Oil Pressure Switch

["Al Powell" <apowell@agcom.tamu.edu>]

OK - we're having those ever-popular oil pressure warning lights.  Of 
course, the analog gauge tells me pressure is great (why doesn't that 
portion of the sensor  ever fail??) so I have ignored it until now, 
but the warnings and beeps are now almost constant. I have reviewed 
the list-wisdom on this (archived since I knew it was coming) and 
my reasoning is thus:

The warning light/beeps appear when I start the car and continue when 
it is at idle, as well as while driving it.  Therefore, I presume the 
problem is in the LOW pressure sensor.  Why?  

Because if it were the HIGH pressure sensor, no warning should 
appear **until 2500 rpm are exceeded** and the tach interface looks 
for the switch closure in the high pressure module.

Make sense?

So - to the car, Bentley in hand, check the illustrations, and as 
usual these are as helpful as.....well, just no help.  The sensors 
are shown floating in some mid-engine never-never land next to the 
lower half of the block, giving me no real clue as to their mounting 
points.  So where in the gee-whiz ARE they?

The only candidates I can find on my 1990 200 are below spark plug #2 
and behind (top one almost touching) the lower radiator hose...of 
course, in one of the LEAST accessible places on the engine.  The top 
one of these two "suspects" has two connections, leading me to assume 
it's the LOW pressure oil sensor.  One connection would be analog 
gauge, the other the hyperactive idiot light.  The lower sensor 
(immediately below, perhaps 1 inch lower on the block) has only one 
wire, so I assume it must be HIGH pressure.

I can't see any brown or white colors associated with these, so I 
can't tell whether they are the culprits or not, but they're the only 
ones which seem to have the right characteristics.

However, the alert reader will note that I have "presumed" once and 
"assumed" twice in this email, as well as trying to reach more than 
one logical conclusion.  Whenever I use either of these terms in 
connection with the Audi, or attempt to use logic, it's a pretty good 
synonym for "I am making a mistake and admitting it in public."

I welcome comments about whether I have:

1)  Figured out the culprit, and;
2)  Located it properly.

Comments direct to me are appreciated, as the evening is young and 
I'll bet some list members are checking email at home right now.

Thanks!
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Al Powell, PhD             Ph:  409/845-2807
Ag Communications          Fax: 409/862-1202
Texas A&M University
http://agcomwww.tamu.edu/agcom/satellit/alpage.htm
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