I have a 04 F350 6.0.
the cruise control in the truck is really starting to bug me. Go on the highway, set it at 75, the truck will go down to 72/73, then accelerate up to 77ish then back to 75, then to 72ish again and so forth. Like its "hunting" almost. I have a Toyota Corolla as a commuter car and when you set it on a certain speed it stays there dead nuts. Anyone know if there is a calibration procedure or something you can adjust to prevent this from happening?

 

I'm not familiar with the system your truck uses. Unlike the gas trucks from 2004 your truck's throttle is completely electronic which means they use a different type of cruise control. I'm going to move this down to the 6.0L forum, maybe the guys down there will have a better idea.

 

If the terrain is rolling, then this sounds about like how my truck behaves as well: Coasts down the hill, doesn't realize it's at the bottom of the hill and angling back up, then I quickly lose 5-7 MPH, and then it boosts up like crazy and surges over the top of the hill, shooting 5-7 MPH past the set point.

I just don't use the cruise control on certain stretches of interstate for this reason. For me, that's I-26 East between Greenville and Columbia. It's a roller coaster and cruise control is better left off. If I'm using my foot, I can keep it in overdrive the whole time, whereas with cruise controls it's downshifting, boosting up big-time, etc.

If it's relatively flat, then it's just fine.

I also think this is just how it's calibrated, and I don't know of any way to "tighten up" settings. I've had other vehicles behave this way too.

 

My truck acts exactly the same. The single biggest gripe I've had with my truck. I suspect everyone's doesn't do it. If they did there would be more talk about it. I've posted up a couple times with no suggestions or insight.

 

Cruise in my 6.0 sucks, my Focus is significantly better.

My thinking has always been that it takes the PCM some time to figure out the load and what to command the VGT to, so it lags behind. Then speed drops out of some defined range that causes it to downshift to accelerate. Then it doesn't back down soon enough and starts coasting, lather rinse repeat until you either take the cruise off or manually throttle it when the next hill starts.

 

Mine has always held it right on where I set it. But I'm a 6-speed stick, also.

 

Mine does the same, especially towing of course.
The big question that could help us all, any TUNES out there that can be groomed to help with this? Maybe one that will hold out longer before the thing downshifts and unleashes the fury?

 

mine also does the same thing. I just let her do what she wants. Lol
good question though. mine never really made me think there was anything wrong with it.
also would like to know about a fix possibly in a tune.

 

I'm dumb, it's not just speed itself that causes it to downshift and surge, it's APP (accelerator pedal position) that does it; the PCM considers both APP and OSS (output shaft speed), as part of the speed control program. I guess part of this problem is the PCM triggers the downshift when the cruise adds pedal to 'catch up" to itself. The tune writer would have to either change the shift mapping, how the PCM interprets APP, or something, but I'm not sure they could do it only when cruise is on.

Edit: apparently the PCM is involved with the cruise control light, it's not a separate function. Maybe someone could write a trans tune that changes the downshift input from APP only when cruise is on.

Here's what I'm reading:
"The PCM sends a serial message over the controlled area network (CAN) to the IC whenever the speed control telltale is turned on or off."

 

I'm one of the fortunate ones also. Unless I hit a pretty large hill mine always hold +\- 1 mph of where I set it and I'm an auto.

 

Same here ... I was really impressed by mine. I was on a 1200 mile trip last month and when I hit a hill it never lost speed, never dropped a gear and never hesitated. I would just see the boost gauge go up and my speed held steady.