When the ignition is on, one of the oil pressure indicator terminals is constantly supplied with power.
If the oil pressure (with the engine off) below 0.3 bar - the contacts of the hydraulically actuated 0.3 bar switch close, thereby creating a connection at «mass» to the second contact output of the indicator, the lamp lights up (flashing).
If, after starting the engine, the oil pressure rises, the contacts in the switch open and the lamp goes out.
Additional control is carried out on models with 4- and 5-cylinder engines, as well as with 6-cylinder engines up to 7/91, by means of a switch with a hydraulic actuator of 1.8 bar, on models with 6-cylinder engines from 8/91, by means of a switch with hydraulic drive 2.5 bar. The switch with hydraulic drive and control unit is included in the mini-check system: if the oil pressure drops below the switching pressure of 1.8 or 2.5 bar, then its contacts close, thereby creating a connection to «mass» to the control unit in the mini-check system, the hydraulic switch 0.3 bar is not switched on at this time.
The mini-check system allows the indicator lamp to flash only when the oil pressure drops below the switching value for more than 1 second, and the engine speed is more than 2100 rpm.
Additionally, the mini-check system emits a warning noise signal. The point of all this is to prevent engine damage due to too low oil pressure (for example, due to its too low level).
Hydraulic switches
There is a significant difference in the switches with the 6-cylinder engine: up to 7/91, the 1.8 bar switch was mounted at the rear at the left cylinder head. Since 8/91, it has been moved down to the left side of the engine cylinders, where, however, higher oil pressure reigns, and therefore the installation of a hydraulically actuated switch of 2.5 bar is necessary.
The insulators of the hydraulic switches are painted in different colors.
0.3 bar hydraulic switch painted brown. On vehicles equipped with a pressure gauge, in place of this switch is an oil pressure sensor, equipped with an additional switching contact for the indicator.
1.8 bar hydraulic switch painted white (with 4- and 5-cylinder engines and 6-cylinder engines up to 7/91).
2.5 bar hydraulic switch painted red (with 6-cylinder engine from 8/91).
Troubleshooting
If the indicator flashes (possible with additional buzzer) with the engine running, stop immediately!
Stop the engine immediately and check the oil level. If he is ok:
Disconnect the wire from the 0.3 bar hydraulic switch, turn on the ignition: if the indicator stays on, this means that there is a short circuit in the wiring to «mass» - not dangerous for the engine.
Disconnect the wire from the 1.8 bar hydraulic switch (or 2.5 bar) and lead to «mass», turn on the ignition: if the indicator remains on or the warning buzzer continues to sound, then the malfunction is related to the mini-check system or wiring - not dangerous to the engine.
If you observe effects other than those listed above, then you may suspect that the engine oil supply is impaired. Next, the car must be towed!
One of the hydraulic switches may be faulty. Sometimes it turns on again if you shake its plug tongue.
If the oil pressure indicator does not flash after the ignition is turned on:
Check the fuse first (chapter «Body electrical system»).
Disconnect the wire from the 0.3 bar hydraulic switch and lead to «mass»: if the indicator now lights up, then the switch was faulty. If it still does not respond, then the wiring or the bulb itself is faulty.
Checking the control unit
Disconnect the wire from the 1.8 bar hydraulic switch and leave it hanging in the air. Start the engine and let it run at about 2500 rpm.
After 2 seconds of operation, the indicator should flash and the buzzer should sound three times. Otherwise, the mini-check system is faulty - replace!
If only the indicator flashes, but the buzzer does not sound, then the wiring from the generator terminal 61 is interrupted (D+).
If the indicator flashes and the buzzer sounds when the wire is connected to the hydraulic switch at an engine speed of about 2500 rpm, then the 1.8 bar switch is faulty (2.5 bar) - provided that the engine oil level is in order.
Leave the engine running at idle, disconnect the wire from the 1.8 bar switch (or 2.5 bar) and leave it hanging in the air. If the lamp and buzzer now work, then the wiring from terminal 1 is interrupted (speed signal) to the mini-check system.
Replacing a hydraulic circuit breaker
Purchase the desired switch with brown, white or red insulation and one o-ring.
Tighten hydraulic switch to 25 Nm.
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