Clunk after going back to 2WD

Stock 2017 Limited highway princess and I seldom have the need for 4WD. Whenever I have the opportunity to "exercise" the 4WD like on sand or logging trails I drop into 2Low and drive in circles and left and right turns all under 5MPH for about 10-15 minutes. This last time while on a beach in Florida, after stopping and going back to 2WD, once I accelerated very slowly, there was a loud and noticeable "clunk" that felt like it came from the transfer case area. Truck drove fine, but I'm wondering what that sound was. Thoughts, please?
Also, can I drive in either 4WD modes in reverse?

 

Clunk is normal, they all do it, time to time. No need to go into 4 low, or drive in circles. Straight line in 4hi is fine. Yes you can drive in reverse in 4wd.

 

Probably an engage from the transfer case?

Yes, you can use 4WD in reverse.

 

The clunk is normal. Sometimes it happens, sometimes it don't.

Why would you not be able to drive in reverse while in 4wd?

 

With selectable 4wd, the front and rear driveshafts will be locked together. When you make turns the front and rear axles will travel in different arcs. Even in loose traction areas, this will create tension in the system from front to rear. When the 4wd is disengaged this tension will cause the locking cog to bind until the pressure to pull it out overcomes the driveline tension. When this happens you will hear a clunk.

You can also get the clunk when you are accelerating or decelerating while switching the 4wd drive system.

To relieve the driveline tension and reduce the clunk, it is good to drive a straight line for a few meters on a loose traction surface and disengage when the not accelerating or decelerating.

 

Try not to bind the front end by turning too tight in 4WD. Clunk is normal.

 

Why driving in circles? Not needed. Yes 4Hi and 4Lo work in reverse also.
The clunking sound was probably just from the transfer case and its normal especially if there is a little binding going on with the axles.

Also its 4x4 not 4wd those are two different systems.

 

Folks really gotta read their owner's manual before hitting up TW lol

 

That would involve effort

 

It is Friday

 

I'm guessing it's normal, I had a 2001 tundra that did the same but worse....sounded and felt the drivetrain was falling out of the truck when engaging and disengaging from 4wd yet the truck always ran fine. However my 2020 tacoma is totally silent. Only way to tell its in 4wd by the green dash led...same when your out of 4wd..no green led.

 

I've got the manual on the desk next to me for when you reply so you can inform me of what page the answers to my questions are.

 

Be cause in the manual that some of the dickheads who troll this forum claim is not read nothing definitive is stated about doing so. Why would anyone take a chance of fucking up a perfectly good truck by doing something with it without prior knowledge or experience doing so?

 

I have done this with great success with all my 4wd trucks over the years. Whenever possible baby that engage/disengage point and life is good.



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Why does it state in the manual, "four wheel drive system" and not 4X4? I found that fact on pages 247- 250 where instructions on how to operate the system are printed. Which, by the way, I have read and must have missed something you picked up. Please enlighten me?

 

Thank you.

 

Disclaimer: I'm not a professional mechanic or an engineer of any stripe.

There's no need to make sharp turns at low speed to exercise 4WD mode. Also, there is no need to engage 4LO per se. Operating in 4HI, with or without turning, will exercise the transfer case shift mechanisms and fully lubricate all the gears and seals. (Ok, technically it's probably a good idea to engage 4LO at least 1 time out of 10, just to exercise the 4LO limit switches & wiper arms, and the 4LO shift fork, but it doesn't affect transfer case seals / gear / chain lubrication, nor front ADD actuator / gear exercise.)

I've found that I can exercise 4HI mode on dry asphalt at freeway speeds (60 mph) as long as 4HI is engaged while the vehicle is travelling in a straight line, and then disengaged, while still in the same straight line, with no more accumulated turns beyond perhaps couple of ordinary single-lane changes. Here's a thread I wrote about why and how I exercise 4HI on dry, straight highways from time to time. It can come in handy if you don't have easy access to dirt roads or a dirt parking lot somewhere nearby.

https://www.tacomaworld.com/threads/4x4-maintenance.788617/#post-28228936

An additional note regarding the frequency & loudness/strength of the 4WD-disengage "clunk" sound - it may seem totally random, but it's not. It is a function of how much tension has built up in the drivetrain (the entire system of gears, shafts, splines, & U joints connecting all 4 wheels) during turns when 4WD (4HI or 4LO) is engaged. This tension starts to accumulate the moment the front tires turn left or right while the vehicle is moving.** (**In the very-uncommon case of a Tacoma or other part-time-4WD vehicle having different-diameter front/rear tires, tension starts accumulating immediately, even in straight-line travel, because the front/rear driveshafts are always turning at different rates if the front/rear tires are not equal diameter.)

As the vehicle turns, the front tires travel slightly farther than the rear tires for a given amount of overall forward vehicle motion. (This is a matter of simple plane geometry.) In order to travel a little farther, the front driveshaft has to turn a little farther (more spins), compared to the rear driveshaft. Because the two drive shafts are locked together (by gears & a chain drive) inside the transfer case when 4WD is engaged, the slight difference in rates of spin between front & rear driveshafts results in tension building up between them.



The sharper the turn, the greater the difference between how far the front wheels must travel (the blue circle tracks,) and how far the rear wheels must travel (the yellow circle tracks.) The sharper the turn, the faster the front driveshaft must spin to keep up with the front wheels as they turn, and the faster the tension between front/rear driveshaft accumulates. (For comparison, picture the above diagram w/ vehicle going straight -- in that case, yellow & blue tracks are perfectly aligned, both driveshafts spin at the same speed, and no tension accumulates.)

On loose/low-friction surfaces like snow, sand, and dirt roads, when the tension built up in the drivetrain exceeds the available tire traction, the affected tire(s) will slip (aka skid or "chuff") on the surface to relieve some (or all) of tension.

Alert / sensitive drivers can hear/feel this from inside the cabin, and someone observing the truck from outside can watch it happen as some tires briefly stop & skid against the loose driving surface. (It also leaves characteristic 4WD "scuff" marks on many surfaces, i.e. dry dirt roads.) (Whether only "some" or in fact "all" of the tension is relieved at any given moment, depends on just how much slip the affected tire/wheel can achieve before it "bites into" the road/driving surface again.)

If turning continues (or resumes, after driving straight for some distance) drivetrain tension immediately starts to build up again until it reaches the threshold at which the stored-up tension forces in the drivetrain overcome one or more wheel traction forces, and the affected wheel(s) again slip/skid briefly and the tension is again released.

This cycle of build-up & release occurs over and over as long as the vehicle is making significant turns on a loose road surface -- it does not occur when the vehicle is driving straight. This is how part-time 4WD is designed to operate.

Having a loose road surface which allows fairly easy tire/wheel slip is very important to the correct, safe, long-term reliable operation of the drivetrain. The Toyota owner's manual for any part-time 4WD vehicle (which is, as of this writing, all of the Tacomas & earlier 4WD pickups Toyota has ever made, regardless of whether automatic or manual transmission, up to & including the 2023 model year) states that it is not advisable to drive on hard/sticky surfaces in 4WD for any length of time. That's because if the wheels cannot periodically slip (on the road surface) as needed, to relieve front/rear driveshaft tension -- the tension can & likely will accumulate, and could put excessive stress on the gears, shafts, U-joints, etc.

Driving around a dry asphalt parking lot at 5 or 10mph in 4WD in a Taco, sometimes you can hear the tires chirping or chuffing on the asphalt as they periodically slip to relieve stress. But the point at which they finally chirp & relieve that stress is considerably higher (more tension) than on a loose/soft road surface. Driving at higher speeds in 4WD on dry pavement while making sharp turns has the potential to accumulate drivetrain tension much more quickly than at low speeds, putting more stress on drivetrain components. (All of the torque-transmitting shafts, splines, gears, chains, & U-joints between the tires & the transfer case. The tension acts on all these parts simultaneously since they are all locked together when 4WD mode is engaged. )

In the interest of having all parts of drivetrain last as long as possible, I don't force a part-time-4WD-engaged drivetrain to fight against the much-higher traction forces of tires on dry pavement, by making turns in 4HI (or 4LO) on dry pavement.

I have experimented with 4HI at freeway speeds (>60mph) (on asphalt pavement), on roads with lots of curves, in very heavy rain, and found that even with water sheeting across the roadway, there is still not enough tire slip to easily & safely relieve driveline tension in 4HI. After just a few thousand feet of travel through a turn or two, upon disengaging 4HI, there is the familiar "thunk" of driveline tension being suddenly released. Wet tires, –- at least, the ones I drive on, stock 2014 TRD OR 265/70/R16 all-terrains –– on hard asphalt aren't slippery enough to allow the fairly-easy periodic release of driveline tension.

Recapping what I said earlier in this post, I've found that the key to doing convenient, on-the-fly 4HI system exercise, even at highway speeds, on dry, hard pavement, is to do it on long straight sections of road, only. Engage 4HI, drive in a straight line.. keep driving straight.. as soon as a significant curve appears up ahead, disengage 4HI before entering the curve.

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Having the above understanding, depending on the road surface, amount of turning, & speed of travel (i.e. say, under 15mph on a dry, pretty-flat dirt road with the windows down, with wind & engine noise low, so it's easy to hear the tires,) it is possible to feel and/or predict whether there will be a hard "clunk" when disengaging 4WD. It depends mainly on how far the vehicle has travelled, while turning, since the last tension-relieving slip of the tires.

The element of apparent randomness that comes in to play -- where sometimes 4WD-disengage doesn't clunk at all, sometimes there's a little clunk, sometimes a bigger one, sometimes it won't disengage at all because there's so much accumulated tension -- comes from the randomly-varying amount of friction at each tire as it moves across the surface. Esp. on loose/slick surfaces, some tires may already be slipping before driveshaft bind-up even starts to occur. So, sometimes sharp turns which just moments earlier caused bind-ups on other parts of road might cause little or no bind up, at least through that particular turn.

The key to overcoming most of this seeming "randomness" of clunk is to get accustomed to how even small amounts of bind-up start to subtly change the steering feel. If there's any beyond-normal tension in the steering (while in motion,) there is going to be some amount of "clunk" when coming out of 4WD.

The reason some people helpfully suggest continuing to drive in straight line at least some distance (maybe 10 to 50 feet, depending) after a turn, before disengaging 4WD, is that by keeping the tires rolling, the friction on them is lower than when stopped, making it easier for one or more of them to slip or "chuff" as needed to release accumulated drivetrain tension before disengaging 4WD.

(If instead you just come to a dead stop in the middle of a sharp turn in 4WD, chances are fair that driveline tension has not had a chance to "chuff" any tires, and now cannot chuff any of them because they are no longer in motion. When you then disengage 4WD, assuming the electric actuator motors have enough torque to push the transfer case shift fork against the stored-up axial (torque) tension in the driveshafts.. the "clunk" happens as the transfer case mode-locking gear collar disconnects under tension, suddenly "popping free" off the shaft gear teeth, instead of smoothly & silently disengaging without force.

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A decent video explaining and illustrating how & why part-time 4WD transfer front/rear driveshaft tension (aka "wind-up," "bind-up", "transmission wind-up," "4WD windup," etc.) occurs. The guy titled the video "Transmission Wind Up," which isn't as accurate as it could be. The tension does not originate in the transmission -- it originates from the difference in speed between the front and rear drive shaft in a part time 4WD drivetrain. But aside from his sloppy titling, the rest of his explanations & demonstrations are pretty clear and accurate.

https://www.youtube.com/watch?v=1EHcrjIlkHk

These additional videos may help people deepen their understanding of how Tacoma (and Tundra) part-time 4WD systems operate, mechanically and electrically. The videos show Tundra equipment but all the parts are almost exactly the same on Tacos, just sized smaller for the lesser vehicle weight & torque handling needs.

At 14:47 in this next video, the lecturer demonstrates the transfer case shifting into 4-wheel drive (HI.) The "Front Drive Clutch Synchronizer Sleeve" slides down, locking the front & rear driveshafts togther to produce 4WD operation.

Later, at 17:45 in this next video, he demonstrates the transfer-case actuator motor shifting the transfer case from 4WD back to 2WD (disengaging 4WD.) This "Front Drive Clutch Synchronizer Sleeve" slides back up, disconnecting the front driveshaft from the rear driveshaft. It is this part through which any accumulated front/rear driveshaft tension will initially release.

Having understood why part-time 4WD drivetrain tension occurs, and where/how it is released, I don't yet know exactly where the accumulated tension goes during a "clunk" release event. Maybe that giant, beefy-looking transfer case chain (24 parallel links in the Tundra version, 16 links in the Tacoma version) absorbs nearly all of the tension release, going from super-tight to completely slack in a fraction of a second, slapping somewhat hard against the outside walls of the transfer case? That seems like a good way to safely absorb thousands of cycles of driveshaft-tension release shock over the life of the truck. Or maybe most of the force travels through the driveshafts until it is stopped by the pinion/ring gears in the front & rear diffs, and to a lesser extent by backlash in the transmission output shaft/gear assembly?

https://www.youtube.com/watch?v=4UoBCIjWIoU



https://www.youtube.com/watch?v=47m7QAPrpsI

 

This guy does a good job explaining the 3rd gen Tacoma 4wd system.

 

If you don't wanna fuck up "a perfectly good truck" don't drive in circles with 4x4 engaged, for starters.

Seems we've all educated you tho. You're welcome.

 

So you read the manual and still decided to drive in circles to exercise the 4x4? Interesting reading comprehension skills friend