When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Understanding the ADD and centre diff actuators in a 3rd Gen
I've been struggling with the 4WD system on my 1996 Surf (4Runner ex Japan), and have found a lot of helpful information on the looooong thread about '4WD not disengaging' (I've copied out almost ten pages of notes to share with my local backyard mechanic...the best we have in town). After reading and digesting that whole thread, and checking on my vehicle, I am left with one major puzzle in my mind about how the 4WD system works. I decided to start a new thread because after extensive searching on the site, i can't find anything that fits (and if there is, please point me to it!).
I'll sketch out how I think it works, and am hoping to get some pointers of where I'm going wrong:
The vehicle is a manual transmission, RH drive, 2.8l turbo-diesel (1KZ series). It has a J-type shifting lever, with a '4WD' push button. No switch anywhere indicating that there is a rear diff lock, so that's out of the picture.
In 2H (normal operation), I understand that the vehicle is in rear-wheel (2-wheel drive), with no power transmission to the front drive shaft, and the front differential disconnected from the front axles. Is this correct?
When pressing the 4WD button:
1) the vacuum-actuator on the front differential links the axles to the diff (somehow...via some sort of sleeve?), meaning the front wheels are now no longer free-wheeling but engaged to the diff. Is this correct?
However...
2) the front drive shaft needs to be engaged, in order to send power to the front diff. This is the bit I don't understand. Does the 4WD button ALSO engage the electrical actuator motor on the front transfer case? Is this correct, i.e. there's two processes that happen when switching from H2 to H4?
Now, is 4H essentially an all-wheel drive, with power going to one of any of the four wheels?
Then, when pulling the shifter DOWN into 4H(L)- this is now a mechanical (I can hear a gear or something engage when I do that) lock of the centre differential. This means that some power goes to the front, and some to the back. Same goes for 4L(L), except that its now in the low-range gears.
Now, many here have said that the operation of the 4WD button is irrelevant when using 4H(L) and 4L(L). How then, if you go from 2H to 4H(L) or 4L(L), is the front actuator supposed to engage the axles? Were they wrong, or does shifting the lever somehow engage another unseen switch that tells the front vacuum acutator to engage?
I recently had the case of being very badly stuck, with no power going to the front wheels in any setting at all. We discovered that there was a hole in the rubber diaphragm of the vacuum actuator, and that's what's got me wondering about how it all works.
My mechanic is convinced that the car doesn't have a centre differential. Is this also a possibility in 3rd gens, and they just have the J-shaped shifter for getting into 4Lo? But then why do I hear something engage when I pull it into 4H(L)?
I'm not mechanically minded by any stretch of imagination, so please bear with me in case I've missed anything obvious. in terms of the repairs on the car, I'll post those in the 4WD disengaging thread once I have a better handle on things. This one's really just for understanding the system.
I've watched a few helpful videos on Youtube to at least improve my understanding of differentials, transfer case, low-range... and this one partially helps me understand the ADD:
However, I'm still confused about exactly what engages with the 4WD button, and the 4H(L) lever... also, the site toyota-4runner.org recons that the J-style shift with push button wasn't included until 1999 onwards... since mine's from Japan, perhaps it was an early prototype.
Still would be grateful if anyone could point me towards the centre differential, as well as how engaging the front diff to the axles gives one 4H/AWD is linked to a simultaneous (or not?) engagement of the front drive shaft via the transfer case.
Ok...
The J shifter controls the center diff. Shift to 4H, it engages the front driveshaft. BUT: I believe the front half shafts won't get any torque to them from the drive shaft/front differential to apply to the wheels, unless the button is pushed, which engages the ADD, locking the two half shafts together, so they appy the torque from the drive shaaft to the wheels. The ADD has a sleeve that locks the half shaft together, which slides over when the button is pushed. The button engages the vacuum, which causes the ADD to slide the sleeve over.
Once the sleeve is engaged, you are in 4WD, which is different than AWD. But you don't drive the front wheels unless you engage the J shift to 4H or 4L. With the ADD engaed you CAN drive the front wheels, but you need to engage the center gear box.
thanks for replying. I have a handle on the ADD and how it engages the front axles to the front diff. I am still puzzled about the centre diff...does this sit inside the transfer case?
In case there is no centre diff on my model (I tried taking pics of the TC but there's not enough room underneath for good pics), what would be the use of engaging the front diff to the axles, unless there was some way to transmit power via the front drive shaft? That is assuming that the 4WD button does only that...
At any rate, my car still needs to be fixed. I jacked it up yesterday and the recently replaced (2nd hand) vacuum actuator has stopped working, so I have an open front differential that's not engaged to the front drive shaft. There's evidently also an issue in the TC, as the front and read shafts are moving together and not independently, even in neutral.
Here's my shifter, just to clear up my description in the first post:
So this lecturer explaining the ADD system on the later Toyotas says that the sequence of engagement is first transfer case to front drive shaft, then engaging the axles to the front diff (note 17:00 onwards)
This would mean either of two things: the 4WD button sets in motion two processes: TC engage and front shafts engage, then manual movement of the shifter locks the centre diff and or goes into Low range (depending on the position) OR, if my car has no centre diff, then the lever must FIRST be shifted, followed by the button.
This would however not match my experience of doing most of my offroad driving in 4H having used only the push-button (resulting in 4 green wheel lights illuminated on the dash).
This explains it well, the surf is a Multimode version, and yes it has a center diff in the t-case that is not engaged in 4WD mode (aka AWD) then you have H4Locked and L4Locked.
Thanks Malcolm... I'd seen that thread, which is why I got confused about my 1996 Surf having a multimode system when it only appeared (in the US?) in the 99s and 00s Limiteds.
That clears up a few things, but I still have a question then about the centre diff: the diff would control sending power to the rear or front drive shafts differentially, and if locked, would send power to both.
Now, when one is in 2H (the front axles disengaged from the front diff), what stops the centre diff sending all it's power to the front drive shaft, since this is the path of least resistance? I guess the centre diff is fully disengaged/bypasses the transfer case.
However, this still leaves me with my question of when pressing the 4WD button to go into AWD, does it ALSO engage something on the centre diff/transfer case, in addition to the front axle actuator?
The centre diff (t-case) has a hub synchro that can engage and disengage the front drive shaft, the front diff also has a shift actuator that engages and disengages the front diff(they work in unison), the centre diff can also be in open diff(4WD) or locked diff (H4L/L4L), when in 2WD the t-case sends power to the rear diff.
Thank you! I guess what I was really asking (in a long way!) was whether the engagement of the front drive shaft at the TC works in unison with the vacuum actuator at the front.
Now to sit with the mechanic and trouble-shoot again...he'd got the vacuum actuator working (replaced) and we tested that the whole system was working as it should, with the vehicle up on stands. However, the lights kept flashing and he suggested changing the 4WD computer, but I wasn't convinced. They said they'd found a dead line in the computer, and so bypassed it with a relay to link the vacuum actuator to the 4WD button... however, that worries me, especially if it has to set two processes in action, in unision.
As soon as the vehicle came off the stands, the vacuum actuator has stopped working and the front drive shaft is locked to the rear, even with the gear lever in 2H/4H position (tested by jacking both front wheel up...they spin independently and are not linked to the front shaft, but trying to turn the shaft by hand shows it's linked to the back). All the sensors were tested (as per the famous '4WD won't disengage thread' https://www.yotatech.com/forums/f2/4...er-ltd-131201/)... so now it's back to testing the vacuum actuator and jumpering the TC actuator. I suspect both are down, and am a bit worried about the TC actuator as after the motor was removed for cleaning (breather pipe had fallen off, water in, thus rust), it started leaking oil! So it was all taken off again and sealed with gasket sealer...but I have a lingering suspicion that something got stuffed in there. I'll update what we find... but if there's anything obvious from what I've described, please let me know!
The T-case actuator needs to be installed with the gear in the correct position to actuate the 4x4 correctly to the proper position the actuator and position sensors thinks its in so everything its timed correctly satisfying the 4WD ECU including the front diff actuator, I believe I read you need to open the small steel access plate on top of the actuator and remove the small gear that moves the rod which actuates the 4x4 hub synchro to remove it, it needs to be aligned when reinstalling the actuator, I assume while in 2WD and with the actuator in 2wd, but unsure. There is a few videos describing this on youtube, you will have to sift through them to find the exact info for your sitch since you actuator may have been installed out of time/phase.
As an update, the mechanic had another look at the transfer case actuator. He had cleaned it out before, but this time took the time to remove all of the old gear grease (as shown in other threads here), and he found a chunk of hard grease blocking one of the positions (his explanation, I didn't see it). Fitted it again and now it engages the 'fork'/arm/hub syncro fully, and this apparently has also triggered the front vacuum actuator to lock the front. A test with it up on stands showed that everything worked as it should. Two remaining issues:
- The 4WD lights still blink on the dash: this is expected because an auto-electrician bypassed one of the ECU ports with a relay as he said he had a dead signal...I"m now thinking it should be looked at again given that the TC actuator hadn't been working properly when he looked at all the wiring. If the ECU is still bad, I'll have to find a crashed surf to replace it from.
- The TC actuator makes quite a loud 'clank' when I engage 4H (pushbutton), which I do not recall ever hearing. It sounds like it could be slack being taken up by the front drive shaft, if that makes sense? Perhaps something is misaligned? When however I use the shift lever to engage 4H(L)- with the button disengaged- I hear nothing. Can anyone explain the mechanism by which the centre diff gets locked? I"d have thought I might also hear the clank if that's the front drive shaft engaging, but could be I'm hearing something else?
Does anyone have a good way to know for sure that the ADD vacuum is engaging, since I can't rely on the ECU at the moment? It seems to be unusually silent, but that could just be because its well-lubed from all the use at the moment.
I certainly am relieved that the whole issue doesn't seem to be from faulty position switches...which I gather are a common culprit and which would be very difficult to replace. I have always regularly used the 4WD functions, so I'm crossing my fingers that these stay good!
For those who are interested, here's the hole that was in the rubber diaphragm of the vacuum actuator: Hole through vacuum actuator diaphragm, the unit was replaced with a 2nd hand one This gave me nightmares: trying to work out why the 4WD lights were flashing after changing the vacuum actuator: ECU on the left. Everything was put back together!
I still remain with the question about how the centre diff works. When you shift the lever down, does it mechanically lock the diff, or does it strike a switch that does something electronically? I've never had any trouble going into the low range gears, but I'd have the same question whether this is controlled mechanically or electrically. Any pointers to where I can learn more about this would be great!
Wow you guys are hard core, no dash removal should be necessary. Yes you are correct!, the manual shifting of the t-case locks the centre diff in the t-case connecting/locking the front and rear drive shaft together, you will notice binding while turning on dry pavement or hard surface when in H4L & L4L because the front and rear are locked together, in 4WD the centre diff is unlocked allowing for slip between front and rear for better turning and control in high speed loose surface conditions, especially good for icy roads. Happy Surfing, sounds like you've learned lots, the 4WD ECU needs to see everything timed correctly to satisfy the system to work correctly, you cannot hack the system or the lights will flash. Curious if it came with REAR DIFF LOCK as that adds to the confusion and 4WD ECU differences/compatibility. Diesel aswell I assume.
2.8l turbo-diesel, yeah, and the torque plus manual transmission make it one of the best cars to drive slowly on tracks and gravel roads of any car I've driven (with the exception of Land Rover Defenders). I think the reason they stripped the dash was because the guys are used to rats eating wires! Something I've fortunately never had an issue with... and a major reason why we keep our yard as owl, snake and wildlife friendly as possible!
Thanks for confirming that the centre diff lock is indeed mechanical. I know the feeling of when the wheels bind, and also it 'hums/whines' at higher speed on tarmac (the reason I know this is because when the 4WD issue first cropped up, I had been in low-range in the bush, but when I tried to get out of 4WD back on the tarmac, the lights started flashing and the diff was evidently stuck somehow...). That's what made me wonder if it could have been electrical or at the ECU, but if it's purely mechanical then I don't know what might have gone wrong. I have a nagging suspicion that all is not well yet. My plan is to get all four wheels off the ground this weekend and test all the modes, and if it all checks out I'll get them to remove the relay and plug back into the ECU and see if it's all clear now and the dash lights work again as they should. I gather the dash lights are a very reliable indicator of the system working (or not).
On a different topic: has anyone ever got a full-service manual from this supplier? I have no idea how to tell what's dodgy or not. The FSMs on this site are from American-built cars and so the engine models and year configurations don't correspond.
Hmmm, crawled under the car this weekend and could turn the front propshaft after putting the lever into in H4(L) and L4(L) without the button depressed... engine was off but car was parked in gear. Could it be that the locking mechanism for the centre diff is broken somehow? Or what am I missing? Does the 4WD button have to be pressed before the diff can lock mechanically?
This was after noticing that I didn't get that binding from the wheels while driving on a hard surface to test... the TC actuator works fine to engage the front shaft with the 2WD button pressed.
The Mysterious Center Diff Lock and other curiosities
Hello @SurfZambia. I came across this thread because I too was curious about how exactly the Center Diff Lock of the Multi-Mode transfer case operated. I have been told that it is locked by a switch that is contacted when the shifter is pulled back from H2-H4 to H4(L) and I have been told that it is locked by mechanical action when the shifter is pulled back. For what it’s worth, the “4WD” button on the shifter is kind of kind of misleading in my opinion. Mainly because when you press the “4WD” button while the shifter is in 2wd, or H2-H4, the vehicle does not actually go into Four Wheel Drive but rather All Wheel Drive. Which means that both front and rear differentials are active but not locked together as they are in true Four Wheel Drive. I also feel that the way the shifter of the Multi-Mode transfer case is designed to mimic the standard J-shifter of a straightforward, mechanical transfer case causes some confusion. The way the Multi-Mode transfer case and the A.D.D. front differential engage is like some sort of black magic. So many different components are involved in the process of changing from 2WD to AWD to 4WD it is overwhelming to try to get your head around it. I have a ‘99 4Runner Limited that has the problem of wanting to be stuck in AWD/4WD all the time with the indicator lights flashing and the front differential disengaged. Since the front differential won’t engage and the indicators are flashing non-stop I don’t know how anyone would know if the center diff is locked or not. I don’t suspect that there would be any binding if it was because the front axles are not engaged. If the center diff locks mechanically then when I pull the transfer case shifter back it should be locked even if the lights are flashing. I believe you posted the question that if the vehicle was in AWD with the center diff was unlocked while the front differential was disengaged wouldn’t it mean that all the power would be sent to the front driveshaft leaving the rear wheels idle. I thought it was good question but I don’t believe it was ever actually answered. Anyway, I would really like to understand exactly how the center differential lock is actuated. I feel that I understand the rest of the system pretty well but that knowledge has eluded me. The way your thread goes silent leaves me wondering if you found your answer and moved on or just gave up. If you happen to catch this please let me know. Peace
02-09-2023, 02:20 AM
