Jump to content

High RPMs downhill

JHarris32

By JHarris32
in 6.6L Duramax Diesel (L5P)

 Share

Recommended Posts

JHarris32 New Member

JHarris32

Posted
Posted

I have a 2020 3500hd duramax. If I am going down a steep hill in normal mode and I’m not towing, the rpms rev up like they do while in tow mode. The truck slows down and won’t coast like my 2011 gasser. I’m just wondering if this is normal? Thank you

redwngr Senior Enthusiast

redwngr

Posted
Posted

That's the grade brake doing it's thing. 

If the exhaust brake is enabled it will assist in this process.

 

Truck is trying to maintain the speed it thinks you are asking for.

No runaway coasting....

 

FWIW, max rpm under decel is higher than max rpm when powered

 

Maximum Powered Speed 3450 rpm
Maximum Braking Speed  4800 rpm

  • 3 months later...
JRSPerf. Enthusiast

JRSPerf.

Posted
Posted (edited)
On 7/22/2020 at 2:39 PM, JHarris32 said:

I have a 2020 3500hd duramax. If I am going down a steep hill in normal mode and I’m not towing, the rpms rev up like they do while in tow mode. The truck slows down and won’t coast like my 2011 gasser. I’m just wondering if this is normal? Thank you

Hello JHarris32.  At your earliest convenience can you PLEASE attach a short video clip of the instrument cluster(rpm guage) while this situation is occurring. Also please note your vehicle's build date, this can be found on the driver's side b-pillar placard. Thank you so much in advance!!

 

On 7/22/2020 at 2:39 PM, JHarris32 said:

I have a 2020 3500hd duramax. If I am going down a steep hill in normal mode and I’m not towing, the rpms rev up like they do while in tow mode. The truck slows down and won’t coast like my 2011 gasser. I’m just wondering if this is normal? Thank you

 

Edited by JRSPerf.
  • 1 year later...
1Joshman Enthusiast

1Joshman

Posted
Posted

High RPMS?  What are the RPMS at high speeds and flat surface?  Say at 70 and 75 MPH.   MIne runs at about 16/0 at 70 and about 16.5 at 75.  This is of course in 10th gear 

  • 2 weeks later...
Another JR Senior Enthusiast

Another JR

Posted
Posted (edited)
On 11/13/2020 at 8:28 PM, JRSPerf. said:

Hello JHarris32.  At your earliest convenience can you PLEASE attach a short video clip of the instrument cluster(rpm guage) while this situation is occurring. Also please note your vehicle's build date, this can be found on the driver's side b-pillar placard. Thank you so much in advance!!

 

 


I’m curious why you ask. The owners manual for my 2021 gas 3500hd says that transmission grade braking only works in tow haul mode. On mine, grade braking works in normal mode on descents. 

Edited by Another JR
Flying MXZ Enthusiast

Flying MXZ

Posted
Posted
On 7/22/2020 at 2:39 PM, JHarris32 said:

I have a 2020 3500hd duramax. If I am going down a steep hill in normal mode and I’m not towing, the rpms rev up like they do while in tow mode. The truck slows down and won’t coast like my 2011 gasser. I’m just wondering if this is normal? Thank you

My 21 does the same.  If it's a steep enough hill I turn on the trailer mode to make it more aggressive.

redwngr Senior Enthusiast

redwngr

Posted
Posted

Not sure if this is what is going on, but if cruise control is set, it tries to maintain set speed on the downgrades too.

 

Same thing (downshift/high rpm) can happen with a 1500 gas truck and no trailer -- if the grade is long and steep enough. 

 

With cruise off, it will likely still add grade and/or exhaust braking if you use the brake pedal (which tells it the vehicle is running faster than you want) 

 

HD's, and especially the dmax, are all about load control.

 

They 'coast' less than a 1/2 ton.

And the 1/2 ton will 'coast' less than the passenger vehicles. 

 

  • Like 1
aliveguy5 Enthusiast

aliveguy5

Posted
Posted
On 7/22/2020 at 2:39 PM, JHarris32 said:

I have a 2020 3500hd duramax. If I am going down a steep hill in normal mode and I’m not towing, the rpms rev up like they do while in tow mode. The truck slows down and won’t coast like my 2011 gasser. I’m just wondering if this is normal? Thank you

Sounds like the Exhaust Brake is holding your speed. Welcome to a HD truck. If you want to continue to accelerate hit the gas pedal for a bit and I believe it will let you coast, or at least let the truck now you are ok with a gain in speed. 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.

  • Forum Statistics

    250.6k
    Total Topics
    2.7m
    Total Posts

  • Member Statistics

    342,993
    Total Members
    8,960
    Most Online
    President Roberto
    Newest Member
    President Roberto
    Joined

  • Who's Online   1 Member, 0 Anonymous, 291 Guests (See full list)

  • Latest Articles

  • Posts

    • bwiese
      upgrading speakers on 2026 std cab WT...

      By bwiese · Posted

      I don't wanna do major surgery/upgrades, behind-the-dash work, add amps, change radio etc. on my '26 std cab WT. On the WT, it has the base 7" IOR infotainment setup, and only two speakers (doors). Since I listen to classical, looked around for speakers that do well and dug thru reviews. Two-way speakers were found to be better than 3-way. So I've ordered a pair of  Infinity Reference REF-9632IX 2 way speakers.  [These may  or may not have been discontinued / succeeded by the 3-way ...9633 ones.]    These speakers are lauded for 3 ohm and 94db sensitivity spec so they'll work well for ordinary base car radios without amps and offer better performance (quality,  strength) than  default base paper-cone speakers. They'll be installed in a few days and I'll let y'all know what I think. Bill W San Jose      
    • RyanbabZ71
      RyanbabZ71's 2019 High Country

      By RyanbabZ71 · Posted

      Thanks!   Changed out the rear shocks today to Bilstein 5100s.  So far feel much better. The stock Rancho were wore out.  Lots of heavy hauling (2 moves, home renovations, etc).  Next up is new BFG tires Monday.  Cant wait the current tires have little to no wet traction and are super loud (hum)    
    • Snowcamo
      No high speed fans...

      By Snowcamo · Posted

      Ok, I got the all the codes. What do you all think would be a place to start?
    • Chuck FB
      Correct way to service 2024 l8t allison trans?

      By Chuck FB · Posted

      How about I just point to the factory pans stand pipe little drain plug and claim its a pan with a real drain plug !. I just couldn't swallow the fact that a Dorman pan which at this point is probably the only pan out there that fits the gas version truck cost that much up here in Canada so I decided initially anyway to skip it and see what I thought of pulling the pan method and down the road if I felt inclined enough then I would either install a drain plug kit in the factory pan if there is enough room for it, or have a bung welded onto it, or put an aftermarket pan on. The thing is if the pan is not pulled then the magnets in the pan can't be inspected and cleaned or if wanting to change out the filter. However of course it certainly would make the job easier to be able to drop the oil first before having to handle the pan if dropping the pan that go around was the goal.    I knew a guy locally that was doing custom field spraying years ago with a pickup pulling a ground driven sprayer, I believe it was a GM pickup with an automatic trans. He had a drain plug in his pan and would drop the oil out of it once a week and top it back up through the dipstick tube to keep refreshing a portion of the oil often and that made the transmission last far longer than had he done nothing. Soft fields pulling 500 gallons of water at the start of each mixed up sprayer tank batch and in low range sucking along. I imagine he got some tips from other custom sprayer outfits doing the same thing at the time and learning from their blown up tranny mistakes. Transmissions then had no lock up converter so would cook the crap out of the fluid slipping that much and it would not have been synthetic fluid either. 
    • KARNUT
      Gas Prices

      By KARNUT · Posted

      3.39 Kroger’s in Montgomery Texas. 

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...