Topic: 4WD did not engage--why?

[PETDOC]

I tried to tow a neighbor's garden tractor up a steep dirt road which in sections was covered in grass. I put my '88 4Runner in low 4WD and half way up the hill when I got to the grassy part the car began slipping. It turns out only a single rear wheel was turning (spinning), as though I was in 2WD. I was baffled and shifted into high 4WD with the same result. After getting my Kubota tractor I towed my 4Runner to the top of the hill and then retrieved the garden tractor.
 Once back on the road I shifted my 4Runner back into 2WD, backed up to disengage the hubs, drove a short distance and put it back into low 4WD. It seemed to be in 4WD. I thought perhaps the shifter may have slipped in between low and high 4WD on the hill, but when I intentionally did this I had no drive at all. I guess the only way to test this is to go back to the hill and try it again.
 Any thoughts on what may have happened would be greatly appreciated.

[Cheesemaker]

Front hubs locked in?  Is your front diff vacuum actuated?

[PETDOC [OP]]

Front hubs locked in?  Is your front diff vacuum actuated?

My hubs are stock for '88 4Runner. They do not require to be manually locked in. The front diff again is stock-I don't think it vacuum activated, bit don't really know.

[Nimyad]

It's probably the hubs not engaging.

[Goldrush]

If you have automatic locking hubs, then yes they are going to be vaccumm powered.

[PETDOC [OP]]

It's probably the hubs not engaging.

So if my hubs did not engage on the hill, but appear to engage after I returned to the road is this an example of an infrequent event or is there a problem brewing?

[Nimyad]

Well with it being vaccum actuated it won't engage without a certain amount of pressure aka low rpms.

[Dingman.]

When i was still ifs my 4wd never worked at first. Something was wrong with the vacuum system.   What i did to fix it was i installed manual locking hubs and attached the vacuum line off the front shift fork assembly directly to the intake so it constantly was locked into position.

There is other fixes too but that is the method i chose, which was really easy to do and solved my problem.

[OOPS]

I though all IFS rigs with ADD came with flanges and not auto hubs. If it has auto hubs they suck, get some manual hubs.

[Dingman.]

I was under the same impression too.

Off to the side of your front differential there should he a front cover where inside there is a shift fork.  There would.be vacuum lines coming out of this

[OOPS]

Just looked an FSM for an 88 runner, they did not have A.D.D. They came with manual locking hubs and Auto-Locking hubs (junk). If yours has auto hubs they a probably messed up inside and need rebuilding.

My suggestion to you is to get some manual locking hubs and replace the  auto hubs, as I have said the are junk.

Manual hubs from 86-95 runner or pickup will work.

Here is a link to the FSM for your rig.

http://www.ncttora.com/fsm/87-4Runner/toyota_truck_and_4runner_88.pdf