Originally Posted by Chaucy
I installed a Superchips Flashpaq 5 tuner on my 2001 F350 7.3 L dually diesel pickup. It has different modes to choose from. When I am not pulling my trailer I selected Performance setting. If I try to use my cruise control at a speed above 65 mph, if makes the engine lunge back and forth within a very small range. If I set it on 75 then it constantly kicks in and out at 74 and 76 mph? anyone else have this problem. Also, if I am towing my trailer do I have to put it into the heavy tow mode to protect my transmission?
This is right in my wheelhouse, as I am a controls and automation engineer. The cruise control is whats called a PID loop controller. The P stands for "proportional". If you are X% below your set speed, it gives you Y% more throttle, (or reduces throttle if you are above your set speed) The ratio between X and Y is proportional. More X equally greater Y.
The tuning factor if the P function is called gain, and is pretty critical to how it operates. Too much gain (too much throttle added for a small drop in speed) will cause overshooting or "hunting". Imagine trying to hold a 60MPH manually only by using 100% throttle, or 0%. Can't be done smoothly.
Too little gain, and you will never give enough throttle to reach your desired speed. Again, try to hold 60mph, but you can only use up to 25% of your throttle. Engineers try very hard to find exactly the right gain to make the car hold speed, ramp up or down quickly if needed, but not overshoot and hunt. This is called "Critical damping", and itself is an artform just to the left of witchcraft. Some guys can do it mathematically. But I'm old school, and just use intuition and 20 years of experience tuning loops. I can get a loop critically damped in a few minutes.
When you add a chip or otherwise substantially increase power, you have altered the tuning of the loop by effectively increasing gain. For example, if you are cruising at 60 again, and reach a hill and slow to 58, the tuning in your cruise control may call for a 3% increase in throttle resulting in just enough power to gain back those 2 MPH quickly, but not to overshoot. But with the chip, you may actually be giving it the equivalent of 5% throttle (by making more power), you gain on 60 faster and overshoot it by 2 mph. Ok, the cruise control pulls that 5% back out (because you are over your set speed) and your speed falls off quickly due to that hill. So now it ads back that 5% and the cycle repeats. Even on level roads, that hunting can occur it the tuning is off far enough. The cruise control loop could be tuned to accommodate that increase in power, but the truth is that chip manufactures never bother with it.
To make matters worse, depending on your gearing and tire sizes, at a given speed, you can be making very little boost. Most stock turbos need about 1100 RPM to start making appreciable boost, and don't fully get spooled up till about 1700-1800.
If you are loafing along with the cruise set at 60 again, and your RPM is down near 1100, you'll have little boost and surprisingly little power. If you start slowing due to that hill again, the cruise will keep feeding in throttle until you get back to set speed. The slower you go, the more throttle it feeds in. At some point, the transmission controller will call for a downshift, but the cruise control loop neither knows or cares. It just keeps opening the throttle till it gets back to 60. With high fuel levels being fed into the engine (from the cruise control) and the jump in RPM from the downshift, you now will make a whole lot of power as the boost comes up, and you start gaining speed quickly. Again, you will overshoot the speed, and the cruise control will pull back on the throttle.
When the throttle pulled back, the transmission shifts back into 4th, your rpms (and thus, boost) drop, your power falls off and you start slowing down, and the whole cycle repeats.
It's just one of those things that comes with chipping a truck. Sometimes it can really affect the cruise. On my Excursion, cruise is useless below 50mph as it will overshoot by a good 10 miles an hour. Above 60, on a flat level road, It can actually hold speed, but put ANY speed change in (a hill or a gust of wind) and it will start slowly start hunting, with the speed changes getting bigger and bigger, till it is effectively swinging between 0% throttle, and 100%. Not a ride you want to give a pregnant woman, I'm here to tell you.