False Coolant Level Warning Symbol on Auto-Check ('88 5KSQ)

["Christopher Ritchie" <critchie1@hotmail.com>]

Thanks to Phil Payne, Stephen Bigelow, Steven Buchholz, Orin Eman, 
and Brett Dikeman for advice on changing the fuel filter.  I really 
appreciate the help available here.
 
Now, after flushing the cooling system, I have another problem with 
my 1988 5000S Quattro.  The "coolant" symbol on the Auto-Check system 
displays about 30 seconds after the engine is started and stays on 
the whole time the engine is running.  It displays the normal "OK" 
when the engine is first started.  The temperature gauge reads 
accurately.  The fan comes on when it is supposed to.  The coolant 
level is above the "Min" mark.  Where the "coolant" symbol comes on 
so soon after starting, I suspected the coolant level sending unit.  
However the sending unit checks out on the multimeter.  No continuity 
when the float is up.  Continuity when it is down.  The float also 
does indeed float up and down with the water level.  One oddity - 
there is continuity between the 2 contacts of the connector 
(blue/yellow and brown wires) to the sending unit.  But only when the 
ignition is turned to the "ON" position.  There is no continuity when 
the ignition is off.  There is also continuity between the brown wire 
and ground.  Also, the fan will not run after the engine is shut off, 
even if the temperature is high enough.  (Isn't it supposed to?)  So, 
if the level sending unit is ok, that leaves the wiring itself, the 
connectors, the temperature sending unit, or something mystifying 
which I am completely missing (most likely).  
 
How is the coolant temperature sending switch tested?  The Bentley at 
96.31 shows a 2 prong sending unit.  What I think is the temperature 
switch is a 4 wire thing on the bottom of the elbow on the block to 
the upper radiator hose.  I am also having problems following the 
wiring diagrams in Group 97.  
 
I thought I once read here that the 3 wire fan thermoswitch on the 
radiator under the lower radiator hose usually fails when the cooling 
system is flushed.  This is because the lower radiator hose is 
disconnected in order to drain and flush the system.  The large 
quantity of antifreeze and water pouring over this switch cause it to 
fail.  The 4 wire connector at the sending unit by the top radiator 
hose was also drenched in this process.  Any ideas?
 
This car was fine and dandy before I started screwing around with 
it.  Now that the weather is getting nicer, my plan was to do 
everything - filters, fluids, lube, clean, test, and adjust - to 
establish a maintenance baseline.  The coolant that I flushed out was 
fine.  But I did not know how old it was.  I hate to fix things that 
do not need fixing only to break them in the process.  (The 
transmission and differential fluids are next.  I already made sure 
the filler plugs will loosen.)

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