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Old 07-25-2021, 03:24 PM   #1
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RPMs... how high is too high?

At different times when towing in mountainous areas and using the tow/haul feature along with the engine break the RPMs could sometimes get on the high side (sounding anyway) after hitting the breaks. When this would happen I'd turn them off and then turn them back on to reset. What's a safe RPM number I should be looking at and when should I be concerned?

(TV = 2015 Chevy 2500 Duramax/Allison)
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Old 07-25-2021, 03:29 PM   #2
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Originally Posted by flintsters View Post
At different times when towing in mountainous areas and using the tow/haul feature along with the engine break the RPMs could sometimes get on the high side (sounding anyway) after hitting the breaks. When this would happen I'd turn them off and then turn them back on to reset. What's a safe RPM number I should be looking at and when should I be concerned?

(TV = 2015 Chevy 2500 Duramax/Allison)
Don't red line it. What does the tach read?
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Old 07-25-2021, 03:33 PM   #3
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There is no red line on the tach. I can't remember exactly where it was on the tach, but I think it was around 4k. I'm heading out into the mountains tomorrow and towing a heavier load so it's on my mind
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Old 07-25-2021, 04:17 PM   #4
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I know the V8 diesels rev a little higher than my inline 6 Cummins but in all reality 3000-3500 rpms ain't much for revs in the world of towing. Try towing up a 10% grade with your rpm's passing 4500 in 1st gear for a mile or so.
I'm also not sure why you should have high revs except for being in TH mode and descending a steeper grade. My CTD will hit around 2500+/- when doing so. Yeah it's a little noisy but nothing like my Hemi 6.4 was.
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Old 07-25-2021, 06:03 PM   #5
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I had a 2004 with an 8.1 and the Allison would not let you hurt the motor down shifting in tow haul. I've got to believe in a 2015 it won't either.
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Old 07-25-2021, 06:05 PM   #6
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At different times when towing in mountainous areas and using the tow/haul feature along with the engine break the RPMs could sometimes get on the high side (sounding anyway) after hitting the breaks. When this would happen I'd turn them off and then turn them back on to reset. What's a safe RPM number I should be looking at and when should I be concerned?

(TV = 2015 Chevy 2500 Duramax/Allison)
I have owned every version of Duramax motor ever built for pick up trucks , YOU cant hurt the motor in engine brake mode. if the motor ever get to the 4500~5000RPM range it will up shift a gear to prevent engine damage. I have never had one of my trucks run away that bad that it needed to up shift . There is two different redlines for these motors one is on fuel and one is in coast no fuel. on fuel they will rev to about 3200~3400 RPM while pulling . engine brake mode it will not inject fuel and it will rev to about 4500 and never hurt the motor. Bottom line let it sing down hill , it will hold back better with more RPM. My last 2016 LML powered truck has 120,000 on it currently and now sits in a friend of mine driveway. I have a 2020 now.
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Old 07-25-2021, 06:19 PM   #7
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When we were coming down the west side of the Rockies on I-70 is one of those times that comes to mind. I never felt out of control but was mindful of the sound and RPMs. Have experienced similar in new england for far shorter durations (like coming over the gap in the white mtns tomorrow). It's helpful to know that the engine will protect itself from my ignorance.
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Old 07-25-2021, 06:27 PM   #8
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You are probably already aware but the transmission needs to be in D for the grade breaking to work properly. At the top of the pass you can set your cruse control at the speed you want to descend and let the truck and tranny do the work.
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Old 07-26-2021, 12:50 AM   #9
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Specifications chart in this link shows max braking RPM is 4800.
https://dieselresource.com/diesel-re...on-background/
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Old 07-26-2021, 07:20 AM   #10
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You are probably already aware but the transmission needs to be in D for the grade breaking to work properly. At the top of the pass you can set your cruse control at the speed you want to descend and let the truck and tranny do the work.
When you say D I assume you mean to not be manually downshifting (which I don't). I just put it in Drive, hit tow/haul and engine break and hit the road. My tach maxes out at 5k.... I think I'd probably have a bit of a panic attach if it approached that number.
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Old 07-26-2021, 10:05 AM   #11
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Yea, when you are going downhill and the truck downshifts to high RPMs and then the cooling fan comes on it feels like you are fixing to launch but all is good. The truck won't hurt itself.
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Old 07-26-2021, 11:21 AM   #12
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use cruise control

Going down the afore-mentioned Eisenhower tunnel west side downgrade (12 miles, 7% grade, 11,200 ft to 6,500 ft) with a 2017 Duramax and 10K fifth wheel, I set my cruise control at 10 MPH less than i wanted to actually go. With that, the tow/haul mode and engine brake, my rig stayed exactly where i wanted it, and the engine did the work. I think i hit the brakes 3-4 times. MUCH better than my pervious gasser which hit 5,500 RPM and scared the bejesus out of us, and still had to overuse the brakes.
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Old 07-26-2021, 07:03 PM   #13
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Agreed. X2. Sounds significant the engine revs and and the cooling fan kicks on for sure.
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Old 07-26-2021, 08:49 PM   #14
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I'm not sure about the DuraMax but my F350 6.7 I can set tow/haul, engine brake and cruise control, the ECM will not let the truck self destruct. I wasn't sure so I started on some small hills and worked up, it does work, I 3/4 time pulling a 36ft 14K 5W. Might give that a try I'm sure all the newer will work the same, the ECM will protect the truck.
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