Motormouth. Take 2.

Cars Monitor Oil So You Don't Have To

November 12, 2000|By Bob Weber.

Q--I read with interest the Motormouth Take 2 (Transportation, Oct. 15) on motor-oil additives. While reading, I recalled an earlier column that indicated car manufacturers are starting to use oil-life sensors. How do these sensors work? W.B., Naperville

A--Before we explain the oil-life monitoring designs, we would like to praise the engineers who have developed them. Not only do oil-life sensors save car owners money, they also help us preserve precious natural resources.

That's because most owners' manuals read as though there are two basic types of automotive maintenance: normal and severe. This has created a motoring population of under-maintainers, over-maintainers and confused car owners in between.

Under-maintainers treat their cars like appliances. You probably know at least one. They use their cars to get to work and run errands. They put gas in them, but never raise the hood to check anything. Under-maintainers believe they are "normal" drivers because they seldom drive their cars at high speeds on the highway, driving them to and from the train station or supermarket--short trips which, they believe, do not stress the vehicle. They may drive only 4,000 miles per year and consider motorists who roll up 15,000 miles a year abusive. Poor, misguided under-maintainers.

Then there are the over-maintainers, who seem to do nothing but tinker with their cars. You see them washing and waxing every weekend. They religiously change the oil and filter every 3,000 miles, as though it may bring them everlasting salvation so they can wax their cars for eternity in heaven. We confess to being a member of this flock.

But most car owners fall in between and are not sure how often to change their oil. The complexity and confusion compound when several drivers share a vehicle. One driver may use the car for numerous short trips (the "severe" type) while the other generally drives many miles at highway speeds (the "normal" type).

Should we split the difference and change the oil between 3,000 and 7,500 miles? Probably. But there is no formula to figure out the optimum oil-change interval. And what factor should we use if we add a quart of fresh oil?

So we sin on the side of waste or negligence. Fortunately for us, salvation has come in the form of the oil-life monitor.

As with many other automotive systems, automakers have different approaches to achieving similar goals, and that goes for oil-life technology. There are two basic approaches, with a third that combines the two.

The General Motors approach is, perhaps, the simplest. The GM Oil-Life System involves math and relies on computerized monitoring of engine revolutions, operating temperature and other factors to determine when the oil should be changed. Beginning with the 2000 model year, cars with this system will be able to go 10,000 to 15,000 miles between oil changes, depending on vehicle and engine.

When the system determines that the oil is approaching the end of its useful life, the "change oil" light comes on for 15 seconds when the engine is started.

Instead of measuring anything pertaining to the oil, the GM system monitors driving habits. There is no on-board sensor, per se. Instead, GM uses a mathematical model to deduce the oil degradation. The model is based on millions of miles of testing.

Though the GM Oil-Life system extends oil-change intervals, it is still biased toward the conservative side. The company doesn't want to be replacing engines left and right.

The high-tech method of measuring the oil's life expectancy uses a sensor to measure the condition of the oil in the crankcase.

As the first on-board maintenance monitor to electronically measure oil quality, the Mercedes-Benz Flexible Service System was the first to use a sensor. The system signals the driver when an oil change is required.

The Flexible Service System can safely extend maintenance intervals up to 20,000 miles, provided the vehicle is driven primarily on the open road at light throttle, says Mercedes. With a lot of heavy stop-and-go driving, the system may prompt the driver for service as often as every 10,000 miles. In any case, a readout on the instrument panel tells the driver well in advance when the next service is needed.

The system incorporating an oil-quality sensor is standard on all Mercedes-Benz vehicles powered by the twin-spark, three-valve V-6 and V-8 engines. The heart of the system is a dielectric sensor in the engine oil pan that sends signals to the Flexible Service System computer based on the amount and type of contaminants in the oil. The sensor also can detect oil level, so the computer can recognize when the oil is "topped up."

Signals about engine speed, throttle opening, oil and coolant temperature are also processed by the computer, so it can distinguish between city or highway driving as well as any unusual engine strain.

Mercedes vehicles without this sensor use an approach similar to GM's, basing the oil life on engine operating conditions.