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95.5-2004 Tacomas & 96-2002 4Runners 4th gen pickups and 3rd gen 4Runners

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Old 10-15-2011, 06:54 AM   #1
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Automatic Diff Disconnect (ADD) modification?

I own a 96 Tacoma and am troubleshooting the 4wd ADD at this time. Upon looking at the system I was wondering why a person wouldn't be able to just run a simple manual vacuum switch to the cab to control the actuator, then just shift into 4wd. I would also believe one would have to regulate the vacuum in some manner? My trouble shooting is leading towards possibly a failed solenoid, or a possible electrical short somewhere in the system. Anyhow, I was curious if anyone out there has tried something different with this ADD system and how they went about it. I do prefer a manual type 4wd system so am curious also if this ADD system can be converted to manual locking hubs. Thanks for your time.
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Old 10-15-2011, 03:20 PM   #2
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Yes, can be converted. You will need the manual hubs, spindles, and non-ADD tube to switch out on the diff.
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Old 10-16-2011, 09:49 AM   #3
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Quote:
Originally Posted by pcurtice View Post
I own a 96 Tacoma and am troubleshooting the 4wd ADD at this time. Upon looking at the system I was wondering why a person wouldn't be able to just run a simple manual vacuum switch to the cab to control the actuator, then just shift into 4wd. I would also believe one would have to regulate the vacuum in some manner? My trouble shooting is leading towards possibly a failed solenoid, or a possible electrical short somewhere in the system. Anyhow, I was curious if anyone out there has tried something different with this ADD system and how they went about it. I do prefer a manual type 4wd system so am curious also if this ADD system can be converted to manual locking hubs. Thanks for your time.
You should be able to rule the VSV's (Vacuum Switching Valve--what you refer to as a solenoid, which it is) in or out fairly easily. If it is good, the fault is probably a sending switch on the TC or the wiring. It would be pretty easy to then put a manual ELECTRICAL switch in the cab to flip the vacuum back and forth, just by switching 12v+ from one VSV to the other.

If you do put in a manual vacuum switch (because the VSV's are shot) and you get the vacuum from the same source, why would you need to regulate it? It's set up with a reservoir in the fender and a check-valve on the manifold line so it is already 'regulated'. This should be pretty easy if you have a manual 2-way valve that can switch vacuum from one line to another. Just don't forget and leave the ADD engaged in 2 wheel drive--can't be good for the diff.
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Old 10-17-2011, 05:13 AM   #4
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Thanks for advice. I checked voltage going to the solenoid, at the connections, on one side getting 12.8, on the other side nothing. So that makes me wonder about the switch on the top of the TC. Will look into that switch, hopefully this week. If it comes down to the solenoid going bad then I am favoring running the vacuum into the cab tied into a manual vacuum control switch. Thanks much again and will keep you posted on status.
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Old 10-17-2011, 08:03 AM   #5
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Thanks for advice. I checked voltage going to the solenoid, at the connections, on one side getting 12.8, on the other side nothing. So that makes me wonder about the switch on the top of the TC. Will look into that switch, hopefully this week. If it comes down to the solenoid going bad then I am favoring running the vacuum into the cab tied into a manual vacuum control switch. Thanks much again and will keep you posted on status.
The voltage is supposed to be only on one side (at any one time). It moves to the other side as you go from 2WD to 4WD. It is never on both sides at the same time.
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Old 10-17-2011, 08:17 AM   #6
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Thanks again. I am heading out right now to throw the volt meter on her in 4wd. Will get back on results.
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Old 10-17-2011, 08:46 AM   #7
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The vsv's have a blue connection on one side and a brown connection on the other. I checked both with truck in 2wd, voltage on blue side, nothing on brown side. Shifted into 4wd high and low, checked voltage, again voltage on blue side, nothing on brown. Soooo I am thinking about that 4wd switch coming off of the TC as the possible culprit or a short in between somewhere. Looking for something in the Haynes that might help. Thanks for the help.
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Old 10-17-2011, 10:58 AM   #8
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Mine is stuck in after I got back from Afghanistan earlier this year. I haven't worked on fixing it yet because I installed manual hubs as soon as I got back.
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Old 10-17-2011, 12:50 PM   #9
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The vsv's have a blue connection on one side and a brown connection on the other. I checked both with truck in 2wd, voltage on blue side, nothing on brown side. Shifted into 4wd high and low, checked voltage, again voltage on blue side, nothing on brown. Soooo I am thinking about that 4wd switch coming off of the TC as the possible culprit or a short in between somewhere. Looking for something in the Haynes that might help. Thanks for the help.
If you NEED the 4WD, in your situation, you can just swap the long hoses to the diff from one VSV to the other and vice-versa (label so you can go back to original setup). That will engage the ADD actuator--or disengage it if you are stuck with it engaged.
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Old 10-19-2011, 07:50 AM   #10
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Thanks again, will give that one a try. Swapping vacuum lines to the VSV"s
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Old 10-19-2011, 01:04 PM   #11
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Anyone know which one does what?
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Old 10-26-2011, 02:47 PM   #12
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I did try swapping the long lines out on the VSV's a few days ago and still couldn't get 4wd. Stepped away from trouble shooting the 4wd for a bit for now, but will be geting back on it. I am leaning towards the 4wd switch which ties into the top of the transfer case even more so now. Have to do something shortly.

I am not sure which side of the solenoid controls 4wd or 2wd. I am geting a good reading on the blue side while in 2wd, but not able to get anything on the brown side solenoid. So I assume if the solenoid is good then blue controls 2wd and brown controls 4wd, but that is speculation. Switching the long vacuum lines did nothing, so who knows. A schematic is not provided in my Haynes manual for the ADD so next time I am in town I will stop at the Toyata dealership and see what they have. We will be back with an update.
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Old 11-06-2011, 01:16 PM   #13
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Update. When all the troubleshooting was over came down to the ADD Solenoid Assembly was bad. OE #85440-35040. $550 and some change. Not going there. When I am ready to spend that amount I will put it into manual equipment and other mods for the 4wd. Probably next spring/summer. Somewhere on that truck is another VSV that has something to do with the EVAP, Dorman 911-603. Look very similar to Toyota ADD solenoids. Bought two for $58 and some change including shipping. Difference is on the ADD solenoids there were air cleaners, none on the Dorman product. Realized this when I went to install, but the heck with it, they going in. So I installed them and they work. But do have another problem with this set up. I think the air cleaner on the Toyota solenoids also act as a vacuum relief for the one option (2wd or 4wd) that you are not using. Therefore on occassion I think that one side is loaded with vacuum and won't allow the other side to pull the actuator over....my theory. I think I have a line on a couple of in-line vacuum relief valves, something that allows vaccum flow to the actuator, but when that solenoid is powered off, will allow the vacuum to bleed off. Right now the vacuum isn't bleeding off, so on occassion I have to pull the lines under the hood to releive the vacuum. I don't expect these valves to be expensive. I will also throw a couple mini air filters in line on each line. Hopefully they won't interfere with vac pressure. With these add ons still will be well below the cost of original equip. There are some other VSV's made by Dorman for Toyota that appear to have air cleaners on them or look more similar to what I took off my truck, but apply to other passenger vehicles. Those look to be in about the same price range as the ones I picked up. I am going to give Dorman a call tommorrow and talk to one of their "specialists" about those others that look more similar. I may just pick up a couple of those if I like what I hear and chalk up the others to a $58 temp loss until I replace the one on my truck for the evap system. We'll be in touch.
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Old 11-06-2011, 01:16 PM
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4runner, 95, 98, add, assembly, control, conversion, diff, differential, disconnect, drive, schematic, solenoid, tacoma, toyota, vacum, vacuum, wheel

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