ABA specific info...... Low pressure switches, normally closed with no pressure, should open as soon as low pressure threshold is reached, these do not operate the buzzer. These switches and this part of the circuit work the same as basic oil idiot light switches have worked since Henry Ford was young. One of either Brown, rated 0.3 bar (4.4 psi) or Blue, rated 0.2 bar (3,0 psi) May be located in the left (driver's) side of the head or on the oil filter flange next to the high pressure switch. ______ High pressure switches, normally open with no pressure, should close when the pressure threshold is reached. When high pressure switch remains open due to a failed switch and/or low oil pressure AND engine rpm is above 2000 the light will light (possibly flash) AND the buzzer will sound. One of: White, rated 1.8 bar (26 psi) Grey, rated 0.9 bar (13 psi) or possibly Black, rated 1,4 bar (20 psi) Located on the oil filter flange. ___________ From http://reflectionsandshadows.com/cabby/dopws.html Cabby specific info....... ---- It is very simple to test the control circuitry.  With the key on and engine off the oil pressure light should be on steady on early cars, or blinking on later cars. Disconnect the yellow wire from the terminal marked "WK" ("Wahrning Kontraption" in German ) on the dual sender on the left side of the cylinder head, (this is the low pressure switch), the oil light should go out.  Start the car with this wire disconnected, light should remain out.  Ground this wire and light should come on and remain on as long as this wire is grounded. Disconnect the blue/black wire from the white, high pressure switch on the oil filter flange and rev the engine over 2000 rpm.  After approximately a two second delay the light and warning beeper should come on and remain on no matter what rpm the engine is running until the blue/black wire is grounded.