2014 GMC Sierra 1500 4.3L Trans temp question (6L80)

Ubiquity · April 13, 2023

I appreciate and understand the analysis you gave to your post.

I don't know to what level my concerns are justified, but one of the reasons I had for going external and independent of the radiator was to avoid/limit the possibility of clogging.

Now that warmer temperatures are forecast I hope to get a good run in tomorrow to help me decide.

Grumpy Bear · April 13, 2023

4 hours ago, 2015regcablongbed said:

@Grumpy Bear

I appreciate and understand the analysis you gave to your post.

I don't know to what level my concerns are justified, but one of the reasons I had for going external and independent of the radiator was to avoid/limit the possibility of clogging.

Now that warmer temperatures are forecast I hope to get a good run in tomorrow to help me decide.

External cooler can clog too my friend.

Ubiquity · April 13, 2023

2 hours ago, Grumpy Bear said:

External cooler can clog too my friend.

Ha haa! Gotcyah, I specifically wrote "...avoid/limit the possibility of..." My ex trained me good don't you know.

Edited April 13, 2023 by 2015regcablongbed
No one respects Rodney

swathdiver · April 13, 2023

If my fluid looked like that after one long highway drive I would be beside myself. The next morning I would be at the dealership having them check the flow of the lines and coolers since I don't have the machine myself, yet!

I drive the previous generation truck which does not have a thermostat for the transmission and where the fluid first goes into a tank on the radiator which allows the fluid to quickly heat up during cold weather. Then it goes to the external cooler in front of the condenser and back into that glorious 6L80.

So mine when driving on an 80 degree day at 70 mph or so after 20-30 minutes runs in the 140s. This is a considerably heavier truck than your pickups, about 1000 pounds more. I'd have to drive 60+ thousand miles for my fluid to begin to look like that.

The new factory thermal bypass valve is fully open at 158 I think. I would like one cooler if possible. We towed a 6K pound boat with the Yukon XL through rush hour city traffic, about 15 miles for 30-40 minutes or so and the hottest the transmission got was 184 once we reached our destination.

If your lines and cooler test good, I'd suspect an issue with the transmission.

It's taken 2 hours to write this with many interruptions. If I failed to answer any questions please let me know and I will revisit this post in the evening.

Willard Norris · April 13, 2023

28 minutes ago, swathdiver said:

If your lines and cooler test good, I'd suspect an issue with the transmission.

The transmission was rebuilt in November ‘22 and only has ~2,500 miles on it.

I’ll be contacting the transmission shop again tomorrow.

thank you.

Willard Norris · April 19, 2023

@swathdiver @Grumpy Bear

I spoke with the transmission shop and they have 3 HotFlush machines in their shop. They flush each vehicle when they do a rebuild, but he said he’d do another flush and drop the filter if I’d like.

- if my cooling fan isn’t working properly (going into high mode at the appropriate time), should I replace the fans before the fluid flush? If the cooler gets flushed and cleaned, but the fans fail to cool properly, then tranny temps while stopped may still run high.
- as a shade tree mechanic, is there a way to re-tune the fans to go into high mode at a lower temp? If so, what would I need to get in order to do this?

Thank you.

Grumpy Bear · April 19, 2023

Do the flush. That is a priority. Then ask the dealer to check that the fan trips are functional and if not get that fixed. The last one, a lower kick in is EXPENSIVE. HP Tunner, license etc or find a shop that dyno tunes and pay them to set it as low as possible. 192 F I think.

swathdiver · April 19, 2023

33 minutes ago, Willard Norris said:

@swathdiver @Grumpy Bear

I spoke with the transmission shop and they have 3 HotFlush machines in their shop. They flush each vehicle when they do a rebuild, but he said he’d do another flush and drop the filter if I’d like.

- if my cooling fan isn’t working properly (going into high mode at the appropriate time), should I replace the fans before the fluid flush? If the cooler gets flushed and cleaned, but the fans fail to cool properly, then tranny temps while stopped may still run high.
- as a shade tree mechanic, is there a way to re-tune the fans to go into high mode at a lower temp? If so, what would I need to get in order to do this?

Thank you.

Well, first see how your cooling fans work. Mine has three relays and I would test them to make sure all three are working before changing out the fans. In my generation of truck, we had two fan options, 500 watt motor and 700 watt motors.

Yes, HPTuner or EFILive and maybe other programmers/companies can change the fan settings. The lower settings only work well in conjunction with a lower temperature thermostat.

Willard Norris · May 19, 2023

@Grumpy Bear @swathdiver

UPDATE: thank y’all so much for all the help with everything. Over the past month I’ve done the following to my truck:

-replaced cooling thermostat with the Mishimoto 180 design

-installed new transmission cooling lines which now comes with a 70° C (158° F) transmission thermostat

-had the radiator cooling fans checked…they work perfectly.

Drove on a 4 hour and 5 hour drive with trans temps hovering around 172-175 during cruise, but would rise to mid 180s when sitting in traffic.

-transmission shop that did the rebuild performed a hot flush and changed out the fluid as well. No metal was found during the flush.

So….I think everything has been done that y’all recommended. The 180 thermostat seems to have been a big help, but I’ll keep monitoring things.

Thank you again!!!

Louis2015 · July 1, 2023

On 4/9/2023 at 8:06 AM, Willard Norris said:

I live in Georgia without too many steep hills.

Any other recommendations?

Hi, I have a 2015 with the 4.3 and 6L80 with 428,000 miles on it. I changed my first transmission about 125,000 and the second around 175,000. My current transmission is doing fine (knock on wood) with just over 250,000 miles on it.

Here's what I did when I changed the second transmission.

I changed the transmission with a Autozone rebuilt unit, but this time I changed the radiator and the fluid tubes going to the front. (the rubber was breaking down and restricting flow) I also deleted the temp thermostat on the side of the transmission.

Oh, and I added a transmission cooler. **Mishimoto MMTC-K2-14 Transmission Cooler**

My 6L80 generally runs 50-60 degrees over ambient temperatures now. It hasn't been over 160-165 in a couple years, until I recently started having trouble with my cooling fans. When the fans aren't working in stop/go traffic in 100 degree temps it recently hit the 200 degree mark. I'm hoping to lice the fan problem soon. BTW, I'm in south louisiana.