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Shifting behavior on hill climb

July 28, 2013, 01:13:31 am

In visiting with Brett a day or so ago on tranny LED readouts, and attempted follow-up today, I have this question you may answer.

I downshifted the 6 speed tranny to 4th gear to enter an uphill drive.  I assume that sets the highest gear to be 4th gear.  Is that correct? 

I thought the manual downshifting put me right then into 4th gear (the rpm moved up to 1500 from about 1200) but I guess in hindsight that I could have been in a gear less than 4th when I manually shifted.  Could that be so?

Now, part way up the hill the rpms dropped back from the 1500 to about 1100.  That makes me think that the tranny upshifted even though I really wanted it to stay at 1500 rpm to pull the hill.  And I had thought that I was already in 4th, set as the maximum, it could not have gone to 5th or 6th and must have gone back to 4th, which was 1100 rpm.   

My goal was to not lug the engine as I went up the hill.  But the drop to 1100 rpm concerns me.  why did it not stay higher, at the 1500 range?  Needed more throttle?

So what was going on here?  Does that mean that the tranny was not in 4th but some lower gear at the 1500 rpm and it sensed it could handle the hill with a higher gear and went up to 4th and the 1100 rpm?  Or, do you think it upshifted, lowering the rpm, when it should not have and I have some problem with the control system? 

Is there some other explanation...operator error comes to mind!  My explanation of what was going on seems awkward, I hope you can interpret, give me some ideas.  I really thought keeping the 1500 going up this hill would have been better than 1100 rpm.

Mike

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Re: Shifting behavior on hill climb

Reply #1 – July 28, 2013, 01:32:56 am

Personally, I let the Allison figure out the best gear to be in on hills (usually 4 or 5), unless I want to show off to the Monaco I pass, then I turn off Economy mode and let it go to 4th and take off.  The only time I manually shift is when the transmission and incline makes it bounce between 4 and 5 or 5 and 6.  Then I manually choose the lower gear and let my foot off a tad just to stop the constant shifting.

And yes, your manual setting is setting the upper limit...but, again, the Allison is smarter than I am, so if I am going 55 mph, I understand setting it to "2" will not downshift to 2nd gear until the transmission declares it safe to do so.  Same with Reverse.  I think someone once translated what happens to the Allison saying "I see you want 2nd gear.  As soon as it is safe to do so (you are going slow enough), I will go to 2nd gear,"

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Re: Shifting behavior on hill climb

Reply #2 – July 28, 2013, 07:33:51 am

I will downshift to keep up the RPMs. Silver leaf tells me the gear I am in. I have run hills requiring2 or 3 rd all the time. Sucks to see a two come up though.

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Reply #3 – July 28, 2013, 08:45:04 am

Our Silverleaf display helps a lot with shifting.  I too shift down when going up and down hills.  For up I try to keep the speed and gear such that the engine stays just a few degrees of its normal temp, which in our case is about 183 degrees.  Normally if I keep the load, as displayed on the silverleaf, to less than 100%, the temp will stay low even on a long climb.  The old rule of 3rd gear for 30s, 4th gear for 40s, etc works well in matching speed the appropriate gear.

For downhill, I try to use a lower gear to keep the speed in the 40s ((or lower).  This would be for long downhill grades.....like miles of 6% or more.  At that speed and transmission gear (4th) the RPMs are high enough to cool the retarder and keep it below 250 or so.  Higher speeds will send the retarder toward 300 degrees very quickly.  On one pass earlier this summer on the North Cascade Highway, I used 2nd and a speed in the 20s to keep the retarder temp around 260 or less. 

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Re: Shifting behavior on hill climb

Reply #4 – July 28, 2013, 10:31:43 am

Mike,

First, I assume you have only the single window Allison shift pad.  If so, it shows the pre-select gear, NOT necessarily the gear you are in. Example:  you are at a stop light.  Window shows "6".  That means under correct conditions, the transmission will shift through the gears all the way to 6th. Clearly, it does not mean you are in 6th gear at the stop light.

If you have the two window Allison shift pad, the left window is the pre-select gear, the right window is the gear you are actually in.

As Dave and George pointed out, there are aftermarket monitoring devices that, amount a lot of other things, can give you the actual gear you are in.

Now, as to your question about the shifting.  If, when you down arrowed to 4th, you were going too slowly for the transmission to upshift to 4th, no it would not have been in 4th gear.  Conversely, if you were going too fast for the transmission to safely downshift to 4th gear, it would not have done so until coach speed slowed.


A reasonable "work around" to aftermarket monitoring devices is to build a chart that shows road speed vs RPM in each gear. I will e-mail you a copy of the one for our 1993 U240-- it would not paste correctly.

But here is the formula for any vehicle: (Engine RPM x 60) / (Rear Axle Ratio x Tire Revolutions per Mile x Transmission Ratio) = MPH
Just make an Excel spread sheet of it.  A quick glance at tach and speedometer will tell you what gear you are in.

Brett

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Re: Shifting behavior on hill climb

Reply #5 – July 28, 2013, 11:12:59 am

Having the great display on our Silverleaf has really helped us to understand the way to manage the transmission and use it sensibly in mountainous conditions.  I don't pretend to understand it all ... but I can sit in my passenger chair and read them out to Jeff when he is driving.  MUCH better than him trying to find the numbers on the transmission keypad of constantly check all the gauges (and much better for my nerves because I don't like heights). We are presently in Leadville, CO ... came up from the Denver side ... and the long ups and downs after the Eisenhower Tunnel were a piece of cake.  Nothing got too warm, our speeds were perfect and I was totally calm.  Silverleaf has got to be one our best upgrades!

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Reply #6 – July 28, 2013, 11:51:07 am

I climbed to the tunnel in 100 degree day and hit 220 on the gauge.  I got to cool down on the down side though.

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Reply #7 – July 28, 2013, 11:59:37 am

only for the World Trans. Not for the HD3060/4060 series.

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Reply #8 – July 28, 2013, 12:44:40 pm

Thanks to all.  Brett, I have the single window.  I wanted to ask you about this yesterday but so much going on we could not.  I need to study what you wrote, look at calculation method too.

For all, the hill is in the mountains and itself only a 300 FT or so climb but to approach that last pass I had been climbing about 2500 FT over ten miles.  Then you slow for a sharp curve for the final climb over the pass.  Everything great until then, where my friend has told me to downshift in advance, that the tranny waits too long to see it in a tougher area and will jerk the coach to try to adjust unless I downshift in advance.

I know that a lot to try to clarify what I was doing but in hindsight I realized I need the help of you folks, so posted.

I think carol and Jeff give me an idea.  Dave m has a great set up, he has shown me, but I seem lacking room for it on the dash.  I wrote John S for more on the silver leaf, wondering if that would fit in the hole in my dash....not sure why it there, surely not for sunglasses as I use it now!  Maybe some monitoring system I do not have

Thanks all,  mike

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Re: Shifting behavior on hill climb

Reply #9 – July 28, 2013, 02:39:32 pm

Get one of the add-ons that will show "gear attained" in addition to "gear selected". Older and newer coaches had both displayed. I am pretty sure SilverLeaf displays gear attained.

On uphill drives, much better to manually downshift to keep RPM where you want them, rather than letting the transmission hunt back and forth. We also keep an eye on the pyrometer to keep exhaust gas temps below 1,000 degrees or so. Higher temps forces us to downshift and drive part throttle.

Since we have the smaller C8.3 engine, we find 3rd gear speeds are about 30 mph, and 4th gear gets us going about 40 mph.

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Reply #10 – July 28, 2013, 07:20:56 pm

Echoing barry and cindy while my coach shows both gear and selected the most important gauges for me are tach and temp. Mr allison tends to lug too much so all my shifting is manual in the mountain pulls and descents. I find 1900 rpm to be the min for cool pulls up and this rpm serves well for unlimited retarder descents.  Its important to be real familiar with torque and max rpm ranges for your engine.

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Re: Shifting behavior on hill climb

Reply #11 – July 28, 2013, 08:14:25 pm

Same as Barry and Chuck for me, manual shifting to keep the RPM up on hard pulls and long down hills helps keep things cool and under control.
Anyone ever been to Brakes Interstate State Park on the Kentucky Virginia state lines, depending on how you come in there are some 2nd gear pulls, and down hills with the retarder nearly full on.

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Re: Shifting behavior on hill climb

Reply #12 – July 29, 2013, 10:57:59 am

Automatic or manual transmission, the best way to climb long grades in hot weather is to find a gear you can accelerate uphill in and then lift your foot and drop RPMs enough to keep the temp reasonable but still keeping the revs high enough to rapidly exchange heat in the radiator.

Won't comment on retarders as we have a Jake but on steep downhills with our U300, I keep high RPM to use the engine as much as possible for braking and let the Jake do the rest.

Wonder what the problem was in Italy? Brake fade with no maintenance on the slack adjusters or a blowout? Must have been going fairly fast to push cars out of the way and then break through the side of the bridge.

Have been on a steep decent in Greece where the bus driver had to hold the shift lever in second gear otherwise it would pop out. He laughed and said the brakes would never stop it and if it popped out, he would never be able to get it back in gear. The bus was so crowded that a pickpocket could not work.

Pierce

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