Cruise control in 5th

[c5dura]

I run over this 6-8% grade hill called the Blue Ridge Mountains everyday when I go to work. I set the cruise to 64MPH at the bottom and unloaded the truck will slowly drop to about 60MPH or so and then drop into 4th and accelerate back up to 64MPH and hold that until I crest the top.

The funny thing is that if I "help" the cruise control by giving it some extra gas, I can maintain 64MPH all the way to the top without the Allison ever downshifting to 4th.

It is seems to me that the cruise control never applies 100% throttle in 5th. Is that likely?

PS, I'm not running in Tow/Haul during this, not sure if it makes a difference.

[ 10-14-2002: Message edited by: c5dura ]</p>

[TLA]

I believe that what you are seeing is typical. Cruise control does not anticipate a need for power, but reacts to that need as expressed by a reduction in speed. There was a 1 mph window for adding power, but that has been increased to 4 mph to reduce the shift "business". The way the engine is set up, the throttle/cruise commands a power increase, but the MAF sensor will not add fuel until is sees an increase in air flow, and the turbo will not spool up until is gets the fuel. The downshift get the air flow up, the MAF adds fuel, the boost goes up, and away you go. With the Juice, there is more HP to work with, and it does add fuel when asked (as evidenced by the puff of smoke), and the downshifting is almost eliminated. Also, rear wheel HP and torque curves are further up the RPM scale than the factory specs show (as they were obtained in a test cell under optimum conditions with no losses for drivetrain, exhaust, etc).
'Hope this makes sense.

[IndigoDually]

In the situation that you are describing the cruise is sensing a drop in speed and it reacts quickly to get you back to the set speed. When it reacts it applies a lot of throttle and the computer senses this and it downshifts to allow the truck to speed up rapidly.
Try this at the bottom of the hill hold your foot steady and allow the truck to lose around 4 mph, then apply a lot of throttle to attempt to regain the lost speed. When the desired speed is attained let out of the throttle just enough to maintain that speed +/- 2 mph. The truck will sense the load and not upshift unless you release the throttle a lot. I have done this when climbing a significant grade, as soon as the downshift occurs, it is rather difficult to get it to upshift again until the hill flattens out some.

Sorry for the long post and I hope it helps,

John

[c5dura]

You guys hit the nail on the head. As I start up the hill on cruise control and slowly lean into the gas when I'm at, say 62 mph, I can accelerate back to 64 mph without ever dropping into 4th.

You're also right on about anticipating the need for power, without cruise, I can maintain 64 mph, even at the very bottom of the hill because I can 'see' whats coming.

Now, if I really want, I can pretty much run up the entire hill at any speed I want, the only restriction being the speed limiter, pretty amazing for 7000+ lbs!

By comparison, my C5 corvette has like a 0MPH window on cruise control, on the HUD (heads up display) that shows my exact speed, it never even drops 1 MPH, when I first start up the hill. Equally impressive is the fact that when I crest the hill (well, I guess the Blue Ridge is a mountain), I never even pick up a single MPH, talk about super fast reaction cruise control.

Btw, the C5 has no speed limiter, and lets just say maintain 150 mph up the inclince doesn't push the LS1 at all (I'm not stock). My rationale is that if I pass a cop going down the hill, there is no way he'd be able to turn around and catch up with me. Besides, at the top of the moutain, I can hop onto I64 going either direction. Not that I have ever had to do that...

[Kennedy]

From what I've seen, the cruise will let you drop to 2 mph BELOW set speed if you exceed it at all. This can be particularly annoying in the small rolling hills as the truck will overrun the set speed on the downhill side, and then not apply until the load pulls it back down about 4 mph. I think this yo-yo effect is part of the programming to correct shift busyness.

[Joe.G]

Yes Kennedy, mine does this also after having the cpu reprogrammed and I hate it. It's back and forth with no consistency in speed, and sometimes it starts hunting for the speed when I'm in rolling hills.

[JEBar]

not knowing exactly what part of the Blue Ridge Mountain range you are running it would be impossible to say for sure but since adding Juice (which I don't see listed among your mods) to my 3500, I've never had it shift out of overdrive while running empty, under cruise on the interstate....that includes considerable running in western NC .... Jim

[c5dura]

JEBar, I'm in Afton, VA. I run up Route 250W which intersects I64 on top of the mountain. 250 is much steeper than I64 in places. I think there is some rule about Interstate not being allowed to have more than like 6% grade or something like that? I'd say that 250 has placed where it is closer to 10%.

Once I get my truck out of the shop from running it into a ditch due to a deer incidence, I'll definately be looking at a power adder. :eek: Problem is that I'm on the fence as to what power adder to get. There are some new ones coming out, but the Juice certrainly seems to be among the more stable/tried and true at the moment.

C5dura

[dnibs6131]

c5dura
Notice that you are in the same boat as I am. I, too, am looking to add power to my Duramax, but don't know where to turn to with all the advertising and hearsay about this mod and that, i.e. TS Performance Chip, the Juice, BD EZ Amp. My concern is what effect any one of them (or others) will have on my warrantee.
I haven't successful "surfing the web" to get Allison info as is mentioned in the article about the BD EZ Amp. :eek:

[62dog]

On a recent trip to SC from north of Toronto I had a chance to put the cruise control(cc) to the test. With two people in the cab and two sport bikes in the bed I was amazed at how well the cc worked. Going down hill is when most cars passed me, as most of my passing was done going up hill! Speeds ranged from 65-70mph on 6-8% grades, infact the truck did a better job maintaining speed than I did. There was no down shifting to 4th gear at all, and that is a 1400km plus trip.

I was impressed with the truck on this first long trip. smile.gif

[Kennedy]

I will say that with increased load like pulling out to the DP Pull Off that cruise works much better. Hearing the turbo spool to 22-24psi boost and "grunting" up a hill in 5th gear (only 19k combined) at 69mph is an awesome experience.

She couldn't hold OD across "the divide", but had I not switched back to manual pilot and let it scrub some mph, she'd have held her speed!

[dnibs6131]

Anyone know of a web site for Allison transmissions.