Part Time 4WD Operation

[Kistle]

I did some searching on this but couldn't find a quick answer.

So when in 2H, where is the power to the front wheels disconnected? Said another way, when shifting from 2H to 4H, which specific part engages to drive the front wheels?

My understanding is that in 2H, the road force (while rolling), turns the front tires and wheels (obviously), which in turn turns the front axles, which in turn turns the front differential, which in turn turns the front driveshaft.

So the entire front drivetrain is turning from road force while driving in 2H, but it doesn't have power applied to it. As soon as you shift the transfer case from 2H to 4H, engine power is transmitted to the front driveshaft via the gears in the transfer case.

Is that all correct? I wasn't sure if there was something in the differential that engaged or disengaged based on "input" from the front driveshaft as a result of putting power to the front driveshaft.

For example, with locking/unlocking hubs, the front driveshaft, differential and axles do NOT spin when rolling in 2H. But in the 4Runner, I believe the entire front drivetrain turns in 2H simply due to rolling roadforce.

Sounds silly just writing it out, but wanted to confirm.

Thanks,

Captain Obvious

 

[Humble Leader]

I don't think the front diff turns. here's why I think that despite not knowing enough of the technical knowledge about 4runner drive trains:

part time 4wd 4runners need to have 4HI engaged regularly to make sure the front diff and any other related seals, moving parts get lubrication. if your explanation above was correct that everything is turning then you would not need to engage 4HI occasionally

in the full time 4wd vehicles you do not need to engage 4HI regularly, all those components and front diff are always moving

so non technical , reasoning based answer

 

[02SE]

I did some searching on this but couldn't find a quick answer.

So when in 2H, where is the power to the front wheels disconnected? Said another way, when shifting from 2H to 4H, which specific part engages to drive the front wheels?

My understanding is that in 2H, the road force (while rolling), turns the front tires and wheels (obviously), which in turn turns the front axles, which in turn turns the front differential, which in turn turns the front driveshaft.

So the entire front drivetrain is turning from road force while driving in 2H, but it doesn't have power applied to it. As soon as you shift the transfer case from 2H to 4H, engine power is transmitted to the front driveshaft via the gears in the transfer case.

Is that all correct? I wasn't sure if there was something in the differential that engaged or disengaged based on "input" from the front driveshaft as a result of putting power to the front driveshaft.

For example, with locking/unlocking hubs, the front driveshaft, differential and axles do NOT spin when rolling in 2H. But in the 4Runner, I believe the entire front drivetrain turns in 2H simply due to rolling roadforce.

Sounds silly just writing it out, but wanted to confirm.

Thanks,

Captain Obvious

The Part-time 4wd 5th gen 4Runner, and in fact most Part-time 4wd Toyotas (the exception being some base models that had manual hubs into the early 90's) going back to the 80's have an A.D.D. (Automatic Disconnecting Differential).

In left hand drive 4Runners like we get in the USA, there is an axle disconnect on the passenger side of the front differential.

The CV axles are connected to drive flanges at the hub (as opposed to manual or auto locking hubs). So the CV axles are always spinning when the vehicle is moving. On the drivers side, the CV axle just spins the side gears in the differential. On the passenger side the automatic axle disconnect prevents the spinning CV axle from also spinning the front differential carrier.

So in 2Hi, the CV axles spin, and the side gears in the diff, but that's it. The carrier holding the ring gear, and the pinion gear don't spin. The front driveshaft from the Transfer case to the front diff flange is also stationary.

So when you shift from 2Hi to 4Hi, the transfer case engages the drive for the front driveshaft, while the A.D.D. simultaneously engages the passenger side CV axle to the differential.

 

[Kistle]

What a spectacular answer from 02SE!!!! Wow!

That sounds really complicated, but I actually believe you are probably spot on! First of all, you couldn't make up an answer that complicated, and secondly, that is very similar to what I was hinting at above...i.e. the engagement of the transfer case sends power through the front driveshaft which in turn engages the axles!

There is no way, however, I would have come up with ADD, or the nuance of left vs right axles!

I need to think about that a little more, but seriously, GREAT explanation!

02SE, if you are still here, care to elaborate on why that is designed that way? And it is interesting that you indicate that MOST part time 4WD systems work that way.

Lastly...be honest....who else here knew that, LOL? I am an older guy who has been working on cars, including vehicle with locking hubs most of my life, and this is news to me!

Thanks 02SE!!

*** EDIT *** I now see that you specified that most TOYOTA part time 4WD systems work that way, not most part time 4WD systems in general! So is this a Toyota specific design? Very interesting. How do other part time 4WD systems work? For that matter, which other manufacturers still use a part time 4WD system? Jeep maybe? Seems like the vast majority are AWD? Or even full time 4WD?

 

[Hungryhawk]

Toyota did some clever engineering with the ADD.
Also clever is the use of brakes in the Trac and Atrac to mimic locking diffs with some advantages and some disadvantages.
All of it dependent on electrical components, so deep water crossings should be avoided.
The center torsen like gear box makes 4th gen and 5th gen Limited amazing trucks.