I'm not a tranny expert, but two things it looks at are the throttle position sensor and the brake switch. The lockup torque converter unlocks when you tap the brake pedal or hit wide-open-throttle. Pitted contacts in the brake switch, or a switch that is misadjusted can make it look like you pressed the brake pedal when you hit a bump.
A bad spot in the throttle position sensor will make it default to a pre-defined defective state to set a fault code in the computer. This will cause the lockup converter to disengage too.
When the converter unlocks, (which it must do at a stop sign so it doesn't snub off the engine), you can hear and feel the engine pick up a little speed but it shouldn't jerk or be harsh. To see if this is what's happening, hold the gas pedal steady while traveling around 55 mph on a level road. Without moving the gas pedal, slowly press the brake pedal about an inch, then release it. You will hear the engine pick up speed, then the converter will re-engage in about one or two seconds. You can see this on the tachometer too if you have one. This should not cause it to drop out of overdrive.
If the brake pedal test feels different than what you've been feeling, it probably IS dropping into 3rd gear. The change in engine rpm is a lot more noticeable than an unlocking torque converter. One thing to look for is a sticking throttle cable that goes from the throttle body down to the transmission. They might have done away with this and gone to computer controls by 2002, but sticking cables were common in the 1990s. The symptom though was a little different. They usually had delayed upshifts that occurred at too high a speed. Once cruising at a steady speed, it shouldn't downshift.
One other possibility that could occur after replacing the transmission or cable is a misadjusted cable. What can happen is you hit, ... Oh, 45 mph for example. The transmission shifts into overdrive and engine rpm drops below 1400 rpm. At this low engine speed, the transmission downshifts to 3rd gear, as designed, but then rpm goes up again and it meets one of the conditions to go into overdrive. This usually makes it go back and forth in and out of overdrive. I suppose a sticking cable could cause this too, but again, you might not have that cable anymore.
I believe there is also a temperature sensor somewhere. I can't remember if an intermittent problem with the sensor causes it to unlock the converter or go to 3rd gear. Also, a misadjusted PRNDL sensor can cause unusual problems. It's a sensor now, not a simple switch like years ago.
Thursday, March 19th, 2009 AT 9:53 PM