As far as I know, and the Bentley confirms that the 85, like the 83.5-84
(which I have), only has one sensor -- the one on the side, which is a
.3bar, the low pressure sender.  They didn't go to two sensors until the
Digifiant, in 86.  That makes this particularly interesting.  They typically
go off at low idle after running hot for a bit.  The oil is thinnest, the
pressure lowest, and the switch goes off and flickers the light.  Does the
automatic drop to idle when going downhill?

At any rate, hooking up a pressure gauge will tell all.  I just went through
installing one (permanently) while also modifying an 89-90 dash to work with
my 1.9(re-wire plus disabling the >2000 rpm pressure input).  With no high
pressure switch on the digijet models, the gauge is nice to have.

Rich
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This is critical to diagnose immediately.  It  is possible that the high
pressure sensor is bad.  It is however possible  that the oil pressure is
low for the high pressure sender which is indicating  worn rod bearings and
if that's the case the next thing that happens is the  number three rod will
come out of
the case right on top with a loud bang.   The only true way to know what's
going on is to connect up a real oil pressure  gauge to the port between the
push rod tubes and get a true oil pressure reading  of the oil when hot at
running temperatures.  PITA I know but unless you  want to see that number
three rod in your engine compartment it's the only right  way.  This is why
it is highly recommended to have a "REAL" oil pressure  gauge mounted on the
dash board, instead of a bobble head, to keep an eye  on the "REAL" oil
pressure of these old high mileage waterboxer engines
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