My car (Saturn AURA) displays an oil life percentage-presumably, when it hits zero, it is time to change the oil. What does it actually measure? My Dad’s Caddilac has a very sophisticated monitorit measures the conductivity (electrical) of the oil, and determines when the oil is so contaminated that it needs changing.
How doe these things work?
I don’t know about your Saturn, but my Honda’s “time for a service checkup” indicator is activated by a certain number of miles on the odometer.
The GM system counts time your vehicle spends in various operation modes, and creates a synthetic score that is then used to determine if your oil likely needs changed.
Here’s a handy GM article:
http://www.gm.com/corporate/responsibility/environment/maintenance/simplified_maintenance_qa_040104.jsp
and here’s the really pertinent part of that article:
“How does it work?
In all internal combustion engines, the primary driver in oil degradation is the combustion event. Every time the engine fires, a small volume of oil is directly exposed to, and destroyed by, this high-temperature, high-pressure event. Furthermore, combustion gases containing acids, fuel, and water get past the piston rings and react with, and degrade the engine oil. In addition to the combustion event, oil temperature plays a role in oil degradation. As oil temperature increases the oxidation rate of the oil speeds up, which can lead to oil thickening over time. On the other hand, colder oil temperatures increase the concentration of contaminants, such as fuel and water in the oil. The GM Oil Life System comprehends these oil degradation processes, and using an algorithm, calculates the optimal time to change the engine oil. While the basic algorithm is the same for all GM engines, each engine gets its own calibration specifically tailored to its unique needs.”