overheating transmission solved

truckguy82 · January 4, 2017

I've never heard of anyone else having this issue, but just something good to know.

'14 6.2 with 6l80. 30k miles

Problem was it was reaching high temps just normal driving. I would see 220-230 degrees frequently under normal driving.

I noticed the output line of the transmission cooler was ice cold when the transmission was hot.

The problem was a malfunctioning thermal bypass valve. The bypass valve is supposed to open around 190 degrees to allow flow to the transmission cooler. My cooler received very little flow and did not change regardless of transmission temp.

I had to bitch and bitch because it did not pull any codes. I had to figure out the problem myself, which was just an educated guess. Finally got the dealer to replace the thermal bypass valve.

Just got the truck back, got it up to 190, and it stayed there, checked the lines and they were all hot....so happy. Finally!

Edited January 4, 2017 by truckguy82

07Softail · January 4, 2017

I pulled that stupid ass t-stat out and made a bypass plug so it forces all of the fluid through the cooler. my trans in the summer never reaches 180 now. The t-sat is definitely not needed here in the south more of a cold northern thing.

RT

truckguy82 · January 4, 2017

I pulled that stupid ass t-stat out and made a bypass plug so it forces all of the fluid through the cooler. my trans in the summer never reaches 180 now. The t-sat is definitely not needed here in the south more of a cold northern thing.

RT

Yeah i live in NJ, if it was even a slightly warmer climate I would gut that thing out without hesitation.

SkiDooNick700 · January 5, 2017

Glad you figured it out!

I pulled that stupid ass t-stat out and made a bypass plug so it forces all of the fluid through the cooler. my trans in the summer never reaches 180 now. The t-sat is definitely not needed here in the south more of a cold northern thing.

RT

Yea I'm thinkin so. Here in MI my trans barley hits 100degrees on my NHT on my 20 or so minute drive to work with several stop lights. I wonder how little it would warm up if I just totally bypassed like you lol. I never owned a vehicle with a trans temp on it prior to this one, so it was interesting to me to see how slowly the trans warms up.

07Softail · January 5, 2017

And this is the first year GM has put them on the trucks....not sure what their reasoning is though.

RT

TJay74 · January 6, 2017

Just a heads up, there is programming in the TCM that is temperature dependent on how the shift profile works. While southern vehicles might be fine, northern vehicles might run into issues if the transmission doesn't warm up fully. I have seen the calibration details and can tell on my truck when it is running under one part of the calibration and then when it flips over to the other once the transmission hits the required temperature.

HondaHawkGT · January 7, 2017

Glad you figured it out!

Yea I'm thinkin so. Here in MI my trans barley hits 100degrees on my NHT on my 20 or so minute drive to work with several stop lights. I wonder how little it would warm up if I just totally bypassed like you lol. I never owned a vehicle with a trans temp on it prior to this one, so it was interesting to me to see how slowly the trans warms up.

It'd have an impact on your fuel economy. It's the primary reason they do it.

j-ten-ner · January 7, 2017

Just a heads up, there is programming in the TCM that is temperature dependent on how the shift profile works. While southern vehicles might be fine, northern vehicles might run into issues if the transmission doesn't warm up fully. I have seen the calibration details and can tell on my truck when it is running under one part of the calibration and then when it flips over to the other once the transmission hits the required temperature.

^^^ yup

On my truck, the converter clutch won't engage before the trans reaches 29 degrees Celsius.

so long

j-ten-ner

11bDad · January 7, 2017

I pulled that stupid ass t-stat out and made a bypass plug so it forces all of the fluid through the cooler. my trans in the summer never reaches 180 now. The t-sat is definitely not needed here in the south more of a cold northern thing.

RT

I added a better tranny cooler and modified my by-pass valve. Now mine never goes above 180* (even in Coastal SC heat) & I can run all day on the interstate @ 75 mph at 140* in the middle of the summer. Flair shifting and wonky downshifting has almost been eliminated also. Glad you were able to get it fixed.

sdeeter19555 · January 7, 2017

^^^ yup

On my truck, the converter clutch won't engage before the trans reaches 29 degrees Celsius.

so long

j-ten-ner

68 degrees Fahrenheit...for those conversionally challenged...at least according to my Edge Insight. ;)

Mine runs cold (2012 6l90e) because of an added cooler, less than 100*F when it's in the 20s. I don't notice anything different, once it reaches that magical 68*F.

Sent from my SAMSUNG-SM-G900A using Tapatalk

Q_2017GMC 1500 · October 5, 2018

On 04/01/2017 at 1:25 PM, truckguy82 said:

I've never heard of anyone else having this issue, but just something good to know.

'14 6.2 with 6l80. 30k miles

Problem was it was reaching high temps just normal driving. I would see 220-230 degrees frequently under normal driving.

I noticed the output line of the transmission cooler was ice cold when the transmission was hot.

The problem was a malfunctioning thermal bypass valve. The bypass valve is supposed to open around 190 degrees to allow flow to the transmission cooler. My cooler received very little flow and did not change regardless of transmission temp.

I had to bitch and bitch because it did not pull any codes. I had to figure out the problem myself, which was just an educated guess. Finally got the dealer to replace the thermal bypass valve.

Just got the truck back, got it up to 190, and it stayed there, checked the lines and they were all hot....so happy. Finally!

My truck is doing the same damn thing...

truckguy82 · October 5, 2018

7 minutes ago, Q_2017GMC 1500 said:

My truck is doing the same damn thing...

Feel the line coming out of the passenger side of the trans cooler. It’s super easy to reach from the front with the hood open.

if it’s ambient temp when your trans is like 200 degrees, then you have a stuck thermostat. It’s an external part to the trans and relatively easy to fix.

The line should be scolding hot when your trans is 200.

I’m actually super happy my post may have helped someone, I went through hell trying to diagnose it and get it fixed.

Angelo Marino · November 26, 2018

Did you do the fix yourself? My trans has been doing the same thing. Took me about a month, 3 trips to the dealership, a call to GM Customer Care, and a bunch of documentation for them to finally start wrenching. Today they called and said my thermal bypass valve needs to be replaced, it's not covered under my powertrain warranty, and its $690 out the door.

If it's an easy fix I will just do it myself. If it's not, I guess I have to bite the bullet.

truckguy82 · November 26, 2018

2 hours ago, Angelo Marino said:

Did you do the fix yourself? My trans has been doing the same thing. Took me about a month, 3 trips to the dealership, a call to GM Customer Care, and a bunch of documentation for them to finally start wrenching. Today they called and said my thermal bypass valve needs to be replaced, it's not covered under my powertrain warranty, and its $690 out the door.

If it's an easy fix I will just do it myself. If it's not, I guess I have to bite the bullet.

It should be an easy fix if you can find the correct part. For some reason there seems to be a mix up with parts and sometimes they end up needing to replace the lines coming off of it.

It’s an external part to the transmission and it’s very simple in operation. $690 is insane to replace it. I have no idea where they pulled that number out of their ass. The part is cheap and so is the labor.

definitely DIY

Edited November 26, 2018 by truckguy82

Angelo Marino · November 26, 2018

1 hour ago, truckguy82 said:

It should be an easy fix if you can find the correct part. For some reason there seems to be a mix up with parts and sometimes they end up needing to replace the lines coming off of it.

It’s an external part to the transmission and it’s very simple in operation. $690 is insane to replace it. I have no idea where they pulled that number out of their ass. The part is cheap and so is the labor.

definitely DIY

GM and the dealership ended up covering a majority of the repair costs. Only reason I can think it was quoted so high is because it included trans fluid. I can only assume they had to change the fluid as well.