At 03:10 PM 11/6/2008, John Rodgers wrote:
>Apparently there is a resistance test circuit built into the thing and
>it takes a few moments for it to do it's job of testing the resistance
>between  the probes in the coolant tank.

Actually there's a blink test built into the gauge -- it's supposed
to blink for three seconds on power-up and any time it's triggered.

The Bosch people took advantage of that with the new-type controllers
-- instead of giving a steady overheat signal, which is what triggers
the blink, they just give a tiny blip to start the gauge blinking,
then another tiny blip a little later on and so forth.  That way the
temperature indicated on the gauge stays about the same instead of
going into the red, which is a glaring misfeature of the older design.

>I suspect that if you do not have the ratio of water to coolant within
>the proper range, that silly light may blink - maybe. IE, to much
>coolant for the water so the resistance is wrong - blinking light; to
>much water, not enough coolant, so resistance is wrong - blinking light.
>This is just my suspicion.

I have read -- somewhere -- a VW troubleshooting tree for that
circuit with a spec for coolant resistance.  Must be in a TSB,
because it's not in Bentley.  But by my own tests the circuit is very
sensitive.  A resistance of less than 180K ohms between the live
probe and any fixed voltage -- ground or 12v or anywhere in between
-- will stop the light blinking by extinguishing the square wave
impressed on the live pin.  That's for the old-style controller; the
new-style should be similar.

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"