For anyone who is unfamiliar with the 4wd handling phenomenon or could use a refresher, here's an attempt at something clear and complete. Otherwise, skip this part.

With a locked center connection -- part-time 4WD (classic 4WD) or full-time 4WD with center differential locked (cdl on) -- the front and rear drive shafts rotate at the same speed, just as if there were one shaft from front to back. During a turn, all four wheels turn in a different size arc and, thus, different speeds. This can't be compensated for by the front and rear diffs because when each side of the diff has traction, any increase in speed on one side is matched by an equal reduction on the other side. Since it's the average speed of the two front wheels and the average speed of the two rear wheels that matters, the effective result is a comparison of one arc for the fronts and one arc for the rears. So, during a turn the rear wheels follow a smaller arc (1 averaged arc) than the fronts and want to turn more slowly. Since the locked drive shaft in between can only rotate at one speed, opposite forces are created between front and rear, with the braking force (skid) being in the front and the drive force (slip) in the back. With same size tires equally inflated, this conflict is near zero when the vehicle is going straight, but starts to come into play as the steering wheel is turned, starting out mild but quickly becoming more severe as the turn is made tighter, resulting in an increasingly heavy feel in the steering wheel and a sensation of brakes being applied. Near the extremes, tires can audibly and visibly skip and skid. On slippery surfaces, this can occur more easily but on pavement more stress is imparted to the drive train, especially at very low speed where releasing energy through the tires is more difficult and there is no momentum to unweight the inside tires during a turn. It also results in handling changes that usually make the car want to turn a little wider (understeer) than it normally would, due to the tendency of a front wheel to skid. (The words bind and windup, in combination with various parts of the drive line - differential, transfer case, axle, transmission - are terms often used to describe this phenomenon. In other domains, a separate phrase - Tight Corner Braking - is also widely used).
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I experimented with this quite a while back, but thought I'd finally write it up.

Too much reading, what's the gist? That the center diff lock button is a useful tool for turning off engine suppression because the common warnings about pavement use in locked mode are over-simplified and often exaggerated. The steering effects in locked 4wd are likely milder than many have assumed (much milder, for instance, than the effects of a locked rear diff) making the occasional, rolling street turn nothing to be concerned about. Some owners of '03 thru '07 models may, at times, have wanted an easy way to briefly turn off engine suppression near pavement but have been hesitant to use the diff lock button due to abundant warnings against it. This thread is mostly for them.

(A shorter, non-4th gen specific version of this thread is here)

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Disclaimer: Anyone who is uncomfortable with any of this should just follow the rule and not drive center locked on pavement. That's the safest route. If this thread makes you unsure and you would like something to make it easy to dismiss, read this.

My incentive to look into this came from wanting to establish for myself the extent to which the CDL (center diff lock) button was useful as a VSC off switch (to turn off engine suppression) since, in 4th gens up to 08, the two come stock as a package deal. Some people had been known to complain of engine power being cut too much in certain slick conditions (possibly more sensitive in later years). That's a potential problem (that, in my testing, is fixed by reasonable throttle) because conventional wisdom on the appropriate use of a locked center severely limits the CDL as a simple solution since the prospect of nearby dry pavement is supposed to make it too risky to use. For example, turning from a slick side road onto a mostly dry, paved road.

Why not just do the VSC off mod and forget about it? Great mod, no doubt, but not everyone may want to do it, and center locked is still the most capable traction mode. The road conditons that may call for defeating VSC are rare and characterized by very low traction. In that situation, I probably want to go all in.

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Obviously, turning the steering wheel, say, an inch away from straight won't create any meaningful rotational difference of the unlocked drive shafts. So where does the meaningful difference start and how can it be measured? When does drive line stress become too great to risk? Barring expensive testing with strain gauges and force meters or, better yet, statistical analysis of drive component wear on large numbers of isolated test vehicles, trying to gauge the effects remains an exercise in uncertainty. It seemed, though, there must be a few practical tests and simple calculations that could shed a little more light.

With apologies to Carl Sagan; extraordinary claims require the maker to put his money where his mouth is. In that spirit, here are a few verifiable data points established from behind the wheel of a full-time 4th gen 4runner with center diff locked on pavement. Full steering lock is about 1.5 steering wheel revolutions (SWR) one direction (slight difference between left and right).

(1) With a little speed and neutral throttle, steering effort at around 1/2 to 3/4 SWR is almost indistinguishable from unlocked 4WD. Adding throttle will induce heavy steering a little earlier. (This would be a better test without power steering).
(2) After driving long enough to execute a few such turns, the center diff unlocks easily. More so, in fact, than after a 40ft circle on dirt or snow*. This is relevant because a well known effect of a bound up system is one that gets temporarily or completely stuck in center lock position. (*Driven 20-30ft afterward to get the wheels straight)
(3) In neutral, with enough speed to coast a little, noticeable braking effect doesn't come on until about 3/4 SWR.
(4) (The smoking gun) The center differential will easily lock/unlock while moving at 1/4 SWR (not recommended). It will lock/unlock at 3/8 too, but starts to get a little clunky. How much farther it might go I'm not willing to find out, but 1/2 SWR doesn't seem out of the question. So, however meaningful those drive shaft rotational differences may be to locked drive line stress, they don't seem to be very meaningful to the locking mechanism, which I presume is a simple dog clutch that waits for nearly identical rotational speeds before it can engage. How close are the rotational speeds? Front/rear drive shaft rotational differences start out small but ramp up quickly: (margin for error should be reasonably small)

1/4 SWR ≈ ..0.3%
1/2 SWR ≈ ..1%
3/4 SWR ≈ ..3%
1.0 SWR ≈ ...6%
1.5 SWR ≈ ..15%

It's interesting to compare against the much higher numbers for the rear diff -- 4%, 9%, 15%, 22%, 40%.

Most street turns can be completed with 1/2 SWR or less. Sometimes 3/4 is required but when that's the case the wheel spends only a moment in that position with the average position through the turn being much less. Consequently, I view the diff lock button as a useful solution to road scenarios that may require turning off engine management.


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If someone decides they're comfortable with the described usage, what are some other considerations? Tire/drive train wear, and fuel consumption are accelerated, but this shouldn't come up enough for that to matter much. Handling will change overall, but most importantly on slick surfaces where the tires will be induced to lose a little traction in turns, reducing steering precision and usually inducing understeer, but, per the above, it should be pretty minimal at around 1/2 SWR. Familiarization with the handling ahead of time is a good idea. When understeer occurs during acceleration it can be especially disconcerting to the unprepared as the vehicle traces a wider path than expected. Steering angle is the most important factor, but even relatively straight roads can present problems if used too long at high speed. The stresses are small, but they are constant under high power output, and transfer and diff fluid temperatures can rise higher than normal if used this way too long. Doubtful anyone would choose to do this intentionally, but forgetting the diff is locked is a possibility.

Other things can affect level of drive line stress -- tire pressure, tread type, pavement condition etc -- so a crisp rule isn't possible but it seems hard to go too wrong around 1/2 SWR with the wheels rolling at a little speed. Steering feel remains a good direct measure though power steering gets in the way of that a little. Just knowing the steering/drive shaft percentages, I imagine, will increase confidence enough for some to put a toe in the water.

So, if you've ever wanted to use the diff lock button on or near pavement to avoid engine suppression, this might be of some interest. I'm not out to convince anyone who doesn't want to be convinced. If anyone feels this subtracts from the value of the Torsen center diff, that's not my view at all. The pairing of the Torsen with a diff lock that engages and disengages quickly, on the fly, hi or lo, is a fantastic combination in my book. Seems a shame to waste that combination where it can be put to good use. (Not only on-road, but off, per point 2, above)


Here's how the measurements were taken. Turning circles were determined by a combination of parking lot step-offs and gps track circles.

Great write-up and explanation.

But... I believe you are way over thinking the whole stress on the drivetrain while locked. My 43rd gen has the old style transfer case that has no differential and is fully locked at all times.

When I'm wheeling in 4wd, there is bind there all the time. Never do I think about how far I can turn the steering wheel before I get driveline bind. This includes 4Lo on Moab's slickrock. I'll go thru an obstacle in 4Lo and sometimes have to go full lock on the steering to follow the trail. On slickrock I can feel the driveline bind and tires will be barking thru the turn (which for those that have never seen/wheeled slickrock, it is NOT slick and has more traction than your average road). I have done this hundreds of times with zero issues.

Also if I haven't wheeled for a while I will kick it into 4Hi and drive on dry pavement. No problems. I know someone that drove home in 4Hi with aired down tires on the highway without an issue. He said it did feel kind of funny, heavy steering. I wouldn't recommend doing this but again, things are tougher than most think.

I'll answer here since I'm going to tout the 4th gen drive train.

No disagreement at all, just a couple of points.

Slick rock can have a lot of grip but rarely is it both level and smooth like a road or parking lot and the uneven weight distribution makes it easier for the drive train to release stress. The lockable Torsen center needs to be lived with for a while to be really appreciated. I suspect if you had easy steering available on a tight, off-road turn with the flip of a switch, you'd often find it hard to resist. Not only easier steering, but more precise, too, for, say, having fun avoiding rocks and bushes in a fairly fast, twisty wash.

It's often mentioned about how having a rear locker reduces trail wear. The reverse holds true for the center (having unlockable). Those tight turns don't chew up the ground as much.

Small points but worth a mention.

Thanks for the input on your experience. I agree, the trucks are pretty tough.

I definitely agree that unlocking the center/transfer case would be nice. This is not an option for me. My point/experience is that the tires slip/bark without the drivetrain experiencing and damage. True, while wheeling, the trail is uneven but not always. Sometimes it is as flat as a road (talking slickrock).

Hard to beat personal experience especially when it's time tested such as yours. No doubt your drive train is plenty up to the task which, considering all the slick rock you've done, is saying a lot. You have the luxury of being able to brush off pages such as those in the disclaimer, but I can tell you, a wide look around the net reveals no shortage of opinion similar to it.

I value the kind of input you have provided quite a lot and I tend to skew in that direction more than most in the 4th gen forum, evidenced by the fact that I wouldn't hesitate to use the diff lock on pavement to shut of engine suppression, which is what this thread is all about. I have a huge number of hours off-pavement in this truck but the hours spent in hard, technical wheeling are no doubt a small percentage of yours. If that was not the case, and I had put in those hard hours using locked 4wd, many of them on slickrock, I'd no doubt feel exactly as you do.

Thanks again for the input.

I appreciate the data that went into this but by no way do I get how it simplifies locking the center diff.
The topic is diff lock on dry pavement, right?
If i am on a snow covered driveway or road w maybe 6 of snow, turning onto dry pavement, I'm in 4Hi or maybe 4Lo at a dead stop w center diff locked, turned wheel at full lock.
I spin through the snow and maybe there is a bind for the length of the 4runner getting onto the dry pavement, maybe 2-5 seconds where the wheels bark then I unlock the center diff having completed the turn, the truck is straight on the paved road and I'm done. Unlock the diff and drive on.
I don't get where that is really overworking the power train as I have done that move, honestly every single time it has snowed since 2005 w/o ever having a problem.

I can see why it doesn't simplify it for you. It's meant to help someone who has considered doing it but may have been hesitant due to abundant warnings against it.

I'm approaching this from the middle of the spectrum of opinion on how risky it may be. If you want to see the other side of the spectrum, which, in general tone, suffers no lack of representation on forums around the internet, including this one, read the link in the disclaimer.

Thanks for the post, though. It gave me an idea for a small edit to improve clarity.

Speaking of edits, if you click edit and remove all the stuff except your post, it might look better. Just sayin'

Good stuff!

I understand the point of this post! I am one of the people who isnt happy with the engine suppression. I live kind of up in the mountains and while we did not get all the snow we really needed it did snow and get cold enough where it was snow packed on the roads for a month and a half. This eventually made some slick roads. I do not have the all time 4x4 so I would only go into 4 hi when I was having trouble with traction. After reading this and other posts similiar to it i feel a little bit more educated as how and when I should use my center diff lock. Before I was frustrated because sometimes I would just spin 4 wheels and engine suppression would leave me at a stop sign irritated. Being lighter on the throttle helps but not always. After reading this I will be using my center diff a bit more so I can really drive the t4r to it's full potential. Thank you for the explanation.

crickey that was a lot !

On a snow covered road, I happily do the "VSC disable mod" by simply locking the center diff and have at it with some 4wd sliding fun.

Good to know about when the roads are covered in snow in one spot and wet from salt, or even dry in others.

Incredibly useful write up (yes I'm thanking you seven years later). I am having a fascinating issue with my 4wd where it will only engage if I disable RSCA. I'm baffled.

Appreciated.

I haven't regularly read the forum for 4 or 5 years, but did so constantly for a decade before that. Never heard of the issue you mention. I'd be inclined to test it many times to make sure it's not random.

But if the phenomenon is real, it is indeed an odd one.

Hope you get it sorted out.

The buttons for the RSCA and Center diff lock wires are swapped. Reach back there and pull the harness/wire out of the buttons, swap them. Voila!