Allison and Cruise Control

[RetiredBob]

Need some advice from the experts, not opinions from the rest of you. When driving my rig (see sig. below) I use cruise control most of the time, set at 60-65. I know you shouldn't use cc if the tranny is continually searching for the right gear, but what about in hilly country. For example, on my recent trip down I-65 through KY, TN and AL, I sometimes experienced steep grades every 4-5 miles. With cc on, the tranny will kick down into 4th and then usually down to 3rd in order to maintain speed. Occasionally, if I see it's a long grade, I will disengage cc and let my speed bleed off, but even then, there is usually a downshift. My question is, What is easiest on the entire drive train: leave it in cc and just let the tranny and engine do their job, or disengage cc and baby it up the grade? Thanks in advance.

[CampDaven]

Not an expert, but know it should not lug. If I am surfing road swells, I may drop from 6 to 5, to 4 if needed. I just feel what is needed from the tachometer. It is kind of like manual shifts on 6% grades, just lesser degrees and less shifts.

However, it is an Allison and will do fine in any case. For me, it is not the tranny, it is not lugging the motor. :-)

[Mike Canter]

The software in the ECU will not let the rpm drop far enough to lug. Better MPG if the engine stays at a lower rpm and goes to high boost and no downshifting. You can stop some of the downshifting on small hills by going into the Economy mode on the Allison pad but on steeper grades I would not use the Economy mode. I have left my Sig in CC on 4% grades without a problem. The big thing to watch is coolant temps if they start approaching 200* then I would get out of CC and downshift to lower the temp.

[CampDaven]

So far we have an 8.1 gas, a MaxxForce 7 diesel, and a Detroit 60 14.0. I suggest the Detroit can lug very well, where the other two should not.

So far, no experts!

[RetiredBob]

Okay, a couple of comments.. My rig has the Allison 1000--no Economy mode. Also, no tranny temp gauge, so I can't monitor the temp (or were you referring to engine temp? Never any overheating problem). I do have an Overdrive Lockout switch which cuts out both 5th and 6th gears.

Still waiting for the "experts" to weigh in!

[jeepersrus]

Ok my 2cents. I found it better to leave the cc on while climbing hills. The engine and tranny know what they need to do and work well together. I don't try to out think the onboard computers. I've been across country round trip 4 times now and the coach performs better on hills while in cruise control.

[Rich-n-Linda]

Originally Posted by jeepersrus

...The engine and tranny know what they need to do and work well together. I don't try to out think the onboard computers....

I could not disagree more with this! You CANNOT just leave a motor home in DRIVE and let the computer do all your thinking and driving for you. These vehicles are NOT a car and cannot be driven like one.

The big flaw in your thinking is that the computer cannot ANTICIPATE anything, it can only react to something that has already happened.

Here's a common scenario...

You are driving on an interstate highway in the mountains. You have your cruise set for 65 mph. In a typical DP, the engine will be turning over somewhere around 1,700 RPM. You start into an upgrade and the coach slows down as gravity does its thing. By the time the computer decides is it time to shift down to 5th gear, it is already too late! The RPM will be down around 1,400 when the shift takes place. Shifting down to 5th brings the RPM back up to 1,700, but the HORSEPOWER PEAK is up around 2,000 RPM! And for those of you who think it is TORQUE that gets you up a hill at speed, you're wrong. It is HORSEPOWER that gets you up a hill at speed, and by letting the computer do all the work, you never got all the horsepower you have available to climb that hill. You will probably end up down around 40-45 mph in 4th gear on that hill.

Now let's look at the exact same scenario, with one critical difference. You ANTICIPATE climbing the grade by manually shifting down to 5th gear at the bottom of the grade. When you do that at 65mph, the RPM climbs to 2,000...right on the HORSEPOWER peak. Gee...do you think that's just an accident? You are now ahead of the game by having your engine at its HP peak as you begin the climb. You may be able to get up the grade in 5th while still at 65 mph! I can do that in our coach (Cummins ISL 400, Allison 3000MH at 35,000 pounds) on grades as steep as 6% by ANTICIPATING what is about to happen instead of just REACTING to something that has already happened.

I typically leave the cruise on when I do this, but I help it a little by actively DRIVING my coach by manually shifting down a gear, as opposed to just riding along and steering while a computer does my thinking for me.

[jeepersrus]

Well after being across country 4 times. Averaging about 9-10 mpg Sounds like the computer is doing just fine. One other thing, I'm not trying to climb the hill at 65mph. I don't even do 65 on flat roads. The engine temp at the most goes up to about 190 deg. It normally runs at 180 deg. The steepest grade I ever climbed was coming off Boulder Dam up to the city of Boulder, and I did that hill at 45mph. Mostly because you just don't have enough room to get your speed up before you hit the hill. If you want to be the first to the top of the hill then go for it, I'll be the guy in the right lane as you pass me on the hill. But I'll be passing you while you're at the fuel pumps.

[Kuzzy]

Well now, here we have exactly what the OP was not asking for. Opinions.

[jeepersrus]

Well now, here we have exactly what the OP was not asking for. Opinions.

If he truly wanted the views of an expert he wouldn't be asking here. He should be on the phone with Allison.

[AttilaKara72]

Below is a thread that might help. If not it is good info to have.

http://www.irv2.com/forums/f125/econ...on-151375.html

[ShapeShifter]

Originally Posted by RetiredBob

I know you shouldn't use cc if the tranny is continually searching for the right gear,

...

I sometimes experienced steep grades every 4-5 miles.

I'm no expert, just giving my opinion (oops! ) but I wouldn't consider downshifting every 4 or 5 miles to be "continually searching for the right gear." I see no problem in that, in fact I'm sure it's necessary to get the right balance of power and economy. (You wouldn't want to spend the 4 or 5 miles between hills in a lower gear, just as you wouldn't want to take the hill in a high gear.)

Now, if the tranny were shifting between gears every 30 seconds going up one hill, that's what I would consider hunting and should get some intervention to keep it in the lower gear.

[wa8yxm]

You describe how I drive.

For the person who said "It is better not to lug" Allison trannies are the best made, the TCC (Transmission control computer) will not let the engine get down to "Lugging" unless it is all the way into its lowest gear and you should not be on hills that steep. (Been there once).

Likewise when going down hill the tranny will not over-rev the engine. Even if you turn the Grade Brake on.

As transmissions go the Allision .. Well even FORD admits it's the best. They call their latest offerering "The answer to the Allison".. Well, if they say so, I think it falls short.

[Kuzzy]

Originally Posted by jeepersrus

If he truly wanted the views of an expert he wouldn't be asking here. He should be on the phone with Allison.

Sorry. No offence meant. Didn't want to see a good thread deteriorate into an argument.