Please explain SR5 4-wheel drive system

[36bound]

Hi all, I'm new to this forum. I've been a Jeep Wrangler owner for years, but I'm about to place an order for a 2012 SR5. Remarkably, not one of the salespeople I have dealt with (or a service writer) was able to answer a few questions that I had about the SR5's part-time 4-wheel drive system. As we are aware, most "4-wheel drive" systems are not actually true 4-wheel drive systems. Even in the Jeep Wrangler line- with the exception of the Rubicon which permits both differentials to be locked-in.

In the 2012 SR5 model, a locking rear differential is NOT available as an option (nor is it standard). This was a bit of a let down to me, but not a deal breaker. But this state of affairs does give rise to the following three questions about the SR5's drivetrain/4-wheel drive system.

1. When in normal drive mode (i.e. not in 4-high or 4-low) which wheel(s) is predominantly driving the vehicle? Is it the right rear? Both rear?

2. When in 4-HIGH which wheel or wheels are predominantly driving the vehicle? Is it the left front & right rear? Right front and right rear? All four wheels?

3. When in 4-LOW which wheel or wheels are predominantly driving the vehicle? Both front wheels only? Both front wheels and the right rear? All four wheels?

Remember, the SR5 is NOT available with a locking rear differential. One would think that a salesperson would be able to explain something as fundamental as how the part-time 4-wheel drive system works on a $38,000 rig.

 

[Hyde802]

1. When in normal drive mode (i.e. not in 4-high or 4-low) which wheel(s) is predominantly driving the vehicle? Is it the right rear? Both rear?

2. When in 4-HIGH which wheel or wheels are predominantly driving the vehicle? Is it the left front & right rear? Right front and right rear? All four wheels?

3. When in 4-LOW which wheel or wheels are predominantly driving the vehicle? Both front wheels only? Both front wheels and the right rear? All four wheels?

Remember, the SR5 is NOT available with a locking rear differential. One would think that a salesperson would be able to explain something as fundamental as how the part-time 4-wheel drive system works on a $38,000 rig.

Ill try for a stab at this. When driving in normal 2 wheel drive, because of the open diff, whichever rear wheel has the least amount of resistance will get the most power. Just like in your jeep.

When in 4wd (do these have a center diff?) same thing applies, power goes to one wheel in the front and one in the rear with the least resistance. If both fronts were turning, you would have a locker. Also, very similar to your jeep.

Hope that helps.

 

[BucknutBob]

To add to what Hyde mentioned, with an open diff, the power is equally divided between the wheels being driven as long as traction is equal. Once traction is broken to one wheel, the power is transferred as previously mentioned. No wheel is dominant, or the specified "drive" wheel--it simply depends on the amount of traction available. With a limited slip diff, the transfer is not total--some power is still available to the wheel with traction when one is slipping, although not an even split. With a locker, the power remains equally divided no matter what the traction situation is--even with one wheel completely off the ground, the power remains constant to both wheels (the only TRUE four wheel drive set-up). A spool is the same as a locker with regards to power transfer, only it has no ability to differentiate speed around corners--both wheels are permanently locked together in the diff.

 

[JB.]

Please explain SR5 4-whell drive system

It's traditional 4WD (front and rear drive shafts rotating as one) with the brakes, when needed, providing resistance via pulsing (ATRAC) to control the slip of individual wheels on the front and rear axles.

So it comes down to understanding basic 4WD, open differentials, and the fact that ATRAC is Toyotas designation for the brake pulsing system.

http://4x4abc.com/4WD101/

 

[KidVermicious]

Just in case it wasn't clear, OP, a system does not need locking differentials to be a "true" 4wd.

 

[BucknutBob]

Not sure if you were referring to MY use of the term "true" or the OP's, KidV, but my purpose for using it was to point out that there is only one way to have full power to all four wheels regardless of the situation, and that is with lockers. With stock open diffs, once one side of the vehicle has lost traction, it might as well be a 2WD (one front and one rear wheel), because the two with traction are basically dead. Just a clarification for my part.

 

[KidVermicious]

Not sure if you were referring to MY use of the term "true" or the OP's, KidV, but my purpose for using it was to point out that there is only one way to have full power to all four wheels regardless of the situation, and that is with lockers. With stock open diffs, once one side of the vehicle has lost traction, it might as well be a 2WD (one front and one rear wheel), because the two with traction are basically dead. Just a clarification for my part.

Yeah, I was talking to him. I figure you probably know how your 4wd works. :thumb3:

 

[Ross]

In the Jeep you have a transfer case, that you use to select between 4 wheel drive, hi or lo, and 2wd. In the 4 runner you have a center diff. It allows power to move around where needed. It is a very good system. It basically behaves like a limited slip diff would in an axle.

You also can lock this diff and then even power goes to the front and rear axles, this would be just like a selectable locker in a differential on an axle.

There is a traction control system that works with the brakes and motor. When you lock the center diff you still have some traction control that may get in your way. It is a great system for some applications and horrid for others. In the previous generation you can disable this function by unpluging a wire. I have never done it but it seems simple.

 

[BucknutBob]

Ross, what you're describing is correct only for newer models--all older ones (not sure when they changed, either 3rd or 4th Gen) used a straight transfer case design just like the Jeep you described--no locking necessary.

 

[Ross]

Ross, what you're describing is correct only for newer models--all older ones (not sure when they changed, either 3rd or 4th Gen) used a straight transfer case design just like the Jeep you described--no locking necessary.

The 4th generation does use the center diff.

 

[Ross]

True 4wd drive to me means power going to all 4 wheels all the time, all 4 wheels drive or power the vehicle. Without lockers this doesn't happen all the time. One tire can spin while the other tire on the axle does nothing. If you are in 4 wheel drive and two wheels are spinning (one on each axle), and the other 2 are doing nothing you don't have 4 wheels driving you vehicle, therefore you do not truly have 4wheel drive.

 

[JohnsD90]

3rd gen was the transition, I have a normal transfer case but by 01 they had center diff.