I recently had a strange experience with my '87 4000 quattro cs. The
oil pressure light started glowing dimly (w/no buzzer) when the engine speed
was held at or above 2,000 RPM for more than a second.
I checked both the low-low and high-low pressure circuits and they seem
to function properly. The low-low one when grounded results in the oil
pressure warning light coming on and staying on bright. When disconnected,
the light goes away. However the light still comes on dimly above 2,000.
The testing of the other circuit yielded similar results. When
grounded, the light (bright) and buzzer come on above 2,000 RPM. When
disconnected the light still comes(dimly) above 2,000 RPM. So clearly the
sensors themselves are not related.
I attached my mechanical oil pressure gauge and it read ~50-60 psi at
idle when warm with 20/50W and well in excess of the 80 psi limit of my
gauge when the engine speed was increased above 2,000 RPM.
I thoroughly read the bently manual and it makes no reference to the
location of the oil pressure warning control unit, only that it probably
needs to be replaced based on my symptoms.
I would gladly pull the damn thing out and just go with an oil pressure
gauge, but I must first find the sensor.