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Old 11-06-2017, 09:09 PM   #1
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On-the-road Diesel Pusher Validation

When we decided to go big and get a Class A in April of this year, we picked a Diesel pusher for a wide range of reasons. We ended up with a big (40', 32,000 lb GVWR) coach and a small engine (330hp MBE 926).

After 10,000 miles and 61 nights in the coach, our family is very happy with our selection. Over the past 2 weeks, we've left the Midwest, spent a few days in the Grand Canyon, and are now outside Phoenix at Lost Dutchman State Park. As the driver, the DP choice was forever cemented after driving I-70 West out of Denver and I-40 South from Flagstaff to Phoenix.

The two-stage engine/exhaust brake made even the steepest mountain downhills a non-event and the torquey little Benz kept us moving upwards without complaint. We didn't win any races, but big mountain driving was a non-event. I've done it in gasser moving trucks a few times and know it can be done safely, but there's just no comparison in comfort or capability.

There's no one-size-fits-all, right choice in the gas vs Diesel debate, but the DP is definitely the winner for us.
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Old 11-06-2017, 09:17 PM   #2
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Old 11-06-2017, 09:22 PM   #3
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I agree. There is one thing about a diesel engine and exhaust brake that everyone should realize. The mfgrs of the diesel engine and Allison auto transmission designed their electronic control modules to "talk" to each other and prevent damage to either component.
The TCM-transmissiion control module will command an upshift when descending a downgrade to prevent the engine from exceeding its maximum RPM when using the exhaust brake. This means the upshift negates most of the exhaust braking effect a great deal, leaving only the service brakes to slow down until engine RPM reduces enough for the Allison TCM to command a downshift again.
When the driver keeps that in mind they can keep RPM's low enough to prevent the upshift by judicious use of service brakes.

The ECM does not do anything so dramatic, it mainly derates engine HP enough to prevent transmission damage on take-off from a dead stop; otherwise a heavy foot could destroy the transmission.
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Old 11-06-2017, 10:04 PM   #4
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I agree. Even though maintenance is higher on a diesel, I wouldn't trade it for a gas coach. The air ride, air brakes, engine brake and quietness makes ours a winner too. Glad to see you are happy with yours.
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Old 11-06-2017, 10:11 PM   #5
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VoiceNinja,
Just for a bit of clarification, you may have been on I-40 in and around Flagstaff going from east to west or, vice versa but, if you indeed went from Flagstaff to Phoenix, you took I-17. I-40 west out of Flag is a horrible section of road. You'd have remembered that. Not a biggie, just thought you'd like to know. As for your choice in coaches and, engines, great move and glad you're really happy.
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Old 11-07-2017, 02:36 AM   #6
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Argh! You're 100% correct, Fire Up! I-17 South, it was!

And, double argh. We did, indeed, enjoy I-40 West out of Flagstaff on the way to the Grand Canyon. My teeth are still rattling...
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Old 11-07-2017, 02:53 AM   #7
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When the driver keeps that in mind they can keep RPM's low enough to prevent the upshift by judicious use of service brakes.
Indeed! I'm usually able to fine "the sweet spot" with RPMs about 200 below the shift point between 55 and 60 mph on a steep grade. Depending on the incline, I may be using just the compression brake (Low) or the compression and exhaust brakes (High) to hold it there.

I've had to stab the service brakes a few times on the steepest descents to keep it from the upshift, but that's been rare.

Great advice and detail, Ray. Thank you!
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Old 11-07-2017, 08:52 AM   #8
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For the last month or two, they have been repaving I-40 west of Flagstaff.
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Old 11-07-2017, 10:05 AM   #9
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Too often people compare the cost of owning a DP to a Gasser and come away thinking that the diesel costs more. In my mind, the value of the air ride, quieter cabin, increased power, much higher load and towing capacity and easy handling, is worth far more than the initial cost and annual maintenance difference. So to me, the DP cost less than a Gasser. (actually it is less, as I would have to make two trips with a gasser to get all my things there)
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Old 11-07-2017, 12:13 PM   #10
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Indeed! I'm usually able to fine "the sweet spot" with RPMs about 200 below the shift point between 55 and 60 mph on a steep grade. Depending on the incline, I may be using just the compression brake (Low) or the compression and exhaust brakes (High) to hold it there.



I've had to stab the service brakes a few times on the steepest descents to keep it from the upshift, but that's been rare.



Great advice and detail, Ray. Thank you!


What engine do you have? Iíve never heard of a 2 position brake being a combo of compression & exhaust brakes. I understand 2 position Jakes and a third position which is the VG turbo but not what you describe.
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Old 11-07-2017, 03:12 PM   #11
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Too often people compare the cost of owning a DP to a Gasser and come away thinking that the diesel costs more. In my mind, the value of the air ride, quieter cabin, increased power, much higher load and towing capacity and easy handling, is worth far more than the initial cost and annual maintenance difference.
Yeah but the "value" of the "air ride, quieter cabin, increased power, much higher load and towing capacity and easy handling" are all very subjective. That "value" will vary from one individual to another and is difficult to put in your checking account. And folks don't just "think" diesels cost more, they do cost more.

The higher acquisition and maintenance costs of a diesel are very objective, real and have to come out of your checkbook.

I guess that's why they make diesels and gassers. And they all perform the same when sitting in the campground.
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Old 11-07-2017, 03:23 PM   #12
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I think your 330 HP is plenty for your GVWR, diesel are all about torque / power not Zero to 60. My coach is only a 300 HP Cummins @ 27,900 GVWR and pulling our jeep Liberty she moves right down the road. I would rather pick a time test engine then go for the latest in HP.


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Old 11-07-2017, 03:45 PM   #13
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What engine do you have? Iíve never heard of a 2 position brake being a combo of compression & exhaust brakes. I understand 2 position Jakes and a third position which is the VG turbo but not what you describe.
We have a Mercedes MBE 926. It has both a true engine compression brake and an exhaust brake. Low activates the compression brake via a 12V signal that triggers an engine oil solenoid and high adds an exhaust brake via a 12V signal that activates an air solenoid. Our compression brake is single stage - it's either all 6 cylinders on or all 6 off.
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Old 11-07-2017, 03:47 PM   #14
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I think your 330 HP is plenty for your GVWR, diesel are all about torque / power not Zero to 60. My coach is only a 300 HP Cummins @ 27,900 GVWR and pulling our jeep Liberty she moves right down the road. I would rather pick a time test engine then go for the latest in HP.


Safe journeys
Absolutely agreed! I don't think I'd want anything bigger. We're remarkably efficient at 55 mph, which is perfect since we seek a lot of non-highway routes.
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