RE: Locking Diffs

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> My two '86 4Kcsq's and my two '87 5kcstq's all have a two-position
> differential lock switch and a lighted image of a 4-wheel set up with a
> spot in the middle for the central lock and one in the rear for the rear
> differential lock.  You engage the central lock on the way to clicking the
> switch for the rear lock.
> 
> I believe the front differential is the one between the front wheels and
> has no lock.  The central differential is the one that locks at any speed
> for "equalized" four-wheel power application.  The rear lock, which
> disengages at something like 15 mph,  makes the rear differential (and, I
> think, the rest of the system) more like a limited slip where one wheel
> can
> turn only a certain percentage of a rotation before the other wheel
> becomes
> engaged.
> 
You are mostly correct ... but actually the stock switch positions are
center locked (position I) and _both_ center and rear locked (position II)
... and unless you got something other than the 4kQs and 5kQs you mention
the diffs do not unlock at any speed unless you move the switch.  

That said, it is a fairly simple matter to get the locking module for the
rear diff on an '89-91 100/200q (perhaps others of the same vintage as well)
which does have the speed sensitive unlocking function.  It provides an easy
means of converting the diff lock switching from a pneumatic switch to
electrical ones, while eliminating the need to fashion some sort of solenoid
as a diff lock actuator.  If you mount the module close to the pneumatic
diff actuator and provide a stout vacuum source you will find that it does
provide a very positive action (except for the urqs and 4kqs that have the
long Bowden cable to actuate the center diff).  The module is usually
mounted under the rear seat in the donor car, pretty much centered over the
propshaft tunnel, and has an electrical connector and 3 vacuum lines
attached.  The cool thing about it (IMO) is that if you want the speed
sensitive unlock you can wire in a speedo signal, but if you want it to stay
locked whenever the switch is on you just leave the speedo inputs
disconnected!

HTH!
Steve Buchholz
San Jose, CA (USA)