You'd never guess the diesel-car market was on the rocks by looking at Mercedes-Benz. General Motors diesel-car sales in 1982, for example, were off a stunning 37. 6 percent compared to 1981, yet Mercedes-Benz - Daimler-Benz AG is the corporate name of the producer in West Germany - actually increased its US diesel sales by almost 3,000 cars, most of them turbodiesels. Among Mercedes-Benz diesel-engine cars, only the low-end 240D is not turbocharged.
(A low-end car at Mercedes-Benz means a price tag of about $24,000. The average price of all cars sold by the company in the US is just under $33,000.)
Clearly, the company has to be doing something right. Right?
Last year the diesel engine accounted for 78.9 percent of all Mercedes sales in the United States, compared to 78.3 percent in 1981. The West German company, for example, sold 65,963 cars here last year, 52,051 of them with diesel engines. In 1981, it sold 63,059 cars, 49,354 of them diesels.
''Well, for one thing,'' says a spokesman, ''a diesel is never an option with a Mercedes.'' If a buyer opts for a 240, he gets a diesel engine beneath the hood. There is no gasoline-fueled alternative.
This avoids confusion because the buyer does not have to make a choice between engines.
The buyer of a Mercedes-Benz automobile first has to decide that he wants to consider a Mercedes. Then he doesn't have to debate a wide choice of gasoline-fueled engines as well as a V-6 , a V-8, or maybe a 4-cylinder diesel. Also, he has to have at least $24,000 in his pocket, or the financial clout to get the required financing for the automobile.
That decided, he picks out a dealership. Then, if he opts for a 300 line of cars, he gets a turbodiesel - nothing else. If he wants a station wagon, he gets the 300TD - a turbodiesel wagon.
The way the company figures it, ''If a buyer opts for a 300TD, it's a Mercedes that only happens to have a diesel engine inside,'' a spokesman explains.
No other company seems to have quite that kind of authority in the marketplace, except perhaps for some of the exquisite, low-volume cars, such as a British-built Aston Martin or one of the Italian exotics.
Daimler-Benz AG of Stuttgart is hardly a high-volume car and truck producer - it also makes off-road construction equipment, among other things - but with auto production approaching 500,000 vehicles a year, neither is it in a league with Aston Martin's 200 cars.
Thus, Daimler-Benz rolls along, not only on the highway but off it as well.
It made the decision a long time ago to ''make it big'' with the diesel. In other words, it brought a lot of class to the oil burner by making it acceptable in the corporate board rooms of the world as well as to anyone else with the wherewithal in his, or her, jeans.
''We expect from 75 to 80 percent of all Mercedes cars sold in the US this year to be diesels,'' a spokesman declares.
Many other diesel manufacturers would like to have that kind of optimism - and the track record to back it up.