Mechanics

SHIFTING PROBLEMS POSSIBLY

1999 Jeep Grand Cherokee • 4.0L 6 cylinder 4WD Automatic • 117,500 miles

Recently purchased this jeep, I noticed tranny seals are leaking but I keep close eye on tranny fluid levels and make sure they are good. But I have noticed especially when have cruise control on the rpm will jump around 200rpm but I can tell its not overdrive bc I have hit the overdrive button and it jumps way higher rpms, so was just wondering if have ever heard of a jeep doing something g like this? It seems to shift through gears smoothly and does not happen hardly at all if I'm just maintaining the same speed Nd not using the cruise control but it has happened when was not. Any ideas of sensors to check or any ideas would be much appreciated thank you.
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Daniel3simons
February 4, 2014.




The torque converter is unlocking intermittently. The clue is your observation of the 200 rpm change which is exactly how much engine speed will change. The torque converter unlocks under heavy acceleration to create a doubling of torque, and it will unlock in what it thinks is a preparation for coming to a stop. Releasing the accelerator pedal to less than about 10 percent will initiate an unlock. Tapping the brake pedal will do that too, although that will also cancel the cruise control. You can duplicate the symptom by holding the accelerator perfectly steady while driving in third or fourth gear over about 45 miles per hour, then briefly tapping the brake pedal. You'll see engine speed go up 200 rpm, then drop back down one or two seconds later when the torque converter relocks.

When this problem starts occurring on its own, use your toes to hold up the brake pedal. If the problem stops, check the adjustment of the brake light switch.

In rare instances it is possible for the throttle position sensor to cause this but usually it's because the signal voltage pops to 0.0 or 5.0 volts. Both of those will be detected as unacceptable conditions, set a diagnostic fault code, and turn on the Check Engine light. If that's not happening, most likely the throttle position sensor isn't the cause.

You can also connect a scanner to view live data to see what is triggering the unlock. The status of the brake light switch will be shown along with various sensor data. By recording a few seconds of information when the unlock occurs, you can play it back slowly, frame-by-frame and see the data in more detail to see what's changing.

Caradiodoc
Feb 4, 2014.


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