Are you scared of the mountains!!

tropical36 wrote:
I agree with the output temp. before the cooler. Now would you happen know how it's done on a GM4L80E, cuz they will go into limp mode, without warning, when too hot, even thought the engine temp. is still OK.

The 4L80E has the temp sensor just after the pump, so it is seeing the temperature in the pan. I think the practical reality is, that even though sensing after the TC may make sense, the internal temperature of the fluid feeding the trans tells you when there is a problem. One of the issues with monitoring fluid temp after the TC is that the later 4L80/85E trans TC is designed to slip all the time. It is controlled by a PWM valve that varies the clutch pressure. The clutch is a patented clutch which is porous, so the fluid actually pumps through the clutch material, so the clutch will keep cool, even though it is constantly slipping. Obviously this generates a lot of heat - hotter than the typical trans. The computer doesn't really need to know that temp, because it measures slip and monitors slip over time. If the fluid starts temp starts to heat to a run away condition, the PWM valving increases pressure to reduce slip. There is an assumption made about the cooling capacity. Since heat is developed over time, the inlet temperature still ends up being a valid indicator.

As far as the computer going into limp mode without warning. There are a lot of things that happen before that point, as the trans tries to save its self. All the clutches for each gear are PWM fed, so the pressure is increased, any time the slip is excessive. Before the trans goes into limp mode, the shifting will become a lot more crisp, as the computer tries to reduce slip. A good OBD interface will let you know what the computer is thinking, as you could monitor PWM comands.