Cruise Control Locks Out Downshifts

[CaptainGizmo]

This topic has been discussed since getting our 2019 Newmar DS 4369 on a Spartan chassis.

I took point on contacting and researching the lead engineer at Spartan, who met with Cummins and Allison.

The issue is that, when the cruise control is on and set, the rig will NOT downshift. This causes a severe overrun down grades; especially over 4 percent. The reason being that the Allison can't be allowed to downshift and bring the engine into the higher RPM range thus allowing the Engine Brake to kick into medium or high.

As you descend, and your speed starts to run away, if you disengage the Cruise, you'll immediately see that the Allison is let go to Preselect a low gear, the trans will downshift, the Engine brake kicks in High, and the speed is scrubbed off as expected.

I've spent the last year conversing back and forth, and even took a Cummins mechanic on an hour long test drive on 6 percent grades.

Well, I just got word from Spartan regarding this issue. There's no solution, but at least we know that it is a deliberate setting. Why, they won't say, and it doesn't make sense to me or the mechanics at Cummins. Nonetheless, guess we'll have to move on.





FROM SPARTAN, ON MARCH 18, 2020:

I do want to assure you that our customers safety is our top priority. Any potential safety concern is documented as such and reviewed by our team of warranty analysts and addressed accordingly. I do apologize for the time this has taken to review with both Cummins and Allison, but the transmission preselect condition you’re experiencing has been deemed a normal expectation.

We have investigated this concern with Allison and have found that Allison’s default value is “no preselect with cruise control”. Meaning that if the cruise control is on/set, the preselect feature is disabled from Allison’s TCM and will not downshift the vehicle to assist with slowing the coach. Since this is Allison’s default value, we cannot change the value without extensive testing and will not authorize a change to this parameter.

To eliminate the condition you’re experiencing, you will need to manually disable or cancel cruise control while going down a grade to allow the engine brake and preselect to slow the vehicle.

[Ray,IN]

That is the normal way CC operates on large vehicles, there is no grade retard like autos. There are 2 choices, accept it or spend the money to have it changed.
Why not use the exhaust brake for downhill grade retard since CC is un-needed?

[JohnBoyToo]

Hmmm, interesting...
and on edit, I somewhat understand how many people would scream and sue if the tranny downshifted and spun the engine to 10K rpms throwing rods and pistons everwhere!

but, while both our cars and even our jeep toad will downshift to drop speed, I really haven't noticed/ tested the coach for that... and I am of the it is what it is side of the ledger

I just tromp the brakes and let off quickly to reduce speed below a safe mph if that will do what I want...

So far, even on 7% grades @ 40000 lbs or so, that's worked for us....


NOW, if they'd do something about the 25 mph uphill like when I was surprised by the Teton Pass grade after a 20 mph corner

[96 Wideglide]

Yeah, shouldn't still have cruise control on when starting down a steep grade !
Guess us that travel in the mountains all the time already know this.

[CaptainGizmo]

You guys completely missed the point.

I’ve been driving mountains my whole driving life of 45 years thank you.

I’ve had every version of cruise control made, including the latest generation that actually applies the brakes.

So, to restate the problem...

When rolling up and down grades, engaging in and disengaging the cruise control is ridiculous.

Flat-landers don’t have this problem; just like we never had a problem East of Albuquerque.

I stand by the fact that the cruise control should be able to allow the rig to downshift WHEN in the right RPM range, to allow the engine brake to kick into high. This should be NO DIFFERENT from normal driving!! Even the Cummins head mechanic of 40 years agreed. AND THIS IS NOT the way “other big rigs” work, according to him.

And to the comment about liability of over-revving, oh come on...gimme a break! My testing scenario is real simple:

Go down at 6% grade at 60 in cruise and watch the speed increase.

Turn the cruise OFF and the trans downshifts and your speed is immediately put in check. No over-revving. It does exactly what it should AS SOON AS YOU TURN THE CC OFF!

I’ve never liked the “It is why it is” status quo. I fight to discover solutions and make things better.

[96 Wideglide]

Yeah, I don't think these beasts have advanced to say a Lexus or Cadillac!

Even my 2018 Kia, if I start down a hill with cruise on, it will shut down the gas, but won't start braking!

[NevadaNick]

Whats wrong with actually driving and operating the vehicle ?

[96 Wideglide]

Originally Posted by NevadaNick

Whats wrong with actually driving and operating the vehicle ?

Thats what I'm forced to do !

[CaptainGizmo]

Guess I have to simplify it even more...

Cruise on = Disables Engine Brake
Cruise off = Engine Brake comes on

[CaptainGizmo]

Originally Posted by 96 Wideglide

Yeah, I don't think these beasts have advanced to say a Lexus or Cadillac!

Even my 2018 Kia, if I start down a hill with cruise on, it will shut down the gas, but won't start braking!

That’s because your KIA doesn’t have an engine brake that comes on EVERY TIME you decelerate!

[CaptainGizmo]

Originally Posted by NevadaNick

Whats wrong with actually driving and operating the vehicle ?

Is that some sort of slam?

So, using all the features and technology isn’t driving and operating your vehicle?

Whatever. Guess you’ll have to define what driving and operating is.

[CaptainGizmo]

Originally Posted by 96 Wideglide

Yeah, shouldn't still have cruise control on when starting down a steep grade !
Guess us that travel in the mountains all the time already know this.

Says who?

Have you consulted with Newmar?
With Spartan?
With Cummins?
With Allison?

Yeah. Didn’t think so.

[jharrell]

Funny my gas rig will downshift with cruise control down a grade to maintain speed.

I had the opposite problem when I first got mine, it was first year with the 6 speed and it would not downshift using the brake with tow haul but would with CC. Actually used CC out west to get it downshift. Ford came out with a software fix after a few months of complaints for that one.

Surprised a Allison / Cummins would not have that ability.

[CaptainGizmo]

Originally Posted by jharrell

Funny my gas rig will downshift with cruise control down a grade to maintain speed.

I had the opposite problem when I first got mine, it was first year with the 6 speed and it would not downshift using the brake with tow haul but would with CC. Actually used CC out west to get it downshift. Ford came out with a software fix after a few months of complaints for that one.

Surprised a Allison / Cummins would not have that ability.

THANK YOU for gettin it!