91 Previa Alltrac, Transfer case and AC problem

[covenant]

Hi, I got a 91 previa alltrac.
There's sound coming from the rear, i think it's from the transfer case.
Like groaning when accelerating or slow down, becomes louder when turning. The more you turn the wheel the louder the sound.
Also, when you turn the wheel while backing up the vehicle, it has the so-called "Tight corner brake" phenomenon, meaning at certain angle of the wheel turn, the vehicle just freezes even with the engine running on idel and your bake not applied, and you need to press the accelerating pedal to some degree to get the vehicle moving, and when it moves, you will hear a clicking sound coming from under the chassis with tangible vibration.

Do the all above mean the transfer case is gonna break down?
Any solution to solve the problem?
Also, can the alltrac be converted to FWD just by remove the front axle or front driveshaft?
Any success examples in such convertion?

Another problem is A/C.
With the A/C switch presses down, the green light on the switch is on. But, nothing happens, no engine idle up, no cold coming out(blower is good), the A/C system just does not come to work. Did some research, says could be the AC amplifier defect. I refer to the service manual, the way to inspect the AC amplifier seems weird to me. See the attached for detail. It tests the harness actually, not the AC amplifier itself, and the conclusion is "If the circuit is as specified, replaced the ac amplifier"! I am not sure if there is some typo here. It means to be "If the circuit is NOT as specified". If there is no typo, so if the the circuit is not as specified even with one difference from the test table, it will indicate that the AC amplifie is GOOD? Really weird.
Thanks!

[timsrv]

What you are describing is consistent with a transfer case failure. The viscous coupling inside goes bad and the result is noise and binding while going around corners. I'm not sure what the "official" word is on driving as a 2wd, but for a time I removed my front drive shaft and drove my 91 All-
Trac as a RWD. I eventually put a new drive line on it everything was fine.

Not sure what to tell you on the AC issue. So far the only AC problem I had with my Previa was a bad expansion valve. Tim

[covenant]

Thank you very much for your reply.
So, I need to replace the transfer case, but the used one usually has around 200K mileage on it. The condition of viscous coupling inside maynot be good. The replacement is the only solution though. Sigh....
By the way, if I just leave the Transfer case alone, what would happen when it completely goes down?

[timsrv]

I think what you are describing is about as bad as it gets.............but it can't be too good for your other drive components. The earlier van transfer cases will fit on the Previa (no viscous coupling), then you could run as a 2wd or a 4wd. You would however need to rig up some sort of shift mechanism though or you would have to climb under the van to shift in and out of 4wd. Using a single speed transfer case off of an automatic would be the easiest, then all you would need to do is run a vacuum line through a solenoid valve so you could switch it on and off electrically from inside the van.

[covenant]

Hi, Tim.
I am working on the AC. I check the AC amplifier, it should be ok. I tested the AC cut relay and magnetic clutch relay, also ok. The FSM suggests that with no engine idle up when AC switch depressed, 1 to look at is AC VSV. But on my 91 previa, I just could not find it, any idea where i can find the AC VSV? Thanks!

[timsrv]

Hopefully somebody else will chime in here as I'm blissfully ignorant of the location of the AC VSV on a Previa (so far I've been pretty lucky with my Previa AC systems). On the typical Toyota AC system the AC VSV is tied into the "actuator" and that is usually mounted to the intake manifold. It's a metal vacuum diaphragm looking thing with a plastic adjustment screw coming out the bottom. Once you find that trace the small vacuum line from it back to a small device with 2 vacuum lines and 2 wires going to it.............that should be your AC VSV. Tim

[covenant]

I did some test today. It turns out to be the ac pressure switch problem. I just short conect the 2 terminals, so the AC system starts to work! http://www.toyotavanpeople.com/forum/images/smilies/icon_biggrin.gif . But, there is still no cold coming out, blowers are good . I checked the refrigerant, it's good under proper pressure. Now, confused again.......:no:

[covenant]

Hi, I found a used transfer case at a local junkyard at $300. I wonder that without garage lift, am I able to replace the transfer case with hydraulic jacks and stands? Will there be enough room? What equipment do i need to proper support the transfer case when lift it up and down? Again, with hydraulic jack? Thanks!

[timsrv]

How did you verify the pressure? Did you check the sight glass on the receiver to see if there are bubbles present?

[covenant]

Sight glass is so blurred, i cannot see anything. It seems the pressure is not good due to the air in the system. So, how do i evacuate air from the system? Thanks

[timsrv]

If you think the system is contaminated then you'll need to evac the system and use a vacuum pump to remove everything. Then you'll need a manifold gauge set to recharge with oil/freon and also monitor correct pressures (low and high sides). If you're doing all that then you should replace the receiver too. If you don't have the equipment and don't want to invest in it then you should take the van into a reputable shop and pay them to do it. Might not be a bad idea to have them check for leaks before they recharge. Tim

[covenant]

I did some quote from some mechanics on the transfer case replacement job. It varys from $200 to $280. Is that a reasonable price? Besides, i need to get the seal between the AT and transfer case from the dealer only? Any other sources? Thanks

[timsrv]

It sounds reasonable to me. The shaft seal should also be available through most auto parts places. The paper gasket may be easier to get from a Toyota dealer??? Prices on parts like these won't matter much so I'd probably get everything from the dealer (but that's just me). Most mechanics like to sell you the parts too, so make sure it's okay with them before supplying them yourself. Tim

[covenant]

Hi, again.
I just wonder that how the power is transfered to the front and rear? Some said when the transfer case, or the viscous coupling to be exact, is cold, the vehicle is just a RWD one. When the viscous coupling warms up, it will bind the front shaft and then we got the AWD. Is that correct? If correct, if the front shaft is removed, will it do any good to eliminate the noise coming from the viscous coupling? or to say, with the front shaft removed, the viscous compling will not work, or it will just work harder? And is it OK to drive the vehicle with front shaft removed for long distance and long term use?
Thanks!

[timsrv]

The way I understand it the viscous coupling controls (limits) the slip between the front and the rear. The common failure is it locking up (no slip), then when slip is required there is binding and noise instead. If you remove the front drive-shaft you are also removing the conflict.......of course then it will only be a 2wd. Tim

[covenant]

I just removed the front shaft. The noise is completely gone, and no binding any more!!!!! :lol::lol::lol:. Everything works fine. There is little snow in winter downhere, so I am much happier with the 2wd as now. I just want to leave it that way. Thanks Tim!

[covenant]

Hi, I noticed that on the 91 previa's transfer case, there is a locking machanism built-in. Is this locking device automatically controlled by the oil pump of the TC? So can I manually lock the TC? How can I do it? I know there is shift fork which can push the clutch back and forth, so how can I pull or push the fork? Do I need special tools? Thanks!

[timsrv]

According to the service manual the fork shaft only has 2 positions (normal and lock). The "lock mode" is for service testing only. I've never messed with it so I'm not exactly sure what (if anything) would happen if you were to drive with it in the lock mode. There is a statement in the manual that says "NOTICE: Use this position only for testing. Do not use it for normal driving." If you want to find out more, you can read about it in the service manual IN-10 through IN-13. Tim

[covenant]

Hi,
Thank you for the reply. In my manual, I just could not find the IN-10 and In-13, nor how to actually change the position of the fork shaft. Will you please post some pictures showing how to change the position of the fork shaft? Thanks a lot!

[covenant]

Hi,I got the locking details from the yahoo previa group,thanks!

[Jonny]

Just in case others are looking at this with their transfer problems, search "disabling Alltrac" and you'll find another useful thread.
My particular problem, the tight turning radius wheel skidding thing, was not solved by simply removing the front drive shaft. No power to anything when I did that. Locking the transfer plus removing the front drive shaft did work. For the record... 12k miles on the locked transfer and no issues. See other thread or repair manual for locking method.
Jonny.