2000 5.4L Triton. After reaching 4th gear (auto) the RPM's will stick at 1200. If I take my foot off the accelerator, it's like I'm in cruise control. Pressing the break does nothing. In fact, the engine will fight the break until almost at a complete stop. If I shift to N, the RPM's race up to 2k and then drop back down to normal (obviously not great for the trans and an incredible annoying way to drive). Oddly enough, this seems to happen once the weather turns colder (October in Chicago area), but then it doesn't matter if it gets warm again...just keeps happening.

This has happened on and off since I have owned the truck (just under 4 years). I have made the following repairs over the last 4 years:

Replaced both exhaust manifolds
Replaced the intake manifold
Replaced the idle speed control
Replaced 'most' of the air-related hoses

Any thoughts?

 

Is the torque converter unlocking?

 

I assume I will need to have a diagnostic tool set up while the truck is running in order to check that.

Let's just assume that the converter is not unlocking. Does that indicate the sensor is most likely shot and if so, is the sensor located on the outside of trans (or where ever) so that it is relatively easy to get to and replace?

Thanks.