Toyota Four Wheel Drive System
Daniel I. Applebaum
Revision 1997.A
Updated: 12 December 1997

This article describes the operation of the Toyota 4WDemand
shift-on-the-fly 4WD system.

The transfer case has a switch that follows whether you are in 4WD or
not.  When you pull the lever into 4WD, this switch activates,
affecting some vacuum switching valves located on the back right
section of the engine compartment.

On the front differential is an "axle disconnect housing."  This is
called the Automatic Disconnecting Differential (or A.D.D.) on
Toyotas.  Basically, one side of the differential has a short axle
shaft that goes into this housing.  The end of this shaft is splined.
On the other side of this housing is an another splined shaft, the
other end of which goes to the left front wheel.  Inside the housing
is a collar, which is internally splined so that it can fit over one
or both of the shafts.

The vacuum mechanism that moves the collar is called a "vacuum motor."
It's advantage over an electric motor is that constant power can be
fed to it, in the form of vacuum, without fear of damage when the
motor stalls.  As the vacuum motor spends 99.9% of its life stalled,
ie. not moving, but under power, this is a good thing.

Based on which vacuum lines are activated by the vacuum switching
valves, the motor can engage or disengage the collar.  In 2WD mode,
the collar is on only one of the shafts, so the two rotate
independently.  In 4WD mode, the collar covers the splined ends on
both of the shafts, therefore linking the two together.  When you
switch back to 2WD, the vacuum motor pulls the collar the other
direction, disconnecting the two shafts.

When in 2WD, the vacuum motor is constantly being fed vacuum to keep
the collar disengaged, when in 4WD, it is constantly being fed vacuum
to keep the collar engaged.  Only when the collar moves to engage or
disengage does the vacuum motor actually move.

Sometimes it takes a while for the 4WD to engage.  In this case, the
transfer case is fully engaged, but the splines on the two shafts in
the A.D.D. housing aren't lined up yet.  The vacuum motor is trying to
move the collar, but it is blocked.  As soon as the splines line up,
the collar will move and lock onto the other shaft.

When the collar is fully engaged, it depresses a switch.  When this
switch is depressed, and the aforementioned switch in the transfer
case is depressed, the "4WD" light on the dashboard illuminates.  This
gives you positive feedback that you are in 4WD mode.

To disengage 4WD just push forward on the lever, at any speed or
direction.  If the system is "binding" it may be hard to do this.  The
drivetrain may be binding if you are currently turning, or are on
drive pavement and have executed some turns.  The engine is exerting
much force either forwards or backwards (engine braking) the collar
will remain bound.

Sometimes it helps to coast, but don't use engine braking.  It
sometimes helps to disengage (press) the clutch.  Sometimes you can
get the lever disengaged but the 4WD light stays on.  I'm not sure at
this point if the transfer is actually disconnected or not, so you
might actually be in 2WD mode, but the front axle is still connected.
Eventually, after turning or coasting, the front axle will disconnect
and the light will go off.

The A.D.D. system uses a flange at each front wheel instead of manual
or locking hubs.  Basically, they're single pieces of metal that bolt
on where a locking hub would otherwise go.