Transmission Questions

Tony629578 · December 9, 2020

Just recently bought a 2015 GMC Sierra 1500 SLE 4x4. I am the third owner. Shortly after buying the truck the transmission grenade’d itself. I’m at 97k miles so still under the 100k mike power train warranty, however I was 21 days outside of the 5 year part of the warranty. I reached out to GM after returning from deployment to try and get some help and hope that they would honor all or maybe partial warranty. Needless to say no help from GM! Shopped around for shops to rebuild and ended just buying a remand one for 2.7k. So here are my questions:

1. Flush the current cooler or buy an upgraded one?

2. If I should buy an upgraded one then how big?

Also should I bypass the current one in the radiator to have a separate cooling system?

3. I’ve heard of people deleting the transmission oil thermometer to achieve a cooler temp. So say do this because heat is the enemy, others say the trans is built to operate at a certain temp.

3.What temperature is the best operating temperature?

4. Should I have the new transmission re flashed or swap the one from my grenade’d one into the remand one?

5. What’s the best way to flush the current cooler if I don’t change it? I’m afraid if I just try to flush it then it won’t get it clean enough and I’ll mess up the new transmission!

That’s all I can think of right now! Thank you in advance for your time and expertise!

JimCost2014 · December 9, 2020

56 minutes ago, Tony629578 said:

Just recently bought a 2015 GMC Sierra 1500 SLE 4x4. I am the third owner. Shortly after buying the truck the transmission grenade’d itself. I’m at 97k miles so still under the 100k mike power train warranty, however I was 21 days outside of the 5 year part of the warranty. I reached out to GM after returning from deployment to try and get some help and hope that they would honor all or maybe partial warranty. Needless to say no help from GM! Shopped around for shops to rebuild and ended just buying a remand one for 2.7k. So here are my questions:

1. Flush the current cooler or buy an upgraded one?

2. If I should buy an upgraded one then how big?

Also should I bypass the current one in the radiator to have a separate cooling system?

3. I’ve heard of people deleting the transmission oil thermometer to achieve a cooler temp. So say do this because heat is the enemy, others say the trans is built to operate at a certain temp.

3.What temperature is the best operating temperature?

4. Should I have the new transmission re flashed or swap the one from my grenade’d one into the remand one?

5. What’s the best way to flush the current cooler if I don’t change it? I’m afraid if I just try to flush it then it won’t get it clean enough and I’ll mess up the new transmission!

That’s all I can think of right now! Thank you in advance for your time and expertise!

Welcome to the site, and thank you very much for your service to our country!!!

Lots of good questions, I will field #4, you will have to "flash" the new transmission so the truck knows it is there, and can go through it's learning curve.

Can probably do #3 also, lots do a thermostat delete on the tranny to keep temps down, would be easy if it is not even in the truck yet.

Thinking you have a 6 speed (#5), so the shop should be able to do the normal flush which pushes out 20 quarts and replaces it with 20 fresh quarts.

Edited December 9, 2020 by JimCost2014

flyingfool · December 9, 2020

1. flush it, and new filter and inspect for metal glitter

2. nope

3. yes, remove the trans t-stat immediatly

4. my trans operates at 124F on the highway. buts its modded, stat delete will give ya 145 F operating temp.

5. youtube it. i use a 3/8 poly hose jammed into the trans cooler , run into a bucket, have helper tell you when fuid stops squirting out, shut off engine, refill. i would use 3- gallons worth for a flush

CamGTP · December 9, 2020

If you install the transmission yourself you must tow it to the dealership to have the TCM flashed with your VIN's operating system so the PCM and TCM has matching calibrations.