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Old 07-31-2013, 03:20 PM   #1
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Need Help Understanding Speed over Hills

2007 Allure 470 (tag axle)
400 HP Cummins
Allison 3000 MH
Six Speed Auto with 2 Overdrive Gears

We have had the coach one year and have always been concerned with what we consider to be a lag driving over hills. Driven mainly in California on US101 and I5, have about 6000 miles for the year

We have had routine maintenance done and have had the Supercharger Tube replaced due to vibration causing a large gap in the side of the tube.
We noticed after this was done we had more power however, with the speed limit being 55 MPH, we found we could do 75 if were not careful, even before that was changed.

This past week we traveled to Pismo Beach from the San Jose area going down US101 and as such you have a incline coming thru San Luis Obispo. We have learned to keep our eyes on the VMS Gauges (Coolant Temp, Oil Pressure and Trans Temp) we noticed that the Coolant Temp was reaching 210 and as such my wife backed off the gas (yes she drives all 5'4" of her) and before we reached the top its was back to 204.

Coming back from Pismo Beach, it was really lagging and we decided to manually downshift, however, I do believe we ever got over 35 MPH.

So my question to all you, is there a operating procedure for driving over inclines and what would be a reasonable speed (50 to 55).

The second question assumes I know what I am doing (I doubt that) If I where to take the coach to someone like Temecula RV, what would I ask them to do and what should I request be done?

Thank you all in advance, this is a great Forum.
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Old 07-31-2013, 04:25 PM   #2
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You need to keep the RPSs in the maximum power range. If you are trying to climb a hill in too high a gear, as you push the pedal harder you put more fuel in, but don't get all the power out. Your turbine won't be making maximum boost and your coolant isn't circulating fast enough to get rid of heat. Down shifting and allowing the engine to rev a bit higher will not only cool better but it will be better for the engine. Remember, diesels have a much different power band than a gas engine, look at some graphs showing HP and torque vs. RPM for your engine and you'll see where it's best to operate the engine for hill climbing power.
In addition, you might try using some Simple Green to loosen dust and grime on the radiator and give it a thorough rinsing. Dirty radiators on diesel pushers can be an issue, more on rear radiator than side mount, but still can have an effect if dirty. 210 is not that hot in a pressurized coolant system, it just shows the engine is working a bit harder to climb the hill. Once you get past 240 or so, then you should pay close attention.
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Old 07-31-2013, 06:36 PM   #3
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I have the 430 model and same engine as you. My trans has only one overdrive on it. When I reach around 232 degrees on the Silverleaf, the light on the dash comes on. this tells me to slow down a bit and keep the RPM's around 1300 to 1500. I will also downshift a gear as not to push it too hard. This usually will allow the engine to cool slightly and the light on the dash to go off. This also only happens when I have a long uphill grade and I'm towing our vehicle. It would not be unusual for the speed to get down to 35-40 miles an hour, if the grade is long with a good percent of uphill. I have gotten down to 20-25 miles an hour coming out of Las Vegas/Stateline heading towards Baker (Baker Grade). It did get hot there a few weeks ago when the temps were in the 100-110. Almost thought about unhitching the tow, to lessen the drag on the coach.
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Old 07-31-2013, 06:36 PM   #4
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A reasonable speed depends upon the weight of the rig and toad, coupled with the size of the engine, the rear end ratio, and the grade you are pulling.

Pulling a 6% grade on a 95 degree day in our rig is frequently a 220 degree event at 35mph if the grade is a couple of miles long. The engine will shut down at 225 degrees. Our thermostat is fully open at 205 degrees.

We have 300 HP and 800 ft lbs of torque pushing 36,000 lbs uphill.

I suspect you have 1100 ft lbs of torque pushing 45,000 lbs uphill. You should go up faster and cooler, but not much.....

It is possible there is nothing wrong with your engine.
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Old 07-31-2013, 11:17 PM   #5
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From what you say, I don't think anything is wrong with the coach. The cool down from 210 to 204 was due to the fan controller speeding up the fan.

If it is the hill I am thinking about, 35 is a pretty good speed especially if you got behind the power curve. In a diesel, once you have lost it, it is hard to get it back. Find out what your torque curve looks like and keep the RPMs at the upper range by downshifting. Don't worry about speed.

The statement about more throttle, more fuel less power out isn't true with the modern electronic controlled engine
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Old 07-31-2013, 11:44 PM   #6
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Quote:
Originally Posted by deandec View Post
I suspect you have 1100 ft lbs of torque pushing 45,000 lbs uphill. You should go up faster and cooler, but not much.....
The 400 ISL has 1200 ft lbs of torque at 1300 rpm and 400 HP at about 2150 rpm according to the Cummins chart.
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Old 08-01-2013, 10:51 AM   #7
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When driving up a long grade I don't watch the speedometer I watch the engine Temp and keep down shifting to keep it under 200 unless I'm near the top of the hill then I'll let it get up to about 207. But then again we're not in any hurry to get anywhere we just want to get there. Also besides simple green you can use a solution of Dawn liquid in a spray bottle when you rinse out the radiator.
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Old 08-01-2013, 11:09 AM   #8
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Now we rarely go up 6% grades
but we've always tried to keep the rpms just above the max torque rpms....
and adjust accordingly --

we will even anticipate a grade and accelerate a bit on the downhill or the flat before the grade - much easier to 'get ahead' of it.... 4 or 5 mph around the speed limit is not much of an issue given the gain in rpms, momentum, etc....
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Old 08-01-2013, 04:54 PM   #9
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Need Help Understanding Speed over Hills

Thanks to all who have responded
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Old 08-01-2013, 08:44 PM   #10
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I have found the "sweet spot" to be 1,700-1,800 RPM climbing a grade. I usually down shift to attain or maintain that RPM range. Once I figured this out my temp light has never come on since and the temp stays steady. It took me awhile to figure it out and I hated seeing that warning light come on.
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Old 08-01-2013, 11:17 PM   #11
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wfalch - I would take your coach to a good Cummins dealer. I have had quite a bit of work done at Temecula Valley RV and can tell you they don't really know diesel motors that well. I am telling you this from personal recent experience. I like the people there a lot. But recently I had to take my rig into a big Cat dealer in (I have a CAT motor) Bakersfield and they fixed a whole bunch of things. Things TVRV missed. What a difference. My C12 motor runs way better than it ever has in the 4 years we have had this coach.

Took me a while to learn the best way to go up hills. It is to manually downshift when you start to lug. I don't know what the max rpm your engine is rated for. Mine is 2,100. So when I start to have to push the throttle down quite a bit and I can feel it lugging I shift down a gear. Sometimes two. In the rockies I have been down to 3rd gear.

When I had recent work done at CAT they sent my radiator out to be cleaned. Wow, what a difference. So far on our current trip which has been from San Diego to Salem OR my temp never went above 202F. Used to go up to approx 208 with downshifting. Just leave in drive and press down and more like 217.

We have 105 more hp than you and 1550 total torque. We go up hills pretty easy. I think we only weight a little more than you do. We are about 42,000 lbs.

We just left the Country Coach factory today in Junction City OR. That is a very good place to get service, upgrades done. Super nice people. Very modern and well equipped facility. Fair charges. It is our second trip there to get things done. Temecuala Valley RV not even in the same league with the factory. Two years ago TVRV replaced my front slide big seal. $2,200. Yesterday the CC factory had to replace the seal TVRV put in wrong. And they did a big laundry list of other things. Total 2,050. And the seal is now done right. TVRV is good for some stuff but frankly in the last two years they have fouled up more than they have fixed. The CC factory also fixed another job TVRV did 5 months ago improperly. They wired up my Hurricane heater wrong. CC fixed that.
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Old 08-02-2013, 10:19 AM   #12
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Caterpillar has published a document called. “Understanding Coach (RV) Performance”that you should be able to find in the internet and it makes for good reading by drivers of all heavier vehicles, but particularly those with larger class A MHs
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Old 08-02-2013, 03:23 PM   #13
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thanks for the information, I don't seem to be having much success finding good service people. I wont bore you with the details, except to say I am a little queasy when it comes to CC. I have seen good things about Temecula RV on this Forum however, I have never used them before. Your point is well taken about using a diesel service center, rather than a do it all service center. I live in San Jose, CA and we are leaving for Reno (Rib Fest) the end of this month, than off to Boise and back thru OR. Any recommendations on a Cummins dealer?
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Old 08-02-2013, 05:23 PM   #14
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Cummins West in West Sacramento. Cummins Rocky Mountain in Sparks.

I have used both.

Both have done repairs for us that fixed the problem. Both prefer to replace parts with new parts rather than fix old parts at lessor cost.

Their goal in my experience is clearly to avoid comebacks and max revenue.
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