Below is info on oil level sensor courtesy of Alex Shepherd, whom I don't know, but is obviously another Alfa enthusiast - perhaps that should be masochist!
Member since Jan-1-04
42 posts Feb-12-04, 11:30 PM (EST)
6. "RE: Oil level sensor - how it works"
In response to message #5
OK, hopefully I can help make some sense of the situation!
The oil level sensor has a bimetal strip and a pair of contacts. It has two wires. In series with the contacts is a small heating wire, wrapped around the bimetal strip. This heating wire has a resistance of approx. 12 Ohm. Note that there is no earthing taking place.
When you turn on the ignition, the check control computer (Alfa Control device) makes its checks. It passes approximately 12V through the oil level sensor (and therefore, through the heating wire). It is able to 'read' the voltage on the wire that returns from the sensor.
If the oil level is low, the heating element heats up the bimetal strip which bends, opening the contacts. With the contacts open, current flow through the circuit ceases, and the check control computer puts on the warning light (and keeps it on until the ignition is switched off).
If the oil level is normal, the oil conducts the heat away from the heater, and the bimetal strip remains straight. The contacts therefore stay closed and the check control computer does not switch on the warning light. Note that the checking of the oil level sensor lasts only a few seconds: the warning light will not illuminate once the engine is running - it will remain either 'off', or 'on' as above.
With the car parked on a slope, the engine oil can be below the level of the sensor. This causes the light to come on during the next start, and stay on until the ignition is switched off. There is obviously no fault in this case.
With the engine oil very hot and slightly low, it is possible for the oil heat to open the sensor. Again, the problem should clear after the engine cools.
Because the heater wire is fragile, it is possible for it to break, which effectively opens the circuit even though the contacts remain closed (the heater is in series with the contacts). This will ensure the light stays on whenever the engine is running.
The two wires to the sensor are very small, and appear as one wire. Consequently they may break, and the connector is another possibility.
Finally, it is important that neither wire is ever earthed (grounded), because both wires are controlled by the check control computer. If someone has attempted to bypass the (faulty) sensor by shorting to ground, or by shorting the wires together, the check control computer will be damaged.
To test for a faulty sensor, substitute a 12 ohm resistor across the two wires. This mimicks the heater resistance (keeps the current within the Alfa Control device's capability) and also simulates closed contacts (correct oil level). If the warning light persists after switching ignition off and on, then check all wiring/connectors, and replace ECU (in instrument cluster) with known-good spare. If the warning light does not reappear, assume that the sensor is faulty.
Good luck (I have had this problem several times with FIAT-group cars!)
'88 Bertone X1/9
'88 FIAT Uno Turbo
'92 Alfa 164 3.0V6