jj

My problem: Cruise control will not hold set speed when going down a long hill. ( If I set it at 70 by the bottom of hill I'm going 80) I just returned from dealer who said they contacted GM Tech Services and were told the engine "LOCK OUT" ? is not installed on 07 Tahoe engines so therefore no speed control going downhill! Is this true or are they trying to blow me off? ...
Thanks for any insights on this problem you might have. J.J.

73shark

True.

shawnvw

The cruise control cannot apply the brakes or downshift for you. It merely applies no throttle when going faster than it was set to

73shark

Does tow/haul downshift any?

jj

I Haven't towed anything on a long down hill grade. But the tow mode does work. You can see the RPM go up when you hit the button.

73shark

Was that when you were going downhill when you turned it on? If so, how fast?

jj

Cruse was set at 70mph on level road. it was a long down grade and the Tahoe picked up speed the further down we got. At 80mph I started using the brakes to slow down.

jj

I think I may have figured out why the Tahoe picks up speed on the downhill side. Old Tahoe uses all eight cylinders to helpslow the vehicle when no pressure on gas pedal. New 07 Tahoe only uses four cylinders when there is no load on the engine reducing engine drag (Good for gas mileage) but not helping slow the car on downgrade. The longer the downgrade the more time the Tahoe has to pick up speed. Thanks to all for helping clarify problem.

in2pro

The engin still has to move all 8 jugs (pistons) no matter what mode its in and the values are still going thru there motion. I suspect it could be a combination of transmission, gearing, wind directionand the load you had that increased speed. I just returned from a long trip using the cruse and many down hill grades and lost speed going downhill when I disengaged the cruse due to wind resistence.

jj

The difference is compression in only four cylinders instead of eight.