4WD AUTO

[dullone]

I have a 2018 Big Horn Crew Cab with the 3.6 Pentastar and 3.55 gear ratio. Any one have empirical knowledge of the affect on mpg when using 4WD Auto ?

 

[Brandon-w]

Classic body or new body style.

 

[MRFREEZE57]

if you leave it in 4wd auto you will be turning the drive line into the transfer case causing more drag.

 

[BOWERSFJ]

You’ll get less MPG’s in 4WD ...


Sent from my iPhone using Tapatalk

 

[kurek]

I have a 2018 Big Horn Crew Cab with the 3.6 Pentastar and 3.55 gear ratio. Any one have empirical knowledge of the affect on mpg when using 4WD Auto ?

Empirically when the vehicle is in 2wd the front axle disconnects the left and right CV's from each other, which means wheel input is resolved in the front diff's spider gears and therefore the front driveshaft and the bottom of the transfer case are stationary.

In 4 auto the front axle disconnect is connected, so the front diff ring and pinion are put into play, the pinion bearing, the mass of the front driveshaft, the mass of the bottom of the transfer case mechanism as well as the bearings in there. The result is some minor wear on those parts and heat build up from the added friction of the ring and pinion being driven "backward" (the gears are mated in the forward direction, not the back side of the teeth) and of course a reduction in fuel mileage from all that extra spinning mass and friction.

As for how much? Not a lot, but more than zero. Never zero and never less than zero.

Sorry I have nothing more precise than that, doubtful anyone outside Ram does.

I suspect that the front CAD disconnects at highway speeds in 4 auto and then gets spun back up by a blip of the transfer case clutches when the vehicle returns to low speeds. The amount of heat that can build up in the diff running off the back side of the ring & pinion teeth is substantial at high speeds and it makes a bit of a whine, and there's really no point at all to having instantaneous 4wd response at highway speeds anyway.

 

[Quint]

Great overall response. My only qualm is with this:

....and there's really no point at all to having instantaneous 4wd response at highway speeds anyway.

There are times that having 4-wheel auto engaged at highway speeds can help when you hit slick spots, such as when your rear wheels catch a patch of ice on a curve or even ice patches under bridges where the water drips down and refreezes on the highway. I can tell a difference between 2wd and 4auto in those.

 

[kurek]

you won't see me going 75mph anywhere that might have a patch of ice on a curve.

 

[ramffml]

you won't see me going 75mph anywhere that might have a patch of ice on a curve.

That's fair; hear in Ontario we get lots of snow and thawing slush on the roads. Often the roads will be slightly/moderately covered or packed down. Everyone is still driving 50+ km/h, but you have to slow down in rougher spots or whiteouts. 4auto is perfect in these conditions.

I'm very impressed with how it works. I was backing up a 2000 pound boat up hill and round a small corner on gravel and grass and rear tires would spin slightly every time I touched the gas. I probably could have powered through it, but instead (for a test) I put it in 4 auto and the truck didn't even think about slipping, it just calmly walked up that hill backwards.

I don't know what/how they're doing it, but that feature is one reason I bought the truck (previous ride was a Jeep GC with permanent 4wd). It's completely seamless and very effective.

 

[Scott S]

With my 5.7 4x4 there may be a slight drop in MPG using Auto 4WD but not really noticeable in the overall picture.

It has been better on fuel than I would have thought actually.

 

[MrZuo]

I have a question about the 4wd auto as well. I can hear a small clunk(ish) noise at low speed. It happens after you stop at the light then go, and can feel the truck a bit jerky.

Any thoughts would be nice.

 

[kurek]

I have a question about the 4wd auto as well. I can hear a small clunk(ish) noise at low speed. It happens after you stop at the light then go, and can feel the truck a bit jerky.

Any thoughts would be nice.

This is pure speculation so only take it as that: on some vehicles with an automatic 4wd feature (I know this 100% with Ford Escape and Jeep Patriot) even though it's not true all-wheel-drive, they simulate the behavior of all-wheel-drive by pulsing the clutch that connects the front and rear during initial acceleration from a stop. This is to prevent wheelspin when starting out on a slippery surface since the automatic 4wd system can't know if it's on ice or not until slip already occurs. So they just assume it might be slippery and start off each launch in 4wd and then let it decide after launch whether it's still needed.

Since there's inherently a little slack in the front drivetrain it makes sense there could be a tiny clunk as that slack gets taken up. Shouldn't be much though - but there's no way to convey in an internet post what a subjective "too much" is.

 

[MrZuo]

This is pure speculation so only take it as that: on some vehicles with an automatic 4wd feature (I know this 100% with Ford Escape and Jeep Patriot) even though it's not true all-wheel-drive, they simulate the behavior of all-wheel-drive by pulsing the clutch that connects the front and rear during initial acceleration from a stop. This is to prevent wheelspin when starting out on a slippery surface since the automatic 4wd system can't know if it's on ice or not until slip already occurs. So they just assume it might be slippery and start off each launch in 4wd and then let it decide after launch whether it's still needed.

Since there's inherently a little slack in the front drivetrain it makes sense there could be a tiny clunk as that slack gets taken up. Shouldn't be much though - but there's no way to convey in an internet post what a subjective "too much" is.

Thanks.

 

[Murphy Slaw]

What transfer case is in the 2018? Mine only engages when there's a slip, so gas mileage isn't affected at all, unless it kicks in.

People complain about the 4 auto BW44-44, but mine works perfect, only when I need it.

But, I'm just an old redneck with some hunting property, not a rock crawler.

 

[kurek]

What transfer case is in the 2018? Mine only engages when there's a slip, so gas mileage isn't affected at all, unless it kicks in.

It could have either case. The year doesn't affect what case is available (except that 5th gens have newer versions of the transfer cases)

There are two different engagements. One separates engine power from the front driveshaft, the other separates the wheels from the front driveshaft. When the selector is in 2wd both of the engagements are disconnected so if you stuck a camera under the truck and went for a drive you would see that the front driveshaft is not turning.

The front axle disengagement mechanism is a splined interface and not a clutch, this allows it to be small and lightweight but cannot be rapidly and smoothly pulsed on and off. So when you shift into 4-Auto the front axle mechanism engages. This causes the movement of the front wheels to feed back into the front differential and turn the front driveshaft and the bottom of the transfer case even when no engine power is being delivered. These parts not only experience friction at the gear interfaces and bearings, they also are spinning at 3x (well, 3.21 or 3.55 or 3.92x) the speed that the wheels are turning and spinning up and down that rotating mass takes energy. Since the truck has a big engine and a low range transfer case these are dramatically heavier parts than their counterparts would be in something like a Subaru that's spinning all 4 wheels all the time with little 1" diameter shafts and 5" ring gears.

When you engage 4-Auto the front axle engages, and then the "auto" part just pulses the clutch pack in the transfer case because that can be done very quickly and the driveshaft, ring and pinion, all those heavy bits are already locked in to the front wheels.

That is why 4-auto necessarily affects fuel mileage, braking distance and NVH (and why there's a 2wd mode in the first place).

 

[MRFREEZE57]

when in 4wd auto, you jack up the front end and spin the wheels the front drive line will be turning to the transfer case, in 2wd just the wheels turn quite freely. still I would rather it have a set of locking hubs so you are not turning the spider gears and cv joints when not needed but I guess many like the idea of shift on the fly 4wd.

 

[warcar1947]

I have a 2017 Limited 1500 4x4, I use 4wd auto when it rains it really improves your traction. I know there is some loss in mpg but I con't detect it.