Warm up the engine. The operating temperature of the oil should be around 80°. Check oil level.
Disconnect electrical cable from oil pressure switch -1- and unscrew oil pressure switch.
Screw a suitable pressure gauge into the oil pressure switch hole.
Start the engine and leave it to idle. Oil pressure must be at least 4-6 bar (on models 316i from September 1988 and 318i from September 1987 at least 4.0-4.3 bar).
Screw in the pressure switch with a new ring and tighten to 35 Nm.
For deviations from the specified value, see the lubrication system troubleshooting table.