Oil pressure warning lamp

The engine oil pressure on the Audi 80 is monitored by two hydraulic switches and a control unit with a buzzer (the latter two are combined in the mini-check system). In simplified terms, monitoring works as follows:

With the ignition switched on, power is constantly supplied to one of the contact terminals of the oil pressure indicator.

If the oil pressure (with the engine not running) is below 0.3 bar, the contacts of the 0.3 bar hydraulic switch close, creating a ground connection to the second contact terminal of the indicator – the lamp lights up (flashes).

If the oil pressure rises after the engine starts, the contacts in the switch open and the lamp goes out.

Additional monitoring is provided on models with 4- and 5-cylinder engines by the 1.8 bar hydraulic switch, and on models with a 6-cylinder engine by the 2.5 bar hydraulic switch.

The hydraulic switch and control unit are part of the mini-check system: if the oil pressure drops below the switching pressure of 1.8 or 2.5 bar, its contacts close, creating a ground connection to the control unit of the mini-check system. The 0.3 bar hydraulic switch does not switch on at this time.

The mini-check system allows the indicator lamp to light up only when the oil pressure drops below the switching pressure for longer than 1 second and the engine speed is above 2100 rpm.

In addition, the mini-check system emits an audible warning signal. Its purpose is to prevent engine damage caused by insufficient oil pressure (for example, due to an oil level that is too low).

Hydraulic switches
Left: 4-cylinder engine: both hydraulic switches, 0.3 bar (1) and 1.8 bar (2), are located on the oil filter flange (3).

Right: 5-cylinder engine: the 0.3 bar (1) and 1.8 bar (2) hydraulic switches are located on the left side of the engine, below the thermostat housing.

Left: on models with a 6-cylinder engine, the oil temperature gauge sender (1) is located next to the crankshaft pulley. The 2.5 bar hydraulic switch (2) is visible directly next to it. The 0.3 bar hydraulic switch is located behind it. On models equipped with additional instruments, the 0.3 bar switch is combined with the oil pressure sensor.

Right: this plug connector (arrow), located at the rear of the left cylinder head, contains all the wires for the oil temperature sensor, the oil pressure sensor and both hydraulic switches.

The insulators of the hydraulic switches are colour-coded.

The 0.3 bar hydraulic switch is brown. On vehicles equipped with a pressure gauge, this switch is replaced by an oil pressure sensor, which additionally has a switching contact for the indicator.

The 1.8 bar hydraulic switch is white (models with 4- and 5-cylinder engines).

The 2.5 bar hydraulic switch is red (models with a 6-cylinder engine).

Fault finding
PROCEDURE

If the indicator lights up (possibly with the buzzer sounding) while the engine is running, stop immediately!
Stop the engine at once and check the oil level. If it is correct:
Disconnect the wire from the 0.3 bar hydraulic switch and leave it hanging free, then switch on the ignition. If the indicator remains lit, a short to ground has occurred in the wiring – this is not dangerous for the engine.

Disconnect the wire from the 1.8 bar (or 2.5 bar) hydraulic switch and connect it to ground, then switch on the ignition. If the indicator remains lit or the warning buzzer continues to sound, the fault is in the mini-check or check system – this is not dangerous for the engine.

If you observe other effects not listed above, the engine oil supply may be impaired. The vehicle must then be towed!
One of the hydraulic switches may be faulty. Sometimes it switches on again if its plug tab is moved.

If the oil pressure indicator does not light up after the ignition is switched on:
First check the fuse (see chapter Body Electrical System).

Disconnect the wire from the 0.3 bar hydraulic switch and connect it to ground. If the indicator now lights up, the switch was faulty. If it still does not respond, the wiring or the bulb itself is faulty.

Control unit check
PROCEDURE

Disconnect the wire from the 1.8 bar hydraulic switch and leave it hanging free. Start the engine and run it at approximately 2500 rpm.

After 2 seconds of operation the indicator should light up and the buzzer should sound three times. Otherwise the mini-check system is faulty – replace!
If only the indicator lights up but the buzzer does not sound, the wiring from generator terminal 61 (D+) is interrupted.

If the indicator lights up and the buzzer sounds with the wire connected to the hydraulic switch at an engine speed of approximately 2500 rpm, the 1.8 bar (2.5 bar) switch is faulty – provided the engine oil level is correct.

Let the engine run at idle, disconnect the wire from the 1.8 bar (or 2.5 bar) switch and leave it hanging free. If the lamp and buzzer now operate, the wiring from terminal 1 (speed signal) and the mini-check system is interrupted.

Replacing the hydraulic switch
PROCEDURE

Obtain the required switch with brown, white or red insulation and one sealing ring.

Tighten the hydraulic switch to 25 N•m.