Software modifications requested by Volkswagen provided “yet another path toward potential input of data as a ‘defeat device,’” said the letter, which was submitted as part of an ongoing lawsuit by a Volkswagen owner in Germany. The letter went on to quote verbatim from United States and European laws prohibiting the devices.

“A supplier has practically no possibility to say ‘no,’ or they lose business from the biggest carmaker in the world,” said Kai Borgeest, a professor at the University of Applied Sciences in Aschaffenburg, Germany, who has testified as an emissions expert at the European Parliament.

Bosch argued that the letter was misinterpreted and applied to different engines than the ones programmed to cheat, according to a court document filed Wednesday as part of the proposed settlement.

The German car industry would probably not be such a powerhouse without Bosch.

The company, which began as an electrical workshop in Stuttgart in 1888, supplied many of the electrical components, like spark plugs, that made mass-market cars possible. After World War II, Bosch was an indispensable partner as the immensely popular Beetle transformed Volkswagen into a global carmaker.

Bosch, which has 390,000 employees, also supplies an array of components such as anti-lock brakes to BMW, Daimler and virtually all of the world’s major carmakers. It would be tough to find a car that does not have some Bosch parts. Bosch, which is owned by a foundation, is one of the world’s largest privately held companies, with sales last year of 73 billion euros, or $78.5 billion.

Since early in its history, Bosch has cultivated a reputation as one of Germany’s most conscientious companies. Although Bosch manufactured military goods for the Nazis during World War II, the company’s wartime chief executive, Hans Walz, helped Jews to escape the concentration camps. Mr. Walz is recognized at the Yad Vashem holocaust remembrance center in Jerusalem.

Beginning in the 1980s, Bosch played a major role in refining diesel technology for passenger cars, providing sophisticated fuel injection systems and computers that sit under the hood and manage the operations of the engine. The technology allowed carmakers to more precisely calibrate the way fuel is burned inside the engine and to reduce the vibrations and sooty exhaust.