Hi gents, wanted to share my entertaining and puzzling adventure with the 4RunnerÂ’s 4wd system malfunctions.

I have a 2020 Venture edition with KDSS. Effectively, itÂ’s a TRD off-road, so it has a manual J-style transfer case shifter. Also has adaptive cruise control.

I wheeled the truck at its stock height for the first year. Never had issues, but ran out of clearance a lot. Put on a lift in May 2021, swapped 33” tires, wheeled some hard NorCal trails much more easily. Handled rock crawling on par with 80 series cruisers.

Strike 1: In July 2021, a few weeks after a big wheeling trip, I noticed a blinking green 4wd sign. Tried engaging 4wd, and it wouldnÂ’t budge. A few days later, it went away and I was able to engage 4H and 4L a few times before it stopped working again. Also, pre-collision control (PSC), vehicle stability control (VSC), and multi-terrain select (MTS) all failed, illuminating a lovely Christmas tree on the dash. Cruise control also failed as its adaptive (though, the adaptive feature can be turned off). Took it to the dealer, who then replaced the front diff actuator (aka ADD), which connects the front half shafts to the front diff, enabling 4wd. Talked to the master mechanic at the dealer who told me that there was no water or anything in the ADD but the motor didnÂ’t run. Fair enough. Did a bunch more wheeling in CA, NV, and AZ.

Strike 2: In January 2022, a few weeks after the latest wheeling trip, got the PCS, VSC, and MTS malfunction lights. Tried 4wd, and it engaged fine. While the Christmas tree was annoying, at least the truck was just as capable. Notably the lights would go off only the first time I start the truck in the day. On next starts, the lights would go away. So IÂ’d start the truck in my driveway, drive to the first stop light, turn it off, and back on, and everything functioned fine. After a few weeks, the issue became more persistent and the lights would not go away for 5-10 starts, if at all during each individual day. Still, 4wd worked, so I let it slide. Finally, in February 2022, the 4wd light went off again about a week before my planned wheeling trip, so I took it to the dealer right away. The same mechanic inspected it, noted itÂ’s the ADD that failed again and filed a claim with Toyota, telling them that they need to get their shit together because the same part failed twice and the dealer doesnÂ’t want a lemon claim. Toyota sent the new ADD, dealer installed it under warranty (though, this time, the motor worked, and something else related to ADD failed). Went wheeling and it performed beautifully.

Strike 3: In March 2022, got PSC/VSC/MTS Christmas tree again. At that time, the vehicleÂ’s alignment was re-done, and the lights went away, only to come back a week later. The alignment shop then performed zero point calibration and the lights went away for good.

Then in April 2022, got the blinking 4wd light again. Played with engaging it back and forth with no result, except in one situation when I put it on 4L and engaged the locker and 4wd engaged (was a one odd situation). Read the forums more and learned that the ADD can seize, in which case, a few whacks with a rubber mallet to the ADD can help. Took out a nice 4lb deadblow, and after a few whacks, lo! it worked. Took out of 4wd, no problem. Engaged again, and it didnÂ’t work again. A few whacks to the ADD, and it worked again, and then a few times again before it stopped engaging 4wd. I kept playing with it more. At this point, I replaced the 4lb deadblow with a 1lb rubber mallet, as I got tired. 4wd would engage after 1-5 whacks to the plastic ADD box, and disengage without an issue. It would engage on its own 1-3 times after the time I whacked it with the mallet.

After a few days of the whacking experiment, 4wd light stopped blinking every time I started the vehicle - even though 4wd would engage only so often without the help from a mallet. Every time I started the vehicle, IÂ’d hear a click on the passenger side of the cabin and no green 4wd light (again, to pop up again when I engaged 4x4). Then, today, when it stopped working again, I took the mallet to it again, and it worked, again. But this time, it never stopped working (so far). Except, the PSC, VSC, and MTS lights were back on (with 4wd working). Then, turned the engine off, started again, and everything works, no more lights.

I appreciate that Toyota should be more reliable than a Jeep, but DAMN, this electrical system is more dramatic than a Brazilian soap opera. Anyone has any ideas what could possibly be going on?

PS, apologies for the long post, but this has to have been documented in history.

I'm wondering if there is something strange going on with the differential and/or axles. 2 (maybe 3?) faulty ADD actuators makes me think either operator error/ modification or an issue with the front differential or axle(s)? I mean, I've seen my share of faulty ADD Actuators from the factory, but I've also seen plenty of abused 4WD systems that made me wonder if it was user error that killed the actuator. Couple questions out of curiosity:

-What type of lift did you install?
-How have you been engaging/disengaging the 4WD? (i.e are you engaging on relatively flat ground, with the wheels relatively straight forward, and allowing the system to fully lock-in/out before you start loading up the drivetrain?)
-Did you install a diff drop?

The 5th gens for the most part (I believe only the Limited trim still gets them, been a few years since I verified it though since we rarely see issues with that) did away with the Torsion T3 LSD that all the 4th gens had standard; it was mostly there to give driving in 4Hi less binding and smoother pavement manners when nearing full lock with the steering by letting the front and rear axles slip something like 15-20%? But one minor quirk is that since the front and rear axles can't slip a little like they can with the T3, it does make the part-time system slightly more finicky as everything really needs to line up exactly. This isn't a really big deal to be honest with you, it's more that the non-T3 systems seem to be more finicky/take longer/pop/clunk into 4WD if they're not switched into/oit of 4WD on relatively level ground with the wheels mostly straight ahead, or driving straight ahead if you're trying to engage it while moving.

I've had customers complain before about "violent shifting" or poor 4WD performance before and when I've gone on test drives some customers will try to engage the system while taking a turn, or on like really uneven pavement, or goosing the throttle before the system fully engages. All of which puts stress on the 4WD system specifically the ADD and could theoretically jam it up/wear it out prematurely.

Something to keep in mind is that the ADD actuator is one of the final steps in the 4WD system when shifting into 4Hi. For example, shifting into 4Hi for a Toyota has been roughly the following procedure all the way back to when the ADD was vacuum operated (there's some minor differences between models and generations of vehicles, but it's basically the same logic path as below for the part time system):

•4Hi is selected -> Vehicle flashes 4WD and 4Hi light -> Transfer Case is comanded/shifted into 4Hi (if requirements are met) -> Vehicle confirms Transfer Case has physically shifted into 4Hi -> Vehicle commands/shifts ADD Actuator into locked position -> Vehicle verifies that ADD Actuator has fully engaged and physically locked the front axles together and the front differential to the transfer case -> Vehicle commands 4WD light and 4hi to stop blinking and remain solid.

The ADD Actuator is always one of the last things to engage when shifting into 4WD and the first thing the system disengages shifting out of 4WD (basically it goes in reverse from what it did to go into 4Hi). All it does is slide a sleeve over to lock the passenger side axle to the driver's side axle (which is driven by the pinion when the Transfer Case is in 4WD). If memory serves me right, there are no sycros in the front differential, it's just a sleeve; but there doesn't need to be because it is meant to be engaged at a stop/slow speeds with the two axles turning roughly the same speed.

So the first thing to do diagnostically is to rule out operator error, then to determine if your modifications could be causing undue stress on the actuator, and then likely crack open the clamshell and injustice linspect it or replace the front differential.

Thanks, man! That’s good information, and I’ve confirmed this stuff with the manual too. To answer your questions:
1) lift is Dobinsons IMS 3” lift.
2) I typically engage 4H when I’m on slippery surface, so like a bland dirt road. I engage 4L when I need more traction or need to slow down. If engaging anything on pavement (just to test), I only engage when going in a straight line since the center diff is locked.
3) I have a Dobinsons 1” diff drop kit.

When it operates, the 4wd works flawlessly. Never had issues or violent shifting or anything. Bulletproof. Also, my transfer case is manually operated (shift level, no actuator to engage it). So if the ADD doesn’t work, I can actually put the truck in 2Low.

The front axle will be cracked open in a month when the truck gets re-geared. Not sure if it’ll do anything.

The transfer case being manual doesn't matter too much in terms of how the system works; one less actuator, but similar amount of detection switches. The way the part-time system works, your transfer case will be in 4Hi or 4Lo, but that doesn't matter much if the front axle isn't locked up by the ADD actuator (3WD I guess? lol).


It does sound like it you're doing everything right and I don't think the lift should be affecting anything in that regard; so I am curious why the pattern. I wonder if the sleeve is maybe a bit sticky or something, like I said one ADD actuator is like "eh, it happens..." two is like, "did I install this stupid thing wrong? WTF?" But I am curious if regearing/rebuilding the clamshell changes anything; let us know if they find anything odd!

Thanks, yeah, that makes sense. What some people run into is the connectors on the ADD motor come loose, so they solder them together. However, if that was the case, my experience with the mallet doesn’t make any sense. It appears that the 4wd system on these trucks is connected to the vehicle stability control and other nannies that can be impacted by the lift, so I’d be curious to see the full operational diagram - it must be checking more than just whether the ADD works, etc.

The ADD is just so simple, it’s just puzzling that it can fail. Theoretically, it’s should be more reliable than the old vacuum system. In practice, I’ve considered installing some kind of pull by wire system, but I think it’ll not going to go well with all the others 4wd sensors.

Well, the drama unveils further. 4wd has worked flawlessly for a good portion of the week till today. Today, I tried engaging in 4L and it didn’t engage and I got the beloved PSC, VSC, MTS lights. A few whacks on the ADD, later, 4L engaged fine. Took it back to 2H, and back into 4L and didn’t engage yet again. A few more whacks, and it engaged. Then stopped engaging again. Then, even weirder, id put in neutral, put in 4L, and put in D/R and it doesn’t engage, then I put it in N, and it engages all of the sudden. Consistently. I’m ever more puzzled at how Toyota 4wd works….

Hmm...

It shouldn't need ADD input going into or out of 4Lo beyond it being engaged... I almost wonder if one of the switches on the transfer case is acting up, but then why would hitting the ADD fix it?

Post Note-

Though I suppose if an ADD actuator switch is intermittently working it could cause the same issue?

You mean the ADD motor prongs come loose, and then comes back into contact when I hit it?

Really good stuff here, so i feel silly adding this...

If replacing the ADD didn't fix the issue, perhaps the issue is upstream. A system/signal that feeds into the ADD, rather than the ADD itself or signals coming out of it. Adding the "whacking trick" to the puzzle leads me to believe it would be a loose wire/sensor/ground feeding into and in proximity to the ADD.

That’s a very good point. I’m just curious how to test this. Especially since everything works fine now (fingers crossed).

I haven't seen the male terminal ends of an ADD Actuator become loose or broken before unless someone really screwed something up. I will mention that once I have seen one of the female terminal ends in the housing not be fully retained, but that was do to someone else accidentally not clicking the terminal back in after running some tests. The majority of the time I have seen poor connection issues it's because the locking tab on the connector housing is broken off. Not a big deal as we have plenty of old wiring harnesses I just scavenge a new one off of; you can also order the connector housing by itself if needed.

It's not a bad thought process and that's what I am wondering myself since he mentioned the weird issue with the 4Lo, which shouldn't have anything to do with the ADD beyond checking that it's engaged. Though the ADD Actuator itself has all the detection switches built into the unit, his transfer case is manually operated so it has "old" style ball-detent switches located on the actual transfer case for detecting positions of the internal components.

Without being at the vehicle to see how the technician tested it and/or seeing the issues the OP is describing, it's a bit hard to definitively point at something.

Thanks, guys. I’ve been considering taking it back to the dealer to see if they’ll fix it under, now, extended warranty (just passed 36k miles). But it would be interesting describing the problem…”4x4 didn’t work, and then I smacked it with the hammer, and now it does, but check it anyway.”

Worth a shot, I mean technically speaking it is the same type of failure as the prior two repairs so it could technically fall under the category of "repeat repair" and thus it would be covered under 3yr/36k regardless because they haven't technically fixed the original concern that occurred under that warranty period (which kind of sucks for the dealer, because they get dinged by Toyota bit and might have to deal with some BS over it; but such is life.)

It also should be covered under drivetrain warranty which is 6yr/60k anyways, so even if you didn't have extended coverage you should be good.

Keep us in the loop, I am really curious what they find on this, maybe politely ask if there's anything in the front axle that could be causing the ADD Actuator to fail prematurely? Most of the time technicians don't like customer's arm chair diagnosing their vehicles, particularly when they start with "well the forum said..." but if you just be nice about it as a question; maybe the tech will be like, "huh... wonder if they have something there?" or bug TAS to see if they have any oddball cases of out of tolerance front axle assemblies. I'm sure the guy is going to be annoyed that he's having to deal with a "come-back" a third time which doesn't look great even if it's nothing he did.

Also, something that I’ve been trying to understand is how 4x4 and PSC/VSC/MTS are connected. I’ve had two situations now where those systems fail, the dash lights up and 4x4 begins to work. Just trying to understand how ECU connects these systems?

Oh that's easy to explain, I should probably do a new version of this thread: Check Engine, TRAC, AND/OR VSC On? Read Me! for the 5th gen section.

But basically it's due to the interlink of the various systems and Toyota/most manufacturers conservative approach to DTCs/Warning Lights. New vehicles that operate under the OBDII protocol are heavily networked, individual modules not only control their designed systems but also interconnect with other modules on the network to make "system hubs" that function as a whole to coordinate certain functions. For example, while most people think the ABS Actuator is just for ABS, it's actually the main controller for the ABS/VSC/TRAC systems; additionally that system needs the ECM, TCM, 4WD, PCS, MTS, etc. (Transmission Control Module) to coordinate when activating systems like VSC, TRAC, 4WD, Radar Cruise, etc. if you remember Toyota's marketing literature for your vehicle; their "Safety Sense" package is not an actual module, but rather multiple modules on the vehicle coordinating with each other to provide you various features and functions.

The Pre-Collision System & Multi-Terrain Select is dependent on the ABS/VSC/TRAC Systems (lets just shorten this to Skid Control ECU). The PCS needs the Skid Control ECU to be clear of DTCs and operating as designed to function correctly, having a DTC like say a "U0114- Lost Communication with 4WD Control ECU" would indicate that the Skid Control ECU has lost communication with the 4WD system/cannot rely on the 4WD system to be operating as designed (it is also possible that since the 4WD module in the 5th gen doesn't output it's own DTCs, that this U-code which is normally a straight communication failure code is used as a "hey I have an issue in the 4WD system, check there" message much like how C1203 is set in the Skid Control ECU on 5th gens when an ECM failure occurs to direct you to check the ECM, also this is why the VSC/Trac OFF lights come on when a ECM related failure occurs).

Since the Skid Control ECU also manages MTS functionality by using the ABS actuator and braking system, it needs to know that the 4WD system is operating as designed and meets the correct required conditions to function. Since it can't determine that because it has lost communication with the 4WD module, it can't operate the MTS system and logs a failure and that system shuts down as well as part of the Skid Control ECU functions. The PCS relies on a properly functioning Skid Control ECU to operate (functions like maintaining distance from the vehicle in front of you in Radar Cruise is done via manipulating the Engine, Drivetrain, and Braking systems), if the Skid Control ECU is not functioning correctly or if a sub-system it manages is not functioning correctly, then the PCS module cannot function correctly and it records a failure and shuts down. Since the original failure comes from a system that isn't related to Emissions Control, the ECM doesn't record a failure usually (and it doesn't set a MIL/Check Engine Light) so you only get the ABS, VSC, TRAC, MTS, PCS, and 4WD lights on.

TL;DR-

I realize that went a bit long and technical, apologies. The long and short of it is that the the PCS & MTS functions are dependent on a functioning Skid Control ECU, if the Skid Control ECU is not able to function correctly (i.e. a 4WD system is acting up which the Skid Control ECU depends on) then all related systems are partially or completely shut down and the warning lights to the related systems are turned on to represent that and let you know.

Hopefully that's helpful.