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High RPMs downhill

JHarris32

By JHarris32
in 6.6L Duramax Diesel (L5P)

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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. 

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