2015 Kia K900: Cruise Control Brake Overuse
January 22, 2015
On the face of it, the cruise control system in our 2015 Kia K900 sedan does an admirable job of maintaining speed. (Note: For now I'm just talking about the regular cruise control or the adaptive system with no traffic detected.)
How good is it? The big Kia's indicated velocity never once deviated from my chosen set-speed by more than a single mph. And this remained true despite a route that included numerous steep climbs and long downgrades at highway speed.
But one critical aspect of the system's performance disturbed me to the point where I felt the need to manually intervene going downhill.
At freeway speeds the system seemed to rely exclusively on the brakes to keep speed in check. At no time did the eight-speed transmission downshift to help out. This approach is the exact opposite of everything I've been taught. It runs counter to every cautionary "Use Low Gear" road sign I've seen on downgrades scattered throughout the western United States.
I first noticed this about a mile down a 6-percent grade with the cruise set at 65 mph. At first I marveled at the K900's speed-holding ability, but then a faint steering wheel shimmy entered the scene, the kind that comes into being when brakes start to get hot.
So I cancelled the cruise control and took over. Almost at once the steering vibration stopped and the Kia's speed began a swift climb. I let it run up to 80 mph before I intervened, which didn't take long because the highly overdriven 8th gear (0.571-to-1) felt barely different from coasting in neutral.
This was a 6-mile descent, so I had time to slow the Kia down to 65 mph, reset and try again. Once again, the Kia's speed held steady, but after 10 seconds or so the persistent steering wheel throb reappeared, slightly more prominent this time. I paid attention to the tachometer and noticed that it hovered near 1,600 rpm, the same speed the engine had been turning on level ground. This meant there had been no automatic downshift. The car was still in top gear.
I shut off the cruise control again. But this time I slid the shifter into manual mode and downshifted into seventh gear (0.652-to-1). The K900 continued to pick up speed when I eased off the brake, so I tried sixth (0.831-to-1). The engine rpm came up again, but the Kia's speed still climbed, albeit to a lesser degree. Another tug brought the transmission into fifth (1-to-1), and this time its descent speed settled close to 65 mph. The engine hummed merrily along with little fuss at 2,800 rpm, and from then on I was able to trim the car's speed with an occasional light brush of brakes.
I was never comfortable letting the K900 run all the way down a long grade on cruise by itself after that. After all, that first time I'd started to sense heat-related brake vibration in the first mile with 5 miles of descent yet to go. And this was a chilly December day with an outside air temperature no greater than 55 degrees. I shudder to think what might have happened had I not realized what was going on and blindly let the cruise control lean on the brakes all the way to the bottom. At best I should think I would have earned myself a pair of permanently warped brake rotors.
Pity. Most cruise control systems don't control descent speed nearly this well. They'll allow the car to gradually pick up speed until the cruise self-cancels to let the driver know it's time to intervene according to how he or she was taught. In such cases I always drop a gear or two and fine-tune my speed with the brakes. I never ride them all the way down in top gear. Never.
But if a cruise control system regulates descent speed as well as the K900, it's hard to fault a driver for assuming that everything is under control. But here that seems to be a dubious assumption. Eighth gear is a highly overdriven 0.571-to-1 cruiser gear meant for fuel economy. There's essentially zero engine braking going on, and seventh gear isn't much better. I had to downshift all the way to fifth gear to achieve equilibrium on the 6-percent freeway grades I encountered.
I understand the need for this much overdrive in the name of highway fuel economy. But in such cases the transmission absolutely must participate in downhill speed regulation when cruise control is engaged. The brakes should never be left to this task by themselves. Top gear in an eight-speed such as this doesn't generate near as much natural engine braking as the three-speed automatic in the Ford Ranchero I drove in high school.
Dan Edmunds, Director of Vehicle Testing @ 8,142 miles