2014 Jeep Cherokee Limited: Cruise Control Braking and Calibration Ruminations
September 18, 2014
There are other things about its powertrain calibration I find suboptimal, too.
First, the sluggish throttle response. Jeep/Chrysler wanted a smooth and refined action to throttle inputs. In this pursuit they damped the throttle so heavily that small inputs made by your foot result in no forward progress whatsoever. Give it a little more pedal and it responds, but it takes its sweet time doing so. This is annoying. Smooth, sure, but annoying.
Manual Mode: Mike also mentioned the sluggish downshifts in manual mode. He's right. You can tap the lever (which is set up in the preferred manner of push-to-downshift) a few times and count off several numbers before your desired cog is finally selected.
When the desired gear is indeed finally summoned, there's essentially zero engine braking for a couple of seconds, then a tepid amount thereafter. What's going on here is an emissions strategy that is not unique to Jeep.
Auto mode: The Jeep's nine-speed gearbox is smooth. And it's only somewhat stingy about downshifting when you roll into the throttle. It's not as bad as other automakers in this regard. However, with nine gears to choose from, the Jeep really has little excuse for not being in the correct gear at all times. This characteristic might be classified as a nitpick relative to the other areas identified in this entry.
Finally, let's talk about the Cherokee's cruise control braking. What's this, you ask? Well, imagine the cruise is set to your desired speed. You approach a downhill grade. In many cars, your set speed would creep up and up helplessly. This is dumb. It's dumb because cars have transmissions and brakes and these systems are, in modern cars, all talking to one another for a variety of reasons. They can — and should — cooperate.
The Jeep, however, is only partially dumb. As you start the downhill descent with the cruise set in the Jeep, the set speed will overshoot only slightly, and then it will recover to your set speed. It will do this even with the adaptive function turned off. Perfect, right? Well, here's the thing: The Jeep is holding your set speed down a grade by applying the brakes.
I probably don't need to point out why this is not ideal. But I will anyway. Dragging the brakes may be great for maintaining your set speed, but it's not so great for pad life. And who knows how hot the brakes are getting in the process. Now imagine a fully loaded Jeep going down any of the long freeway grades in the U.S. with the cruise set. Yipes!
The better way to hold your set speed would be to downshift (hello, automatic transmission), because downshifting delivers engine braking (that is, in most cars...). Only bring the brakes in as a last resort to supplement (not supplant) engine braking. But in many cases, a good dose of engine braking alone is all it takes to maintain your set speed, and this puts zero additional burden on the friction brakes at each wheel.
In fact, engine braking places no additional burden on anything at all. Except possibly Jeep powertrain calibration engineers.
Jason Kavanagh, Engineering Editor@ 4,164 miles.