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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.  

why not invest in a non contact infrared thermometer...

measure the temp of the cooling line coming out and going back into the transmission...

there have been instances of the check ball that lives in one of the cooler lines becoming dislodged and plugging off the transmission oil cooler flow..  

the infra red temp guns are at most parts stores for 30 to 35 bucks..



i use a 60 buck version from harbor freight to get into tight spots



Here's the list of GM transmissions and their in/out line sizes and flow direction.

GM Transmission Cooler Fittings

Powerglide - OEM
Fitting Location Thread Size Flow Direction
Upper ?" Pipe Return/in
Lower ?" Pipe Cooler feed/out


Powerglide - Dedenbear
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Return/in
Lower ¼" Pipe Cooler feed/out


TH200C Metric, 2004R
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Cooler feed/out
Lower ¼" Pipe Return/in


TH350, TH250
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Return/in
Lower ¼" Pipe Cooler feed/out


TH400
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Return/in
Lower ¼" Pipe Cooler feed/out


700R4/4L60/4L60E/4L65E
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Return/in
Lower ¼" Pipe Cooler feed/out


4L80E (1991-1996)
Fitting Location Thread Size Flow Direction
Upper ¼" Pipe Return/in
Lower ¼" Pipe Cooler feed/out


4L80E/4L85E (1997 & later)
Fitting Location Thread Size Flow Direction
Rear ¼" Pipe Return/in
Front ¼" Pipe Cooler feed/out



take the return line off and extend it into a 3 liter bottle or a gallon jug to check the cooler flow with a chunk of hose to verify cooler flow is making the complete circle..

it will burn out  in under 15 miles without cooler flow..  

with the second gear burned up.. its more than likely cooler flow problems..

many techs check at the radiator..    not at the end of the return line at the transmission..

if it got that hot.. it may have already damaged the second gear planets again..


there is an article on this exact problem.. but i won't be able to search it out till tonight..  

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