This only applies to our I shift EX, I can only surmise all I shifts with cruise control, will be the same. CVT might have the same software but of course it would not work on a manual.
I raised the point of speed control by cruise control either by braking or engine/gearbox. The general opinion was that although cruise control maintained speed on the level or uphill, it did not control either brakes or gearbox when going downhill.
I have to travel the A30 fortnightly and for the past two trips I have been able to use cruise control a fair bit of the journey from Launceston to Truro. On both occasions the cruise control has stopped the speed increasing downhill. Travelling westwards, on Bodmin Moor there is quite a steep hill off the moor into the Fowey valley and up the other side where the Jamaica Inn is located. Cruise control set at 70mph, over the brow and down the steep climb, I noticed that the car held 70 mph, no running away. On the trip yesterday I made a conscious effort to pay attention to what was happening. There was no lurching, nor a sudden rise in engine revs just a definite holding back. Also, I looked at the gear indicator and saw that the car had of it's own volition changed down into 4th gear and as the hill bottomed out and we started to climb, engine revs increased and gears changed up to 6th. Moving further along into the road works, which are subject to 40 mph there are two moderate downhills. In both cases the car, just ran way no control from the cruise control, so I had to apply the brakes. I believe I am correct in the fact that when the throttle is completely closed no fuel is injected into the cylinders, however despite this the car will increase speed unless brakes are applied on a downward slope.
Does the Jazz have some clever software in the cruise control to prevent 70 mph being exceeded downhill. There is no doubt that the cruise control definitely controlled downhill speed when set at 70mph.