it in just yet because of Covid. But after building up several “reasons” I made the appointment.

Skipping the other reasons (TSB for the water gurgle in dash and recall reprogram for the particulate filter) the major item was the shudder and a hard shift. It ended up being replaced with a new transmission under warranty. Here is what they found:

“GDS running noticed slipping shift on 1-2-3 shift. Checked fluid and level was ok but very burnt. Removed transmission and disassembled. Found heavy metal debris in pan. Found 2-6 clutch burnt and had been very hot. Metal in torque converter, pump, and valve body. Pump body scored metal in all clutch packs. Installed Serta transmission, programmed TCM and performed fast learn.”

Wow! I did not think it was that bad! Yes it is 4x4 Diesel and I do tow a tandem 14’ trailer but it is no where near the 7,700 lb limit for the truck. I have never received the overheat warning on the transmission on the DIC. What could have destroyed it before 45k miles? Could it be I run a Pedal Commander and it can’t handle the rapid spin up of the throttle?
 

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Did you leave the PC on it when you took it in?
 

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The stock transmission settings are soft and allow way too much slip. It seems transmissions vary though, some slip more than others. I remember how much mine slipped on the 3rd to 4th shift. The grunty diesel does not need "help" from the transmission. Solid shifts work great with the 2.8. Tune that trans as soon as you are out of warranty.
 

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2021 Black \/6 4WD Z71 Crew Cab Short Box
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Thanks for the follow up. The more intel we have the better.
Mine is a little clunky from 1-2, not just when cold either so your post has caused me to want to bring it in before I blow through my warranty.
 

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2018 zr2 ccsb duramax
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Yes I've seen some confused 6speed owners thinking they have 8speed problems but no real major problems or failures
 

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Yes I've seen some confused 6speed owners thinking they have 8speed problems but no real major problems or failures
Quite a few 6 speed torque converters have experienced "shudder". GDE did a good write up/post mortem report on one they looked at.


Running the blue label Mobil 1 LVHP fluid in my 6 speed, it works better than it did on the OEM fill. Any fully synthetic trans fluid should be an improvement though. Tuning the trans (GDE) made a night and day improvement. Shifts are firm but smooth, early and correct, no mushing around and just what the doctor ordered for the diesel's power curve.
 

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Quite a few 6 speed torque converters have experienced "shudder". GDE did a good write up/post mortem report on one they looked at.


Running the blue label Mobil 1 LVHP fluid in my 6 speed, it works better than it did on the OEM fill. Any fully synthetic trans fluid should be an improvement though. Tuning the trans (GDE) made a night and day improvement. Shifts are firm but smooth, early and correct, no mushing around and just what the doctor ordered for the diesel's power curve.
I would have to agree with you. I have had the same positive experience with the trans tune and switching to the synthetic trans fluid.
 

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Discussion Starter · #11 ·
Yes it is the 6 speed. The dealer (whom I trust) said they have never heard of this with the diesels. They said they have had several of the V6 models come in but I was their first diesel.

it shifts like new now. And I am going out on a limb here by saying it shifts “different” than new. Previously, if I was below 35MPH and decelerating down a slight hill, it would hard-shift down a gear. But now, even only level ground and at lower speed (<20MPH) such as a parking lot, it will drop 1-2 gears but very softly and smooth. It is not an annoying downshift.
 

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Yes it is the 6 speed. The dealer (whom I trust) said they have never heard of this with the diesels. They said they have had several of the V6 models come in but I was their first diesel.

it shifts like new now. And I am going out on a limb here by saying it shifts “different” than new. Previously, if I was below 35MPH and decelerating down a slight hill, it would hard-shift down a gear. But now, even only level ground and at lower speed (<20MPH) such as a parking lot, it will drop 1-2 gears but very softly and smooth. It is not an annoying downshift.
Very glad to hear that!

I think the transmission itself, the "hardware" is fine. The "software" (TCM settings) were awful in the older trucks. I do believe GM updated the TCM settings on the new trucks. For those who do not want to tune their older trucks, getting the updated trans settings is likely a worthwhile endeavor. The transmission can work just fine, it just needs to be tuned correctly. The way my truck came as delivered vs the way it works now, it is like a different transmission is installed.

Speaking of annoying downshifts, one of the favorite things the tuner did (that we asked for) was to turn off grade braking in "D". No more aggressive downshifting and revving the engine to the moon. If I want grade braking, the trans can be put in "manual mode" and I can control the braking better from there, usually with Tow/Haul engaged for the exhaust brake. Manipulating the transmission and EB to do my bidding and not what it seemingly wants to do occupies me on long trips through the mountains :LOL:

As we know, for open road driving, all the trans really needs to do is get into 6th gear, then it's work is done for the day as long as the road is open and one is going at freeway speeds. Driving from CO to ID, have literally driven for 500 miles in 6th gear the whole time with no downshifting. Ah, the open west :cool:
 
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16 Canyon SLT, V6, 4X4, Longbed, Cyber Metallic Gray
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When towing, do you tow in alot ofbsteep grades?
Hard getting a compromise between a car and a truck transmission.
Factory leans toward making it act like a car transmission, example, all the posts of people leaving chevy for ford or Toyota because the transmission doesn't act like a car transmission, but for us that use a truck as a truck, we want long life expectancy out of the transmission, more than how smooth the shift is.
 

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Yes it is the 6 speed. The dealer (whom I trust) said they have never heard of this with the diesels. They said they have had several of the V6 models come in but I was their first diesel.

it shifts like new now. And I am going out on a limb here by saying it shifts “different” than new. Previously, if I was below 35MPH and decelerating down a slight hill, it would hard-shift down a gear. But now, even only level ground and at lower speed (<20MPH) such as a parking lot, it will drop 1-2 gears but very softly and smooth. It is not an annoying downshift.
You got a good trans, a CERTA unit, that means it's a GM certified rebuilt with all the updates available at the time. Usually better than new.
 
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When towing, do you tow in alot ofbsteep grades?
Hard getting a compromise between a car and a truck transmission.
Factory leans toward making it act like a car transmission, example, all the posts of people leaving chevy for ford or Toyota because the transmission doesn't act like a car transmission, but for us that use a truck as a truck, we want long life expectancy out of the transmission, more than how smooth the shift is.
Yep, smooth means slipping, and slipping means wearing. Feeling a trans shift (even the TC) was a good thing.
 

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Likely all those issues are just downstream failures caused by your shudder. The converter failures on the 6SPDs seem to launch debris everywhere and wear out the pump (low line pressure) and plug up the valve body (poor shift quality/no shift condition). Some of the photos of the failed converters online show metal to metal contact (no friction material) and it just gets better from there. Pretty typical but unfortunate failure mode, although it seems like not a ton of people as a percentage of the whole population are having this happen. I have to say I'm curious though, if folks are very observant and note the shudder early on, would changing out the transmission fluid have ultimately saved this unit?

At that mileage though... Impressive.
 

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Let's take a step back here. First off, super rare issue; clearly.

“GDS running noticed slipping shift on 1-2-3 shift. Checked fluid and level was ok but very burnt. Removed transmission and disassembled. Found heavy metal debris in pan. Found 2-6 clutch burnt and had been very hot. Metal in torque converter, pump, and valve body. Pump body scored metal in all clutch packs. Installed Serta transmission, programmed TCM and performed fast learn.”
Symptoms: slipping clutches, burnt fluid, "heavy metal debris" (steel, not aluminum or clutch material), a single badly abused clutch, debris distributed by fluid to various subsystems, pump body scored.

Burnt fluid comes from heat comes from friction. The "heavy metal debris" could be clutch parts or pump parts based on the description. The badly worn clutch is the smoking gun.

Diagnosis: For some reason, the 2-6 clutch failed. My guess (based solely on the tech's description) would be a failed 2-6 clutch solenoid; clogged or incorrectly assembled from the factory. Everything else came from that. Failure was likely in the form of partial actuation. It would probably extend/retract 90%, and that meant the 2nd/6th gears were always slipping a bit. That slippage lead to heat, lead to failure of the fluid (failure of lubricity), and either the pump started shredding or something in the 2-6 gear set got hot and started shredding. Those steel bits start eating up other parts, and you end up with the scenario you found yourself with.
 

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You all may find this article (marketing material, LOL) interesting:


This one from Gears Magazine as well about 1/3 of the way down the page under -- PUMP FAILURE/ PRESSURE REGULATOR WEAR

 

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Likely all those issues are just downstream failures caused by your shudder. The converter failures on the 6SPDs seem to launch debris everywhere and wear out the pump (low line pressure) and plug up the valve body (poor shift quality/no shift condition). Some of the photos of the failed converters online show metal to metal contact (no friction material) and it just gets better from there. Pretty typical but unfortunate failure mode, although it seems like not a ton of people as a percentage of the whole population are having this happen. I have to say I'm curious though, if folks are very observant and note the shudder early on, would changing out the transmission fluid have ultimately saved this unit?

At that mileage though... Impressive.
I had the converter replaced in my Canyon diesel just shy of 60,000. It started acting up again after another 40,000 and the trans shop I took it to suggested just changing the fluid. Did a fluid change myself and shudder was gone. That was around 10,000 miles ago and no issues since.
 

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You all may find this article (marketing material, LOL) interesting:


This one from Gears Magazine as well about 1/3 of the way down the page under -- PUMP FAILURE/ PRESSURE REGULATOR WEAR

We have the little brother, 6L50. But yep, lots of times aftermarket trumps OEM on this kind of stuff.
I really don't know if we have the same TC but I doubt it.
 
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