RPMs shoot up in cruise control?

jeep
grand-cherokee

#1

Ok I’ll try and make this brief,

About a month ago I’m driving at 60 down the highway with cruise on when all of a sudden the RPMs start from 2,000 to about 5,000. The Jeep doesn’t go any faster and as soon as I take it off cruise control (tap the brakes, hit the off button) or even holding down the Decel button for a few seconds it acts normal unless I hit the off button then immediately turn cruise back on it jumps right back up, but if it’s off for about 3 seconds it’s fine.

Now being an amateur mechanic my first thought was rebuild the transmission. So I did. And well that was a waste of $3000 because it’s still doing it.

It’s a Grand Cherokee Overland 5.7L V8 Hemi.
Thanks


#2

If so, then either the tach is misreading, or the transmission is downshifting. or something is wrong with the transmission.

Do you hear it downshifting? do you hear the RPMs increase? Does this occur on a hill only?

Does this occur with driving with the cruise control off?


#3

I can hear the RPMs it literally sounds like a jet engine just quieter of course.
It doesn’t feel like it shifts at all, it’s all smooth from start to finish.

It doesn’t happen if cruise is off at all, and when it does it’s flat as Kansas.

The transmission was rebuilt about 2 weeks ago by a professional not me.


#4

I’d take it back to the guy who did the rebuild. Could you demonstrate this for him? How repeatable is it?


#5

He hooked up his transmission reader before it was rebuilt and said the reasons weren’t bad at all, he rebuilt it and said when he opened it up it wasn’t in bad shape at all but it was definitely getting old. When he completed it he said it went great and nothing unexpected happened. He does a free service after a rebuild and told us to get it done about 2500 miles after the rebuild so I’ll ask when that time comes.

It’s pretty random so we’d have to drive around for a bit, but on my way to work it’s 45 minutes and generally it happens anywhere from 0-4 times one way.

And it seems to only do it at higher speeds it’s never done it going 30-40 I’m not sure if that’s relevant.

Now one of my coworkers told me to replace the throttle position sensor, do you think that’s a possibility?


#6

I hope some of the experts here will weigh in.

I’m not an expert, but there is no way the RPMs could rev up like that unless the transmission downshifted for no reason, assuming the transmission is in good shape.

In some cars the cruise control can command a downshift if the speed increases and zero throttle does not slow it down (eg, going downhill). Perhaps the problem is there, which would point to the car’s computer, assuming the computer handles cruise control. (as opposed to a separate module, which was used in older models)


#7

I so rarely use cruise control if mine malfunctioned I would just not use it. I frequently drive a 1/2 mile+ 25mph street with 3 school zones. If the school zones are active I don’t use it because it will not hold 20mph. I do use it at 25mph because the street is a popular hunting ground for local cops. Other than that my driving is in town and residential streets plus country roads with many curves and elevation changes where cruise would be useless/dangerous.