Auto vs 2H Settings...

[Knotsure]

I don't understand the I don't need auto4 in the rain argument. The option is there why not use it. I stop and go a lot on my way to and from work (90 mile roundtrip) so I value that added traction when taking off from a light and making turns from the grocery store near my home, which is a madhouse anywhere near rush hour. Trying to get out in traffic when the road is wet and heavy with traffic is a pain.

 

 

But hey we all have our own preferences.

 

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[sdeeter19555]

What I think happens when you flip it to auto.... you engage the transfer case actuator into 4wd as every other truck has done for the last 20 years, but there is a clutch somewhere in between the front differential and the transfer case. Normally the clutch is disengaged, and when you lose traction the clutch engages. I'm pretty sure that's out most clutch type limited slip differentials operate.

 

I mean my wife's acura rdx, i know when i was hooliganing in the snow it gave me a service message that the awd clutch fluid was too hot.

 

Subaru's, i know you can pull a fuse to disengage the awd clutch that sends power to the rear tires.

 

Both these "awd" vehicles are front biased systems have a clutch that transmits power to the rear.

 

How that's different from our trucks I can't figure out.

The Subaru systems have varied a lot over the years...from a full time 4wd (Subaru XT-6, front wheel slip caused clutches to engage the rear differential), to a full time AWD (Older Subaru Outbacks, always in 4wd but disengaged during turns to prevent tire scrub), to the modern asymmetric AWD (back to front wheel slip causing electronic engagement of the rear wheels, with brake caliper engagement on slipping wheels).

 

I think I understand your description of this newer auto 4wd in the trucks. I feel it probably would never show any problems short engagement mechanisms or chain stretch.

 

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[truckguy82]

 

Think about it though. If they were the same then it would be safe to drive in 4hi all the time and it certainly isn't.

No it wouldn't.

 

4hi locks the front and rear together

 

4auto does not

 

It's ok not to know the answer....you can just say I don't know

 

We just need someone who knows more about the systems than you or I

[sdeeter19555]

I don't understand the I don't need auto4 in the rain argument. The option is there why not use it. I stop and go a lot on my way to and from work (90 mile roundtrip) so I value that added traction when taking off from a light and making turns from the grocery store near my home, which is a madhouse anywhere near rush hour. Trying to get out in traffic when the road is wet and heavy with traffic is a pain.

 

 

But hey we all have our own preferences.

 

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I guess owning a gutless 6.0l (agressive torque mangement) in a heavier 2500 doesn't give me that opportunity for frequent wheel spin on wet roads. I've never needed 4wd on wet pavement...not saying someone else doesn't, just that I never have.

 

I do agree with you, it's an option and is meant to be used...if inclined and it makes you feel safer, use it. It's not affecting anyone else...

 

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[truckguy82]

The Subaru systems have varied a lot over the years...from a full time 4wd (Subaru XT-6, front wheel slip caused clutches to engage the rear differential), to a full time AWD (Older Subaru Outbacks, always in 4wd but disengaged during turns to prevent tire scrub), to the modern asymmetric AWD (back to front wheel slip causing electronic engagement of the rear wheels, with brake caliper engagement on slipping wheels).

 

I think I understand your description of this newer auto 4wd in the trucks. I feel it probably would never show any problems short engagement mechanisms or chain stretch.

 

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So basically if what I said about how our trucks engage the front in 4auto mode, it's essentially no different from awd? Is that what I gather from that?

 

I mean from all that I can tell so far, with my limited understanding, is that the dial on the dash could be justified in saying

 

2hi

Awd

4hi

4lo

 

The ultimate drivetrain

 

I just can't find a difference between 4auto and awd

[SouthernSilveradoGuy85]

No it wouldn't.

 

4hi locks the front and rear together

 

4auto does not

 

It's ok not to know the answer....you can just say I don't know

 

We just need someone who knows more about the systems than you or I

 

4auto does lock the front and rear just like it would in 4hi or 4lo. The difference is that 4auto isn't full time 4hi, only if it feels the truck lost traction. With it engaged it's just in 4hi, but only when it needs it.

[truckguy82]

 

4auto does lock the front and rear just like it would in 4hi or 4lo. The difference is that 4auto isn't full time 4hi, only if it feels the truck lost traction. With it engaged it's just in 4hi, but only when it needs it.

Exactly, my question is how is that operation any different than what an AWD system does, or atleast some AWD systems....

 

You have to know awd systems pretty well to answer that. I do not

[sdeeter19555]

So basically if what I said about how our trucks engage the front in 4auto mode, it's essentially no different from awd? Is that what I gather from that?

 

I mean from all that I can tell so far, with my limited understanding, is that the dial on the dash could be justified in saying

 

2hi

Awd

4hi

4lo

 

The ultimate drivetrain

 

I just can't find a difference between 4auto and awd

I think it's reverse of what most AWD systems do, they drive the front wheels and engage the rear wheels, these auto 4wd systems drive the rear wheels and engage the front as needed.

 

I'm going out on a limb here, but I think the main difference between a AWD and a true 4wd is low range capability and the ability to solidly lock the front/rear differentials together through gearing in the transfercase. An AWD, even though the clutch can provide limited lockup, is still something that could be overwhelmed.

 

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[SouthernSilveradoGuy85]

Exactly, my question is how is that operation any different than what an AWD system does, or atleast some AWD systems....

 

You have to know awd systems pretty well to answer that. I do not

 

The only thing I know is that a 4x4 (even one with the auto function) system and an AWD system just isn't designed the same. A 4x4 system isn't made for dry pavement while AWD is so there's something different. According to you, they're the same so you should be alright with driving around on a sunny day in 4hi, but it wouldn't be alright. So it's not the same system. Just because they both send power to the front doesn't mean the designs are the same. Think of it almost like an OHC engine and a regualr engine where the cam is above the crank. They both open and close the valves, they just do it in a different way.

[kstruckcountry]

Keep in mind that I don't know how the transfercase guts operate in one of the auto 4wd trucks (assuming it's an electrically-actuated clutch of some sort).

 

The only added wear I could see in auto 4wd is when the transfercase engages...and whatever mechanism engages it. Otherwise, it would engage momentarily until it senses no speed difference between the front and rear wheels, then disengage like a normal 2wd. I'd be more worried about chain stretch in the transfer case from the potentially frequent and abrupt engagements than wear of any other component. The truck needs to sense slipping between the front and rear wheels, so there is going to be a shock load applied at some point.

 

I see two questions for the auto 4wd trucks...

 

1) do they have a front differential engagement deal like the non-autos? In other words, would the front differential be spinning all the time anyway or does it also disengage when not in auto 4wd? I can't see how it could have the front differential engage/disengage everytime 4wd was demanded (too slow for that, at least mine is). If they don't have engagement at the front differential, then those components spin all the time anyway.

 

2) what is the life expectancy of the transfer case and its actuator?

 

 

 

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In Auto4 the front differential is always turning. In 2hi, it is disengaged. This is much of the reason for the mileage drop in Auto4.

 

The transfer cases hold up pretty well, engagement is very smooth.

 

Auto4 is very different from all wheel drive, and other than a shared goal they really couldn't be any more different. Auto4 isn't really limited slip. It can't send any more than 50% to the front wheels, since it is based on a classic 4x4 system. It ramps up output to the front wheels until they match speeds, whether or not that takes full lockup depends on the conditions. The pickup isn't really sensing the torque required, it's sensing the speed difference.

 

A center differential in an AWD can generally send, or "vector" more than 50% to either end of the vehicle. Subaru's center LSD is capable of being driver controlled in some models to control handling characteristics. But generally, the goal of the center diff in in AWD setup is to get the torque to where it's needed. With one end of the car on ice and the other on pavement, the majority of the torque is going to be applied to the end on pavement.

[truckguy82]

In Auto4 the front differential is always turning. In 2hi, it is disengaged. This is much of the reason for the mileage drop in Auto4.

 

The transfer cases hold up pretty well, engagement is very smooth.

 

Auto4 is very different from all wheel drive, and other than a shared goal they really couldn't be any more different. Auto4 isn't really limited slip. It can't send any more than 50% to the front wheels, since it is based on a classic 4x4 system. It ramps up output to the front wheels until they match speeds, whether or not that takes full lockup depends on the conditions. The pickup isn't really sensing the torque required, it's sensing the speed difference.

 

A center differential in an AWD can generally send, or "vector" more than 50% to either end of the vehicle. Subaru's center LSD is capable of being driver controlled in some models to control handling characteristics. But generally, the goal of the center diff in in AWD setup is to get the torque to where it's needed. With one end of the car on ice and the other on pavement, the majority of the torque is going to be applied to the end on pavement.

Doesn't subaru only have that center limited slip on the sti? I was fairly confident that's one of the major reasons they are so coveted. Limited slip on front, rear, and center.

 

Just want to be sure you are talking about awd systems in general and not one of the best systems money can buy

[truckguy82]

 

The only thing I know is that a 4x4 (even one with the auto function) system and an AWD system just isn't designed the same. A 4x4 system isn't made for dry pavement while AWD is so there's something different. According to you, they're the same so you should be alright with driving around on a sunny day in 4hi, but it wouldn't be alright. So it's not the same system. Just because they both send power to the front doesn't mean the designs are the same. Think of it almost like an OHC engine and a regualr engine where the cam is above the crank. They both open and close the valves, they just do it in a different way.

So you have no idea, thanks, should have said that awhile ago

[kstruckcountry]

Doesn't subaru only have that center limited slip on the sti? I was fairly confident that's one of the major reasons they are so coveted. Limited slip on front, rear, and center.

 

Just want to be sure you are talking about awd systems in general and not one of the best systems money can buy

You're right, Subaru itself has multiple systems in use currently, the driver controlled center diff isn't available on many models.

 

This article explains them really well: https://www.cnet.com/roadshow/news/not-every-subaru-all-wheel-drive-system-is-created-equal/

 

Basically VTD is the closest to an Auto4 type of operation, except that it's always "on" and a completely different design.

 

The STi is the only driver controllable system I'm aware of currently.

[SouthernSilveradoGuy85]

So you have no idea, thanks, should have said that awhile ago

 

I do know they're different and are designed to do different things so apparently I know more about it than you do if you want to try to get personal. Seems like you're the one trying to think you know how it is when you don't. At least I can admit if I'm wrong and don't mind saying I don't know something. Have fun putting it in auto4 for simple rain. I don't know how I made it home in my '98 Z71 that didn't have the auto feature or my '86 that was just 2wd. I guess I'm lucky to be alive.

[Sean Neilan]

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