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Old 02-05-2008, 06:03 PM   #1
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We are new to diesel and have a couple of questions. Our new motor home is a Jayco Seneca on the Kodiak C5500 chassis with 6.6 Duramax engine and close ratio Allison 6 speed tranny. We know the Duramax does not have a true exhaust brake, but perhaps better said an engine brake involving the turbo. Our questions focus on driving down long, steep, high speed hills.

1. - when traveling down a long fast hill, is it ok to keep the engine brake on all the time or will this damage the turbo or engine?

2. - on descending REALLY steep hills, will the engine brake work when the transmission is put in 3rd. or 2nd. gear?

3. - we hear some diesel pusher drivers turn their exhaust brake on whenever they are driving in the mountains and keep in on all the time. Is this a good idea for the Duramax?

Any other tips would be truly appreciated.
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Old 02-05-2008, 06:03 PM   #2
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We are new to diesel and have a couple of questions. Our new motor home is a Jayco Seneca on the Kodiak C5500 chassis with 6.6 Duramax engine and close ratio Allison 6 speed tranny. We know the Duramax does not have a true exhaust brake, but perhaps better said an engine brake involving the turbo. Our questions focus on driving down long, steep, high speed hills.

1. - when traveling down a long fast hill, is it ok to keep the engine brake on all the time or will this damage the turbo or engine?

2. - on descending REALLY steep hills, will the engine brake work when the transmission is put in 3rd. or 2nd. gear?

3. - we hear some diesel pusher drivers turn their exhaust brake on whenever they are driving in the mountains and keep in on all the time. Is this a good idea for the Duramax?

Any other tips would be truly appreciated.
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Old 02-06-2008, 06:23 AM   #3
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Hi,

While I can't speak from driving experience I can say that you are right that your Duramax uses the turbo to act as an engine brake by varying the vane pitch.

To answer your questions, I would suggest you review the owner's manual regarding operation of your engine brake. There is some good info. in there.

Here are some points to remember...

While using the exhaust brake, the proper gear range selection is important. The exhaust brake is most effective if the gear range selected is the lowest possible range that does not allow the engine rpm to go more than 200 rpm above rated (full load) rpm. If your vehicle has a DURAMAX® 6600 Diesel Engine, do not exceed 3600 rpm.

If your vehicle has an Allison® 1000/2200/2300/2350/2500/2550 series automatic transmission and the exhaust brake button is pressed, the transmission downshifts to a factory default setting of third gear.

The accelerator pedal must be released in order for the exhaust brake to engage. The exhaust brake will automatically shut off when the antilock brake system is active.

The exhaust brake will become active in cruise control when you exceed the set speed by 3 to 5 mph.

I hope this helps and maybe someone else will reply who has some actual driving time behind the wheel of one of these rigs.
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Old 02-06-2008, 03:43 PM   #4
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If you go to the Allison transmission FAQ page you will find that you cannot exceed the computerized RPM with the grade retard feature. The Allison will upshift into the next higher gear if the RPM's exceed the computer parameter.
Arguably the best internet site for Duramax powered trucks is: dieselplace.com. Several GM technicians participate there.
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Old 02-07-2008, 05:54 AM   #5
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ray,IN:
...you will find that you cannot exceed the computerized RPM with the grade retard feature. The Allison will upshift into the next higher gear if the RPM's exceed the computer parameter.
</div></BLOCKQUOTE>

Yep, that's exactly right. The Allison's control systems won't let the operator do anything on purpose or by accident that would overspeed the engine or harm the transmission. It's foolproof.
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Old 02-07-2008, 05:34 PM   #6
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Mr. Goodwrench - welcome to iRV2. We're glad to have you aboard!

All of these reasons are why I love my D-max/Allison combo.

Can you coment on the injector warranty question in the other thread?
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Old 02-09-2008, 02:38 AM   #7
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Sure.
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