Hello,
I recently acquired a new (to me) 1991 4Runner that has the A.D.D system for disconnecting and connecting the 4WD.
The problem:
While driving with the transfer case in 2H, the front axle makes this horrible grating/humming noise and vibration (its so bad it vibrates my knuckles while holding the steering wheel) but it instantly goes away when it is put into 4H or 4L.

What i have done so far:
I looked around on here and found some useful articles on the ADD system and its function. What I did was put the drivers side tube for the front diff actuator directly connected to a strong vacuum source with a check valve and connected the vacuum resevoir to the disconnected VSV (the red one) so there would not be a leak, this cured the problem of the noise and vibration from the front axle while in 2H.....only for a few hours.....now it does it intermittently and even worse than before (sometimes even for just a second) I think that the actuator on the front diff is bad seeing as i checked almost every vacuum hose i could see and none were broken (yes even the ones to the actuator) or that the previous owner drove it like this while it made this horrible noise for so long (he was a super space cadet, no understanding of mechanical anything, could probably burn a pot of water if he tried) that the locking mechanism is just FUBAR inside there, but i drained the fluid from it and it was a little dark, but no abnormal amount of metal shavings were suspended in it (?) and it did not smell funky or burnt.

Any ideas/suggestion/theories would be appreciated since I know nothing about japanese auto makers and their love of vacuum controlled systems (i am an old school BMW Audi Volkswagen guy)

Jon H.

HAPPY HOLIDAYS!

oh and it has the 3.0 3VZ-E, if that helps

anyone?

Usually it's the other way around, people have noises and vibes when they shift into 4WD.

I don't quite follow the other part, were you working with the Lock VSV or the Unlock VSV ?

my problem is the oppisite... i have gears out of a 91 4runner in my 87 and mine shakes when its in 4wd or when i lock the hubs and its in 2wd. any ideas on that??? its pretty horrible at about 25 mph

i disconnected the unlock VSV (the red one receives voltage while the transfer case is in 2H) and I connected the actuator directly to the manifold, but it still makes this noise/vibration. I know that it usually happens when trying to go into 4WD, but my 4WD engages just fine (4H and 4L) it only happens when I am drivng from 25-55mph in 2H ONLY, but once it starts doing it, it will make the noise all the way down to 5mph and will not make it again till I speed up to 25mph.
I have one more idea up my sleeve, I completely forgot to loop the vacuum canister into the system while trying to lock it into 2H, so I will try that and see how it works.
I am at my wits end here with this because how can something that slides a collar on a shaft make so much damn noise?!?

I guess it is the actuator, it does have 290,000 miles on it and its probably can not hold vacuum worth a damn. now only if i had a jiob to buy these parts.....ah the holidays are the best