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Just a note of caution...I have a 2005 Sequoia and the ADD actuator is behind the front axle and I could not remove the cover without removing the actuator.

CAUTION: when I tried to remove the vacuum hose, the plastic nipple on the actuator cap broke off.:( I tried to epoxy it back on, per another post, but to no avail. In the end I had to remove the actuator (requiring a change of differential fluid and a new rtv silicon gasket) epoxy the nipple hole closed and modify a brake adjustment barrel for a bicycle to create a new nipple for the vacuum hose and drill a new hole for my "new" nipple.

It saved me $250 for a new actuator but it was a pain to fix a little broken nipple.
I'm curious, how long did this hold up? I am just now trying it myself (using a random metal sleeve and a whole pile of high test JB Weld). I also roughed up both parts with sandpaper before doing it. I already had to replace the actuator that is bolted onto the transfer case (it had gotten water inside and seized up--fortunately I was able to get the replacement at a yard for $20).
 

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Hello everyone. Hoping to get a little help troubleshooting my 4WD issue. Blinking 4Hi light, no lights at all for 4lo. With key on, engine off, I can hear the actuators working on the t-case and front diff. With 4Hi button pressed in to engage, the front driveshaft is engaged. When the button is out to disengage 4Hi, the front driveshaft is disengaged. I jacked up the front wheels and they spin freely regardless of whether 4Hi is engaged or not. This leads me to believe that the front diff actuator has failed. What are your thoughts? The 4WD system on my truck is used frequently, especially during the winter months. The system worked fine earlier in the day and then started blinking later in the day. Thank you for your help.
 

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Hello everyone. Hoping to get a little help troubleshooting my 4WD issue. Blinking 4Hi light, no lights at all for 4lo. With key on, engine off, I can hear the actuators working on the t-case and front diff. With 4Hi button pressed in to engage, the front driveshaft is engaged. When the button is out to disengage 4Hi, the front driveshaft is disengaged. I jacked up the front wheels and they spin freely regardless of whether 4Hi is engaged or not. This leads me to believe that the front diff actuator has failed. What are your thoughts? The 4WD system on my truck is used frequently, especially during the winter months. The system worked fine earlier in the day and then started blinking later in the day. Thank you for your help.
I think you are right, something is preventing the vacuum actuator on the front axle from doing what it is supposed to do.

From what I have read, the actuator itself is almost never the issue, it is far more common for there to be a problem with the vacuum lines and relays. You may have to trace and/or test all of the individual vacuum hoses and relays. Unfortunately I don't have any kind of a schematic for how the lines go.

(On my 2001 Tundra, the plastic nipple on the front axle vacuum actuator had broken off and needed to be replaced - I used a small metal sleeve and some JB Weld).

Some people have gotten fed up with the complexity of the A.D.D. system and gone the low-tech way (I can't post a link yet, but just Google: Toyota Tech defeating the ADD system).

Good luck!
 

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I had the same 4hi/4lo lights flashing on my 05 Tundra. The rotary contacts on the cap for the transfer case actuator were a little corroded and I cleaned them up with real fine steel wool. I added a little dielectric grease, reassembled and it is good to go now.
 

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Hello everyone. Hoping to get a little help troubleshooting my 4WD issue. Blinking 4Hi light, no lights at all for 4lo. With key on, engine off, I can hear the actuators working on the t-case and front diff. With 4Hi button pressed in to engage, the front driveshaft is engaged. When the button is out to disengage 4Hi, the front driveshaft is disengaged. I jacked up the front wheels and they spin freely regardless of whether 4Hi is engaged or not. This leads me to believe that the front diff actuator has failed. What are your thoughts? The 4WD system on my truck is used frequently, especially during the winter months. The system worked fine earlier in the day and then started blinking later in the day. Thank you for your help.
Did you ever get this fixed? Same symptoms as mine
 

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I have a 2001 Tundra. I have removed, cleaned, and bench tested the transfer case actuator. Also trimmed cracked section off hose. 20amp fuse for 4WD tests good. When I press 4HI there are no noises of running motors and No Lights. I also tested for voltage on the Actuator motor pins = 0 VDC. I am planning to inspect the Front Axle Actuator. Is there an interlock that could remove power from the System?
 

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I know this is going to sound lame but my brother in law works the service counter at a toyota dealership and he said they see this problem quite a bit. Their fix is to engage the 4 hi button in park, then when the light starts flashing put the truck in reverse on dry pavement and cut the wheel all the way to the right or left while backing out slowly.

It fixed my problem. He said for some reason if you do not use your 4 wheel drive regularly that the grease migrates in the mechanism (or something like that). Anyway it was a superquick easy fix for me and after trying to figure this out for about a month of crawling under my truck and tapping the ADD, I am happy to have this behind me.
This worked for me as well on a 2012 Sequoia!
 
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