99 Tahoe Trans Temp Dilemma And Problem

hondaman74 · April 2, 2007

I have a 99 Tahoe 5.7 4L60 4X4 4dr with 150,000+ on it, with the factory tow package. I just recently purchased a 5,200lb travel trailer and will be picking it up next week. To prepare for the newly added trailer I started with adding a Trans temp gauge, changed all the fluids, and so on. I placed the original Trans temp sending unit in line just before the radiator. I test drove and immediately started seeing temps of 250+, well that’s as far as the auto meter gauge would go, I am assuming hotter. This was with around town driving only. I did some research and found that this was extremely hot and damage could occur. But I wondered a truck with 150+ on it that shifts like it was new and has never had a problem, how could the temps be so hot. After doing some ore research I decided to add a second or third cooler if you include the radiator. This helped a little and does not react as fast anymore but still will climb to 250 with in 4-5 miles. I decided to move the sending unit to the 1gt gear pressure port in the side of ht trans and now it runs around 185-190 coolant temp. There is only one problem. It will still continue to climb and during a drive today 30 miles at 70mph it rose to 230-240 and would not change with it locked up and running great. I have flushed the fluid with over 4 gallons of new fluid while I installed the new cooler and still no change.

Here are my questions.

1. How is this possible? It runs great, shifts great, and has never had a problem but will consistently run 200 plus with no load, even at highway speeds it still continues to climb in overdrive. Never slips or will shift incorrectly.

2. Do you think the sending unit is picking up hear from the case and no so much the fluid. It only seems to go one way HOTTER. Unless it sits off for several hours.

3. What other that the TC would cause temps this high. I am afraid if I pick up my trailer on Thursday and cruise home in D3 I will do some serious damage.

4. Do you recommend switching to synthetic at such high mileage with this trans only using Dex3 it entire life.

5. What is your recommendation if nay to help alleviate the problem?

I am not a novice in the automotive world. I work for a major import manufacturer and have taught automotive for over 8 years, but this one has got me stumped, how doe a trans running this hot shift so well. Oh I have switched sending units and gauges with my coolant temp and it has the same reading so it’s not the gauge or sending unit.

Any ideas would be great I am really concerned about this one.

Thanks

Keith