I am looking for information about the normal operating temperatures of my 98 TJ 3-speed automatic transmission. I think it is a TF999. It's mated to the 4.0 engine and an Atlas II T-case. I've installed a tube-finned cooler and a deep fluid pan. The sending unit is mounted to the outflow line from the pan to the cooler. When I am crawling, it runs between 180 and 200 degrees. When I am cruising dirt roads or long slow inclines, it climbs to 220. I am worried that this may be outside of the normal operating range and I could fry it. Any thoughts would be welcome.

So there I was...............................220 should not be a problem . I have been told not to exceed 240 from my tranny guy. I run a 350 in front of a 4L60. I also run a Derale Cooler with an electric fan on top of the stock Radiator cooler.

The sending unit is mounted to the outflow line from the pan to the cooler.


this location will give you a 'skewed' high temperature. Try mounting the sending unit in the pan for the best reading. Makes sense as the pan is where the tranny pump draws fluid from..

Thanks for the advice. It sounds like it is operating within tolerances. I think I'll try wrapping the sender and wire in some insulation as an easy first step. If the reading remains skewed, then I'll relocate it to the pan. Thanks again.

Placing the sender in the outlet line (farthest forward) will give you the highest readings, as this is the fluid coming right out of the converter.

You can place the sender in either line, the pan, or even some of the service ports in the case, as long as you understand what the gauge is telling you.