2006 GMC Yukon Transmission Problems - How Hot Should It Run?

I have a 2006 GMC Yukon XL and its transmission started slipping at just under 60K miles. My local repair shop (who I trust) suggested a transmission shop that they work with a lot. Since I tow a trailer with the vehicle it seems reasonable enough to go with their suggestion to rebuild the transmission using upgraded “heavy duty” components and install a transmission cooler, which I had them do. If you are interested in seeing what they installed, I have uploaded the repair tag at http://www.steveandvicki.com/Yukon_Transmission_Repair.pdf. Right after we got the car back we took the trailer on a trip and very soon the “Transmission Hot” warning light came on. I happened to be driving very close to the transmission shop at that time so I stopped by and they replaced the transmission fluid with synthetic fluid and said that would remedy the situation. However, very shortly after leaving the shop the warning light came on again. At the time I asked the owner of the shop if the heavy duty transmission might just run hotter than the original and he told me that it should not. Well, I brought it back in and he disassembled and reassembled the transmission and even moved the placement of the transmission cooler but, long story short, the transmission hot light still comes on. Then he changed his tune told me that the heavy duty transmission should run hotter, contrary to what he told me originally. He went on to tell me that there was no way to adjust the temperature sensor that controls the warming light so he convinced me to install a supplemental temperature gauge for the transmission. He said that the sensor was set at the factory to trigger at 180 degrees and that the transmission was likely just running a few degrees over that. So, we made a deal that he would install the gauge for $250 and if the transmission temperature stayed below 190, which he said was a safe number, then I would pay for the gauge, but if the transmission ran over 190 degrees, he would cover the cost because that would indicate that there was probably something wrong with the transmission. He did the work but now he has once again changed his tune and is telling me that it is no problem for the transmission temperature to run as high as 250, which is 60 degrees higher than what he told me originally. So, the question is, 1) should a heavy duty transmission with a cooler run at a higher temperature than standard transmission and 2) how hot is too hot for it to run? Any guidance would be greatly appreciated.

I have a 97 Chevy 4X4 1500 Ex Cab. I tow a 25ft 5th wheel. My 4L60E was rebuilt with all the upgrades for towing and cooler. Mine runs 150 when not towing. Its gone as high as 220. Mostly when towing it stays around 180-200. I know my gage is right on. I had the guy that rebuilt it check it with his laser temp probe.

I have asked around and most say 225 up 260. I will be interested in what is said here too.

Anyone else care to comment?

260 degrees is WAY, WAY high, even for synthetic. Did the trans guy bypass the radiator cooler when he installed the external?? Does the trans run too hot only when towing or is it running hot while not towing??

To answer your questions specifically:
1: Heavy duty or not, its the fluid which breaks down from the heat, not the internals of the transmission. Excessive heat can also damage the rubber components (Piston seals etc). Heavy duty or not, all the rubber is the same. When I make a transmission heavy duty I replace/upgrade things like the friction elements (Bands and clutches), hard parts like shells, planets, and one ways (Sprags). I tweak valve bodies and separator plates, pumps and pump housings. None of these things are too affected by excessive heat. Frictions can and will be damaged by super heating caused by slipping. They can actually get so hot that they literally weld themselves together. They would have to get a lot hotter than 260 degrees. What you are doing by running at 260 is quickly shortening the life of your fluid. It is breaking down quickly.

2: How hot is too hot?? Let me put it this way, you want to keep your fluid temp UNDER 200 degrees. A GOOD heavy duty external cooler should accomplish that. If your truck was in my shop right now I would first be checking the external cooler itself. Is it adequate enough for your transmission and your towing specs?? I would have a scanner on it checking for any problems in the converter clutch. (Faulty converter clutches can quickly overheat a transmission). If alls good, I would probably recommend a larger external cooler, preferably one with its own cooling fan. They run around $200.

Answer my questions at the top, it would really help me help you.


HI transman. The OP has not been back. So I would like to ask you this. My external cooler is hooked up with the radiator cooler. When towing I have my temp will hold at 210 for many miles. Then go back down to 180-190. Should I add a bigger cooler or a fan? Also my trans guy put the temp probe in what he called the presser test port. He took out a plug on the left side of the trans. Is this ok? I wanted in the pan. Thanks

transman above has all the answers you need, answer his few questions and he’ll get you on the right track.
I agree with keeping trans fluid temp around 200 or so. I wouldn’t be surprised or worried to see 250 degrees as you crested the Grapevine towing a horse trailer, but the temp should come right back down again. If it’s at 250 while you’re driving in town and you didn’t have this trouble before, I’d expect the trans shop to remedy this.

Bodyman, Depending on how much weight you are pulling. In the OP’s case, I would leave the radiator cooler in line with the external only because the external probably doesnt have its own fan. This way his trans has the benefit of the large radiator and its cooling fan/s for cooling. An external cooler w/o its own cooling fan relies on airflow coming into the front of the vehicle mostly from the forward movement of the vehicle. So, towing heavy at slower speeds will generate more heat and cause the trans temps to be hotter. As far as yours goes, 210 degrees will wear your fluid out quicker meaning more frequent fluid and filter changes. If you can get it down another 10 degrees or so, you’ll be even better.


transman618, thank you for you detailed response. The transmission warning light only goes on when towing. You asked if the trans guy bypassed the radiator cooler when we installed the external, but I am not sure what you mean. I do know that the transmision cooler is installed in front of the radiator. Does this tell you if he “byassed” the radiator cooler?

Is the fluid level correct? Could the dipstick be incorrect?

We have definitely checked the fluid level a number of times. Not sure if the is the dipstick could be incorrect. What would make it read incorrectly?

Thank You Transman. If I am towing I keep a close eye on the fluid. I check every 2 fill ups. I think before I travel again I will mount a fan that I can control. When I see the temp going up, I will turn it on.

Over filling causes over heating which expands the fluid to be even more over filled and to compound the over heating. You might drain and refill the transmission to the low end of the NORMAL range and observe the temperature for any improvement and post the results. An incorrect dipstick/tube, an improperly installed tube or a damaged dipstick can indicate an incorrect fluid level.

Given the history of the truck, repeated teardowns, and some of the associated BS is it possible that the rebuilder may not be an automatic transmission whiz?
Maybe the overheating is caused by slipping due to something not being set up correctly.

I don’t know that considering Firestone as a first choice for a transmission rebuild would be the route that should have been taken.

In retrospect, and having forgotten something, are you saying that Firestone is the middleman in this deal?

In other words, did Firestone pull the transmission and send it out and on the followups have you actually seen this transmission being torn down again and by whom?
Is the person feeding you the changing lines the Firestone place or the transmission shop?

ok4450 - The guys at this particular Firestone location have actually been great, however they didn’t do the work. They subbed the work out to Vaughan’s Transmission in Petaluma, CA who they use for all their transmission work. Vaughan’s have been doing transmissions for over 30 years so you would hope that they would know what they are doing.

Person feeding me the changing lines is the owner of Vaughn’s. Actually it is in Santa Rosa, not Petaluma.

Has the transmission cooler been flushed thoroughly. When a transmission fails it throws a great deal of clutch fiber and metal down to the cooler where, if not flushed out, restricts the cooler and shortens the life of the replacement transmission.

Rod, I am not sure about this, but I will certainly ask.

Actually, thinking about it, the cooler was added after the transmission failed so this should not be an issue.

They still should check cooler flow in the radiator cooler unless they bypassed it when they installed the external. What exactly failed in the transmission which caused it to be overhauled??