Mark, the three green lights works as follows, from left to right:

1. Front differential lock (if fitted - usually only on european vans).
First stage is a flashing LED which is operated by a vacuum switch near
the operating knob as soon as the knob is pulled out (this is because
extra warning is needed on the front diff as engaging it causes
significant steering loss!). As soon as the lock shaft is engaged by the
vacuum servo (may take a few driveshaft revolutions) a plunger switch
fitted above the shaft comes into play and makes the flashing LED solid.

2. The middle LED is for the manually operated four wheel drive which
was fitted on some vans (again mainly european) instead of the more
usual viscous coupling.  Very rare!

3. The rear diff lock operates the right hand LED using the same plunger
switch method as the front diff lock, but there is no flashing stage
operated by vacuum.  I assume you only have a rear diff lock, so it is
likely to be this LED which is relevant to you, and you probably have
only one of the three possible operating knobs fitted.

You can check the lock easily by jacking up both wheels on one side -
turning one wheel should cause the other to turn if the diff is not
locked.  If the lock is working OK it is easy to unscrew the plunger
switch and check it out (push in to make contacts).  You should also be
able to be able to see the vacuum servo pushing the shaft in to lock the
diff.  If you need to replace the servo it is less expensive than I
thought it would be (�27 in the UK, thats about $42).  If the shaft is
seized where it enters the diff housing (not uncommon) it can be a
problem to free it off - get back to me and I will give you some tips on
how to free it without removing the transmission (I've just done it on
mine successfully, but it wasn't easy). Hope that helps

Don McCallum (1992 1.6 Turbo Diesel Syncro, North Wales, UK)

Mark Watts wrote:

> I need help in trouble shooting the diff lock on my 86 westy synchro.
> I had a
> quatuum station wagon synchro so I know what to look for when it's
> locked.
> Right now it doesn't seem to be getting any vacuum so I'll start there
> first.
>
> BTW the dash has three green lights for the diff  lock but the owners
> manual
> only has one???? The vacuum switch only has two positions though... In
> my
> station wagon you could lock the center diff and then both the center
> diff and
> the rear diff if needed and the lights would come on accordingly.
>
> I know the viscous coupling works as I went through loose sand at the
> beach no
> problem this last weekend.
>
> Thanks in advance.
>
> Mark
> 86 westie synchro (paid $4000! and passed all the regular checks!)