4 Lo for downhill; whirring sound the whole way?

JeepScares

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Hi,
We have been driving our new JLU Rubicon 3.6L with automatic transmission for only a short time, and I finally had a chance to try out its 4 Lo setting.

We were descending a set of steep switchbacks so it seemed prudent to utilize the drag of the drive train. We stayed well below 10mph, but the entire time we descended the drive train made a distinct "whirring" sound. It was a high pitched tone that I might associate with dragging the system at a very high rpm.
Later in the afternoon we made a short climb over some rocks and I had occasion to switch back into the 4 Lo setting. When I was actually driving the gearing against the incline it seemed quiet compared to when I has using it to slow the descent.

I am imagining that the extremely low crawl ratio in this new system causes the gears in the transfer case to spin like a gerbil wheel when it is trying to keep the Jeep from careening down a hill.

Is the noise I am hearing a common experience when working your way down a hill in a new JLU?

Thank you.





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DWaX

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What is the range of the rpms when you begin to hear the sound?

WaX
 
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JeepScares

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Engine RPM was down near 1000rpm. Basically the engine was at idle and I was tapping and releasing the brake the whole way down.
 

D60

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The planetaries are pretty noisy, yeah
 

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use hill descent feature.
 
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JeepScares

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use hill descent feature.
Does that feature interact with the transmission in a direct way?

It is my understanding that the hill descent is simply a computer controlled brake by wire system.

I am trying to better understand the what sort of experience to expect with regards to sounds from the drive line, and its components that are new to me.
 

limeade

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When using engine and gearing (4 low) while descending, you will hear the drivetrain make the "whirring" noise, it's perfectly normal.

The hill descent control feature is very nice and effective, but is even noisier.

All normal sounds while off-roading and using the features of your new jeep.
 

Arterius2

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Does that feature interact with the transmission in a direct way?

It is my understanding that the hill descent is simply a computer controlled brake by wire system.

I am trying to better understand the what sort of experience to expect with regards to sounds from the drive line, and its components that are new to me.
Were you in manual mode?
If so, That’s the sound of your engine braking noise. Normally when I do this, I can see my RPM shoot up to not-so-fun levels so personally I would avoid doing that on an auto transmission.
If it’s on auto then it must be something else.
And yes hill descent basically activates the brakes for you and could easily overheat them, so I also wouldn’t use it for long descents, I almost lost all my braking power doing this and was scared shitless.
 

limeade

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Were you in manual mode?
If so, That’s the sound of your engine braking noise. Normally when I do this, I can see my RPM shoot up to not-so-fun levels so personally I would avoid doing that on an auto transmission.
If it’s on auto then it must be something else.
And yes hill descent basically activates the brakes for you and could easily overheat them, so I also wouldn’t use it for long descents, I almost lost all my braking power doing this and was scared shitless.
I don't see a problem using it in manual mode. I use manual mode in 4 low all the time. I simply shift up or down to match the speed and to the terrain and traction level and to keep the RPM's at an acceptable level.
 

Arterius2

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I don't see a problem using it in manual mode. I use manual mode in 4 low all the time. I simply shift up or down to match the speed and to the terrain and traction level and to keep the RPM's at an acceptable level.
Yes, that’s the correct way at it.
But what I’m saying is that if you say left it in first gear in 4LO coming down a steep hill, and didn’t shift up, your RPM will shoot way high and you will hear that loud wheeerrrrring sound OP described. It’s the sound of engine braking excessively.
 

limeade

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Yes, that’s the correct way at it.
But what I’m saying is that if you say left it in first gear in 4LO coming down a steep hill, and didn’t shift up, your RPM will shoot way high and you will hear that loud wheeerrrrring sound OP described. It’s the sound of engine braking excessively.
I see what you're saying and totally agree with that.
 
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FWIW, the sound I am trying to describe is not related to the engine spinning at high rpm. The engine never rev'd up to high rpm.

The sound was apparent even when the transmission was in neutral, which is a circumstance that occurred when I shifted into 4 Lo and later when I shifted out of 4 Lo.

The sound was a bright and shiny tone that I never heard on the old rusty beasts I have driven in the past.

Thanks for the suggestions!
 

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FWIW, the sound I am trying to describe is not related to the engine spinning at high rpm. The engine never rev'd up to high rpm.

The sound was apparent even when the transmission was in neutral, which is a circumstance that occurred when I shifted into 4 Lo and later when I shifted out of 4 Lo.

The sound was a bright and shiny tone that I never heard on the old rusty beasts I have driven in the past.

Thanks for the suggestions!
I would bet it was the engine cooling fan, ours runs like crazy when we are crawling off road.
 

sf5211

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Just a guess but maybe the torque converter? I don’t go off road so I can’t answer but any manual transmission folks getting that sound?
 

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