Clunk after going back to 2WD

Nomenclature... 4wd, 4x4, selectable 4x4, part-time 4wd... they're all the same. They are a 2wd to 4wd locking drive system. It doesn't matter what you call it, as long as you don't call it All Wheel Drive, AWD. That is a similar but different system.

 

You appear kinda' anxious to bail your trolling ass out of making stupid statements. Still waiting for the manual page number that says not top do whatever I do.

 

He was confusing 4WD, which we have with AWD which is full time all wheel drive.

 

Remember folks, this is what the OP is lacking.

 

Go from 4H to 2WD while moving... take your foot off the gas pedal then make the switch... no clunk! I think the clunk happens when there's a load on the drive line.

 

Driving circles while in 4wd is a great way to get clunks and more wear and tear on the drivetrain.

 

Unless it’s an AWD system because that’s a totally different beast…

 

Real men dont read manuals, nor ask for directions.....

 

You can get much louder clunks and a differential oil shower if you burn rubber while turning in 4wd also…. Can’t do that with a AWD!

 

Thanks for writing this up. It confirms and explains a lot of what I have noticed in my 2018 TRD OR since buying it a couple years back. My worst clunks have always been while re-engaging 2WD after making sharper turns in 4HI (sometimes several minutes after). It's easy to feel when the system is binding, but even if you take the turns at less-than-full-lock, you'll still get the clunk. It intuitively feels as if there is tension building in the system, but I never quite knew how to explain it. Your comment about "where does the tension go" is relatable, as I think about this every time it happens. It's obviously not good, but it can't be that bad in and of itself, either. All of the energy being released into the transfer case chain, and then loudly popping out makes sense to me.

I take my truck off-road fairly frequently. Usually just logging roads, but sometimes some loose dirt/mud, or larger rocks. I quickly realized you basically need to A) not turn too sharp when 4HI or 4LO are engaged (nothing really bad happens, but you can feel that the truck doesn't like it) and B) drive in a straight line for as long as possible before disengaging back into 2WD. On top of that, it's best to do it when not accelerating or decelerating, as you said. I have gotten the worst clunks of all while stopped, but I would not necessarily assert that disengaging in a straight line at 10MPH is any better. Would be interesting to test.

The one situation where I have trouble is when it is dry, and very cold (<20 degrees). I'm usually on my way to work, and it's usually rained a day or two before, so the prospect of ice on the road is real. In this situation I would like to use 4HI in case my tires slip while in a shady turn at speed. If you do not hit ice, you have essentially driven on a completely dry/sticky road for 20 minutes straight, up to 60MPH+, and through turns. If you do not touch ice, the wheels never get a chance to relieve the stress, and it was all for naught. I tend to clunk the worst when pulling into a parking lot under these conditions. You can feel the tension and lack of steering response in close quarters at low speeds. It is almost impossible to navigate a parking lot in 4HI but, if you do not turn it off while in motion a bit before the parking lot, you will have to do it either while stopped, or after some tight turns. Still figuring out how to navigate this one. Luckily, winter is almost over.

 

For those cold <20º F trips on way to work, you might consider experimenting carefully with going in & out of 4HI at highway speeds, using the straight sections where there is less need for 4WD traction as opportunities to relieve the bind-up tension accumulated during turns & curves. I.e. as curves / possible ice appear to be coming up, switch into 4HI, stay in it as long as needed, then at the next straightaway (without slowing down or stopping) switch back to 2HI on the fly to relieve tension.. then switch back in to 4HI at the next curvy/possibly icy section, etc. Rinse, repeat.

Depending on the length of drive this could put a lot of cycles on the electronic transfer-case & front diff actuators. As mechanical devices, with plastic gears at that, they have a finite number of working cycles. I don't know what the expected cycle life is -- 10K cycles? 50K? 250K? You could run some quick math on your use-case & see how much additional cycling your actuators would have to do. Anecdotally, most of the actuator failures I read about on TW here seem to be caused by water intrusion, not mechanical failure/wear of the electric motor + plastic worm-gear drive.

EDIT:
Some cursory research on the lifespan of small, lightly-loaded plastic gears yields a wide guesstimated range of anywhere from 10^5 to 10^8 cycles, depending mainly on the torque loading of the gear set. If Toyota engineers were reasonably conservative in designing the gear ratios to keep areal loading of the gear teeth/spirals to moderate values, a figure of 10^5 (100,000) cycles seems reasonable. Of course a lot depends on the exact type of plastic; GRP (glass-reinforced polymer) is going to last longer than plain nylon, for example. I've only been inside a Tacoma transfer-case actuator once. As I recall, the gear plastic seemed like pretty ordinary white (?) nylon, not GRP (glass reinforced polymer.)

In the case of switching frequently in & out of 4HI while travelling straight at moderate to high speed, I think that operating regime is probably among the easiest / lowest torque loading for the actuators because the moving parts & actuator sleeves/forks in the front diff & transfer case are warm, lubricated, and in-motion. (Compared to say, trying to switch out of 4HI while stopped mid-turn in winter weather after a brief drive on dirt road, with the gears under tension, at rest, & their lubes very cold.)

As for the electric motors, 100K cycles of ~1 second each is only 27 hours of accumulated run-time. A decent-quality small electric motor should be able to handle that without a problem. Given how infrequently these motors are used it's no wonder they probably fail much more often due to moisture corrosion than to operational wear-and-tear.

https://gearsolutions.com/features/experimental-tests-to-predict-plastic-gears-lifetime/
https://www.mdpi.com/2073-4360/14/10/2064
https://www.geartechnology.com/ext/resources/issues/0585x/Chen.pdf

 

Idk, when I had my gen1 tundra you couldn't engage, or disengage 4wd if under power. With my tacoma it doesn't matter if on the gas, or not. The 4wd system in these trucks work seamless with zero noise.

 

i think they mean AWD not 4WD.