Loud thunk when trying to engage 4wd

Question: Where do you live? This sure as hell doesn't apply to where i live.

 

My truck also does "thunking" when I flip it into 4 high sometimes. I think it's from the front drive line and tires slipping maybe, it just makes a loud noise as it suddenly engages.

 

Ditto. I guess I've been doing it wrong for 30 years. 40MPH on JUST snow (not ice) is fine and 4WD can help. Ice is a totally different thing and not the subject of this thread (presumably), because if he was going 40MPH on ICE, he wouldn't have to worry about the clunk because before long his truck will be in a ditch totaled.

 

I couldn't leave the house for 5 months if it wasn't safe to drive in 4HI over 20 mph. Holy balls. Snowy, icy roads are meant for 4HI.

 

That isn't true at all.

 

Mine only clunks after disengaging 4hi at a standstill and then accelerating. Disengaging while moving with gas out and clutch in seems to do the trick.

 

I noticed my 12’ clunked a few times during the Texas storms but largely in part to my slow speeds and turning. Not very good to go full lock on 4hi

 

Update: took it to an off-road shop here in Utah and found the vacuum line was unplugged on the actuator and all is full of dirt so getting it replaced should do the trick they say

 

I got the loud clunk this past January. Great timing for NWPA weather being in the snow belt. Was at the end of my driveway and needed 4WD to get out safely as I hadn't a chance to plow. Loud clunk, got out of the driveway and immediately took it out of 4WD. Started moving slowly down the road and wanted to see what it'd do if I went back into 4WD. Another loud clunk and then some lighter clunks. Tested a few times after and the same thing. Also, 4WD would not engage. Took the truck to my mechanic, he looked at it and ended up calling the local Toyota dealership. They told him it was the Transfer Case Actuator and that it was about $1300 just for the part.

I've since been doing some searching and it appears I can get the part for $150-$200 and appears to be fairly simple to replace. I'm just wondering if anyone here agrees with the diagnosis? I'm looking to trade the truck in and would like to get this fixed first.

Thank you.

 

To those who are having 4WD HI and/or LO engage / disengage issues: it can save considerable money & wasted effort if you take the time needed to understand how the moderately-complex part-time 4WD system in 2nd gen tacos works. It's pretty common for dealerships & mechanics to misdiagnose 4WD. The best way to reduce the risk of them doing so is for you to understand in detail how all the 4WD parts and systems interact. This thread about 2nd gen 4WD is excellent: https://www.tacomaworld.com/threads/4wd-system-operation-help-thread.742420/

If you don't already understand what 4WD "binding" or "wind-up" is, explainer videos like this one will go a long way towards helping you build a fundamental understanding of why an otherwise-normally-functioning part-time 4WD system may be difficult (or even impossible) to disengage in certain circumstances. (Note: the video calls it "transmission windup" but it's actually "transfer case windup".):

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

As to safe speeds for engaging 4HI: I routinely exercise my 4HI by engaging and disengaging it while going in a straight line (very important) on flat, dry asphalt at freeway speeds -- i.e. in-gear, cruising at 60MPH, with light accelerator (i.e. at the typical 2.2K engine rpm for 60MPH in 6th gear in a manual-trans taco with stock gearing and 265/70R16 tires.) I have never experienced any clunks, jolts, bangs, steering disturbances, etc. when exercising 4HI in this manner. When doing this on dry pavement it is important to disengage 4HI while still on a straight section of road. (Before any significant curves / turns are needed.)

Note, I would never try to engage 4HI at high speed if there is significant torque load on the drivetrain, i.e. accelerating up a steep incline, towing a heavy trailer, etc.

I have experienced 4HI disengage clunk sometimes at low speeds, if there is bind-up in the transfer case due to my having recently made unrelieved turns. (Unrelieved = tires did not chirp or scuff to release drivetrain tension induced by differing wheel speeds.) This is probably the most common source of 4WD clunk.

I suspect, but don't know for sure, that there may also be 4WD "clunks" related to the front propeller shaft slip joint (aka spline joint): If 4WD doesn't get used regularly I wonder if it's possible that this joint gets sticky and/or dry, possibly causing it to go "clunk" because the joint either doesn't have enough grease, or what grease is there isn't evenly/fully distributed within the spline joint.

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There are some cases where it is perfectly justifiable to use 4HI at speeds of 35-50mph, for example on 2-lane, reasonably flat, not-too-twisty dirt roads where there is plenty of safe maneuvering room and good visibility, and extra traction is needed. Other examples would be driving in light snowy conditions on pretty-flat and reasonably wide roads where it may be safe to be doing 30-40mph. In a desert / dune-running situation, course depending, one could easily do 45-60mph (and more) in 4WD without undue safety concerns.

 

I will add that if your tires are not matched and/or are unevenly worn it will put excessive strain on the driveline even going straight, on 4wd/AWD vehicles it's very important to have matched tires to prevent driveline bindup.