At 06:16 PM 3/26/2011, Scott Daniel  - Turbovans wrote:

>why is the blue wire D + tied into the Oxygen Sensor elasped milage
>counter box and light ..
>and why does it say 'cat elapsed milage counter' at the box, track 46,
>page   97.107  ?

Short answer is that the switch at track 46 (that's the box under the
van) will light the OXS light when it trips, by grounding its
negative lead.  But *also* the wire at track 45 dives into the
fuse/relay panel on plug C, comes out on plug A at track 44 where
it's referred over to track 50.  There at the plug B connector it
joins the D+ wire heading from the instrument panel back to the
alternator.  This whole charade is for a bulb check - any time D+ is
grounded and lights up the ALT light, it will also light up the OXS
light.  If you haven't noticed that this happens, goes to show how
good people are at noticing things they see every day (or that they
drive an '89+ which doesn't have an OXS light).

The diode inside the switchbox at track 45 is so that the OXS switch
won't attempt to mess with the D+ line when it trips: D+ can pull OXS
low, but OXS cannot even try to pull D+ low (which it could have
trouble doing if the alternator were charging, could be some magic
smoke let out there somewhere as well as a false ALT light.  Which
might no longer be false if the magic smoke came out).

Yrs,
d