Tips on Buying Cars: Cruise Control on Manual Transmission, manual transmissions, cruise control

Question
QUESTION: Hello

The cruise control on my automatic tranmission car seems to change the gears automtically to accelerate when ascending or slowdown when descending hills.  Can cruise control in manual transmission cars do this?  Or can it only operate on flat roads?

Thanks for your time.

Craig Daly
Canberra, Ausralia

ANSWER: Dear Craig,

Thank you for your question.

Cruise control in manual transmissions will adjust the speed depending on grades, but will not change gears.

Cruise control only maintains the speed, the automatic transmission will change gears due to its programming depending on the conditions.  This downshifting is being controlled by the automatic transmission, not the cruise control.

The cruise control unit only controls throttle opening to ensure a constant speed.  Therefore, if you are in 5th gear with your manual transmission on the highway and you start going up a large hill, the vehicle will stay in 5th gear and only the throttle will adjust.  The manual transmission will be not change gears.  If the adjustment is too large (i.e. large hill), the acceleration

There is also a safety mechanism on your car where if you push on the clutch of your manual transmission, the cruise control will disengage to avoid overreving the engine.

In other words, the cruise control only replaces your right foot and will adjust just as you would if you were controlling the throttle.
Therefore, if you were driving on a highway in top gear and you approached a hill....what would you do?  Push more on the throttle right?  This is what the cruise control will do....

If you have any questions, please contact me.

Best regards,

Paul

---------- FOLLOW-UP ----------

QUESTION: So the cruise control and automatic transmission are operating independently in my automatic transmission car?  The reason I ask is that if I on downhill, without cruise control, without applying throttle I accelerate (part gravity part inertia no doubt).  If I have cruise control on, I can feel the gears changing down to regulate speed which remains relatively constant. If cruise control only regulates throttle, then shouldn't I still accelerate on descent?

Answer
Dear Craig,

Yes...correct, the cruise control and automatic transmission operate independently.

You have to remember that the cruise control will maintain speed, so if you are going up a hill, the system will increase throttle to maintain speed.  The same works when you are going downhill.  Without the cruise on, when you go downhill, gravity is causing you to accelerate.  With the cruise control on, it will maintain your set speed, therefore, it will close the throttle and downshift if necessary to maintain your speed.

If you have any questions, please contact me.

Best regards,

Paul