Originally Posted by BlackWorksInc

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.

Originally Posted by bayaz

More story here.

I decided to remove the diff drop kit after some additional research unrelated to the 4x4 issue (even aussies don’t use the “diff tilt” kits that are sold in the US, as they affect front pinion angles, eat away clearance, and don’t provide much drop). After I removed the diff drop, I drove the truck around, engaging 4wd periodically. Much to my surprise, the 4wd engagement action slightly changed - it occasionally skipped a beat, even though it didn’t happen pre-diff drop removal. Had to restart the truck and repeat the whacking procedures a few times. Works fine now. While this isn’t conclusive, it led me to believe that there is some relationship between the 4wd system engagement and tilting the front diff. Just an observation. More experimenting to go on.

Originally Posted by Jakeepoo3

Inside A.D.D. unit the final limit switches indicate complete engagement of the splined collar that links axle to the diff. Without full engagement signal or a fault in the electronics of the module not relaying the signal of engagement to the 4WD module. Not knowing if the A.D.D. had fully engaged the 4wd module keeps waiting for a switch signal that will never come. This will not allow you to shift into 4lo. Another symptom right? Well if you beat on it with a hammer and get intermittent communication between the two modules I would expect faulty contacts or corrosion involved within the A.D.D. You mentioned they're new though.. also your skid control module will be disabled if there is no communication with the A.D.D. module 🤔 which is the possible cause of your Christmas tree lights. Like blackworks mentioned shifting to 4hi and 4lo need to be done properly to prevent hard shifting and premature wear and tear.. especially 4lo. To shift into 4lo flawlessly come to a stop on a slight incline, shift into neutral let off the brakes enough to get a slight roll and the make the shift. You really have to jam the the sh1t out of it otherwise. 4hi, slow to below 40 mph, straight, let off throttle, complete shift before using throttle again. Keep us updated this is a fun one!

Originally Posted by bayaz

Thanks. That’s the procedure I follow to engage 4H and 4L too.

As far as ADD and Christmas lights, I’m certain there’s no corrosion. I asked the mechanic each time if there was water and there was none. Moreover, the ADD motor itself was functional the second time it was replaced (Toyota said the signal being passed to ADD wasn’t being read). The current motor is only two months old, and I haven’t submerged the vehicle deep enough to flood it even if the breather failed. Further, I had weeks upon weeks of having full Christmas lights AND fully functioning 4wd. Christmas lights were fixed with a zero point calibration.

I’m curious what else goes into engaging ADD. There are 6 prongs (including ground) in the connecting harness.

As far as diff tilt/drop, I’ll be honest, I’m glad I removed it. Now the u-joints and front drive shaft are back to the stock design. U-joints have way less flexibility than the CVs. The actual “correction” to the CV angle was negligible. People who measured it, found the difference to be only 1-2 degrees. Further, I’m glad to gain back the clearance under my skid. My front skid has taken hits hard enough to sheer off 3 captive nuts and bend one of the mounting points. With a diff drop, it was sitting on spacers, reducing clearance and creating additional weak points. I’m glad the drop is gone.

Originally Posted by Jakeepoo3

Yes correct. Your four wheel drive is functional without the skid ECU/master solenoid. But are your VSC and other abs related functions all working when the ADD module isn't? "(Toyota said the signal being passed to ADD wasn’t being read)" and the technician tested the entire circuit to pinpoint the failure point? By chance do you know if he tested the new ADD module before installation? Battery voltage alone will fry it. Here is the circuit and connector diagram attached. I reckon you could bust a multimeter out and and test the circuit and communication between modules.. Did you keep your old module? With some skill and knowledge you can open it up and inspect all the contacts and switches. Sometimes a good cleaning is the only thing wrong with it. You can also hook it up to a power source with the PROPER relay and test it yourself, Toyota won't do this though they will only replace it.

Originally Posted by KKTRD-PRO

Dang, what a nightmare. Isn't there some sort of lemon law that can be invoked here?

Originally Posted by bayaz

That diagram is golden. I don’t think Toyota tested it like that. All the tech did was order and install a new one and see if 4wd worked. Good because it’s speedy but bad because the issue appears to be more than just an ADD failure. I wish I asked for the old actuator, but I never did. I’d be curious to test it myself.

Originally Posted by KKTRD-PRO

Dang, what a nightmare. Isn't there some sort of lemon law that can be invoked here?

Originally Posted by BlackWorksInc

I am curious what they meant by "the signal isn't reaching the ADD," because that makes it sound like a 4WD Module issue or a wiring harness issue not sending a signal to the ADD? Which seems a bit odd to me, but I suppose is possible?

I have had to de-pin... I want to say pin#3 on the ADD and either leave it open or ground it to diagnose whether or not an issue was ADD related or not. But aside from the actual motor being driven, everything else on the connector should be an Output not an Input as it's feeding signals back to the 4WD ECU on the status and position of the actuator once it's been commanded to engage/disengage.

Maybe? I mean this would technically be a 3rd repair attempt by Toyota... I'm not too familiar with lemon laws and that would be up to the OP if they want to pursue that option.

Originally Posted by bayaz

Thanks. That’s the procedure I follow to engage 4H and 4L too.

As far as ADD and Christmas lights, I’m certain there’s no corrosion. I asked the mechanic each time if there was water and there was none. Moreover, the ADD motor itself was functional the second time it was replaced (Toyota said the signal being passed to ADD wasn’t being read). The current motor is only two months old, and I haven’t submerged the vehicle deep enough to flood it even if the breather failed. Further, I had weeks upon weeks of having full Christmas lights AND fully functioning 4wd. Christmas lights were fixed with a zero point calibration.

I’m curious what else goes into engaging ADD. There are 6 prongs (including ground) in the connecting harness.

As far as diff tilt/drop, I’ll be honest, I’m glad I removed it. Now the u-joints and front drive shaft are back to the stock design. U-joints have way less flexibility than the CVs. The actual “correction” to the CV angle was negligible. People who measured it, found the difference to be only 1-2 degrees. Further, I’m glad to gain back the clearance under my skid. My front skid has taken hits hard enough to sheer off 3 captive nuts and bend one of the mounting points. With a diff drop, it was sitting on spacers, reducing clearance and creating additional weak points. I’m glad the drop is gone.