Weird Cruise Control Issue

[HookMeUp]

I have the truck in my sig, left for Colorado on Saturday and ran cruise all the way across Nebraska no issues. We get into the Denver area and it starts acting up, on the slightest incline it would down shift to 5th for about 10 seconds and then shift back into 6th. If I shut the cruise off problem went away, on slight inclines press down on the accelerator, no down shift, turn cruise back on, it would down shift every time! Truck has about 2,300 miles on it. Is the system still learning or is there an issue?

 

[jhenderson]

Thinner air means less power. Even with a turbo there’s a loss, just less than a naturally aspirated engine. The truck senses the load vs hp and downshifts. The back and forth likely means you were on the edge for the parameters the ecm uses under cruise control. If you engaged tow/ haul it most likely wouldn’t have shifted out of 5th to begin with.

 

[SnoKing]

The Aisin is not a learning transmission like the 68RFE. There are reported issues of lazy boost at higher elevation. If I tow at 60 at or around 6000 feet or above my boost sticks at 10 lbs and speed falls off to 56-57 before it down shifts and takes off again. Speeding up to 62 resolves this issue. In TH the Aision shifts to 4th at 45, 5th at 55 and 6th at 65. Driving close to these speeds seems to lead to hunting. SnoKing

 

[HookMeUp]

Doesn't do the downshift if I'm not using the cruise control.

 

[SnoKing]

Doesn't do the downshift if I'm not using the cruise control.

Put the boost meter and watch it and see if it sticks at around 10 lbs.

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[HookMeUp]

I'll have to watch that next time I'm out and see if it does stick.

 

[jhenderson]

My truck downshifts on a hill near my home when it’s in cruise at the speed limit. Above by 10mph it doesn’t. It stays in 5th in tow/ haul under the same conditions. If I’m not using cruise it doesn’t shift at the speed limit. I’m sure it’s programming.

 

[Learjet]

I saw the same thing you reported last year in the mountains. I just locked out 6th and the truck was happy.

And even at sea level it will downshift sooner when using the cruise control...not sure why.

 

[6V92TA]

all I have to offer is with your gearing and shift parameters that are built in, and the fact that cruise control does not anticipate hills,its probably normal for your truck.. as you said, if you use the accelerator a little bit it doesn't do it... whereas that cruise control isn't going to do anything until it senses a loss of speed and starts feeding in more fuel and it still isn't holding a speed,then it downshifts, gives itself some more fuel and then shifts itself right back where it was until the process repeats itself.

as someone else stated, you can always lock it out..

lets face it, 342 gearing and 6th gear , what RPM does that thing run at highway speeds. I know it is below peak torque, simply because my truck has 410's and at 70mph its running 1900 to 2000 rpm which is the heart of the torque band.

 

[HookMeUp]

At 75mph = 1700 rpms.

 

[SnoKing]

all I have to offer is with your gearing and shift parameters that are built in, and the fact that cruise control does not anticipate hills,its probably normal for your truck.. as you said, if you use the accelerator a little bit it doesn't do it... whereas that cruise control isn't going to do anything until it senses a loss of speed and starts feeding in more fuel and it still isn't holding a speed,then it downshifts, gives itself some more fuel and then shifts itself right back where it was until the process repeats itself.

as someone else stated, you can always lock it out..

lets face it, 342 gearing and 6th gear , what RPM does that thing run at highway speeds. I know it is below peak torque, simply because my truck has 410's and at 70mph its running 1900 to 2000 rpm which is the heart of the torque band.

Max torque is at 1600 RPM

Here are specs for an early year like a 2014. Later ones had small increases. My 2015 is 865, now they are 930.

Engine: Cummins 6.7L inline-six
Turbo: Single with variable nozzle
Head Material: Aluminum-alloy
Block Material: Cast iron
Bore: 4.21 inches
Stroke: 4.88 inches
High Output Horsepower: 385 hp at 2,800 rpm
High Output Torque: 850 lb-ft at 1,600 rpm
Base Auto Horsepower: 370 hp at 2,800 rpm
Base Auto Torque: 800 lb-ft at 1,600 rpm
Manual Horsepower: 350 hp at 2,800 rpm
Manual Torque: 660 lb-ft at 1,500 rpm

 

[HookMeUp]

Did some experimenting today, with the cruise on it will only build 10psi of boost in 6th gear. Without cruise on same hill required 13psi of boost and no downshift. Why would they program it like this when the Cummins with no load will take the hill easily in top gear?

 

[SnoKing]

Did some experimenting today, with the cruise on it will only build 10psi of boost in 6th gear. Without cruise on same hill required 13psi of boost and no downshift. Why would they program it like this when the Cummins with no load will take the hill easily in top gear?

Try the same hill at a faster speed on cruise. If I speed up from 60 to 62 towing the problem goes away. SnoKing

 

[6V92TA]

Did some experimenting today, with the cruise on it will only build 10psi of boost in 6th gear. Without cruise on same hill required 13psi of boost and no downshift. Why would they program it like this when the Cummins with no load will take the hill easily in top gear?

because it isn't programmed to anticipate climbing hills... your eyes will start pushing the gas long before the cruise control adds more fuel, when climbing a hill.. it just isn't that smart..

using sno kings power output and a gear ration tire size engine speed calculator, with 342 gears and a 33inch tire you be running at 1400 rpm at 60mph. below the torque peak by a couple hundred rpm. https://www.reillymotorsports.com/speed-tire-rpm-calculator.html

 

[SnoKing]

because it isn't programmed to anticipate climbing hills... your eyes will start pushing the gas long before the cruise control adds more fuel, when climbing a hill.. it just isn't that smart..

using sno kings power output and a gear ration tire size engine speed calculator, with 342 gears and a 33inch tire you be running at 1400 rpm at 60mph. below the torque peak by a couple hundred rpm. https://www.reillymotorsports.com/speed-tire-rpm-calculator.html

1400? Towing? Not at 60MPH, in TH it will not shift to 6th until 65 MPH.

 

[kthaxton]

It sounds like a similar issue a few of us have discussed in the past. It is not "normal" or part of the programming as mine does it too, on the same grade I do every day on my commute. Some days it does it, some days it doesn't. It happens somewhat frequently in the warmer months, and is almost non-existent in the winter....but again, it is intermittent.
It is either some kind of programming glitch, noisy signal from a potentiometer (throttle?) or a sticky spot in the lever that slides the turbo vane. These are my best guesses.

As others mentioned, mine will stick around 8-10 PSI, then downshift, then boost goes back to normal then shifts up again. When it works normally, it will stay in 6th and boost will be in the low-mid teens on this same grade.

However, I am not convinced it is specific to the Cruise, I think it is just more obvious with cruise on....there have been times where I was not in cruise, and seemed to have to give it significant throttle before boost goes up, or a downshift.

Dealer has looked at it three times. No fix yet.....although annoying, it's nothing I felt throwing a fit about.

 

[HookMeUp]

I have to take mine in for the U11 recall soon, and thought I would mention the issue and have them take a look at it.

 

[kthaxton]

Report back if they find anything.

 

[HookMeUp]

Sorry for the late reply, but this issue has been resolved. When I took it into the dealer for the U11 recall, I asked them to look at the cruise control issue. I guess there was a TSB on the issue and flashed the ECU. I didn't get a chance to test until a couple weeks ago, it no longer shifts back and forth.

 

[Learjet]

do you know the TSB #