Issues with Cruise Control and gear shift in 98 Sunfire

[Grus]

My 98 Sunfire has issues with the Cruise Control.
Whenever I use the cruise control (which is every day), and the gearbox decides to shift gear, the cruise control turns off - it does this especially around 45-50 mph which unfortunately are the speedlimits when I'm driving to work.
If I lock it into third manually, the cruise control works, but unfortunately it increases the rpm - thereby decreasing the mpg - and more importantly, it takes the automatic out of automatic transmission

Do anyone know why the cruise control turns off at gearshift?

98 Sunfire Convertible, 2.4L, 4 speed automatic

 

[melsg5]

Why is the transmission shifting gears? Are you going up a hill which causes it to downshift and then the cruise control disengages?

 

[Grus]

It is just the natural flow for the transmission - as the rpm are quite low at that speed aparrently. So as your example, if the road becomes steeper and more power is needed it will need to use a lower gear. Most of the time though it would be caused by me accelerating to the desired speed and turning on the cruise control, where after the transmission decides to go a gear up to keep rpm down - unfortunately it causes the cruise control to disengage.

 

[ericv1984]

That's weird. Whenever I've had cruise control on any of my cars, if I go down a hill, the engine had always automatically slowed itself down to the speed I set cruise control at. And if I go up a hill, the engine compensates and speeds up to the set speed but doesn't shift any gears.

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[tomgp]

i would check the tranny oil and see if its burned,if the tcm = transmission control module senses a slip it will disengage the cruise control.