The two auto assembly plants look the same, sprawled 15 miles apart on opposite sides of the Hudson River.

General Motors and Ford. Tarrytown, N.Y., and Mahwah, N.J.

This month, however, Ford will shut down the Mahwah plant, the company's largest, while GM's Tarrytown plant still runs full tilt.

The story of Mahwah and Tarrytown appears to be more than the head-to-head competition between two cars, Ford's Fairmont and Chevrolet's Citation, although that is the primary reason why Tarrytown won and Mahwah lost. The Citation, with its advanced front-wheel drive engine and improved gasoline mileage, is selling fast. The more conventional Fairmonts and Mercury Zephyrs made at Mahwah are not.

That was not the only reason that Tarrytown has been successful and Mahwah has not, however.

Ford officials say that Mahwah had a history of problems with quality, absenteeism, production costs and labor-management tension. Despite improvements in recent years, Mahwah still ranked near the bottom, Ford claims.

The Tarrytown plant, on the other hand, is one of GM's success stories: the site of a six-year-old experiment by GM and the United Auto Workers Union to "humanize" the assembly line. The company and the union now trade ideas for solving plant and labor problems, and in the process, morale and workmanship have steadily risen, according to union and company officials.

Ford officials put the "problem" tag on their own plant in April, when they announced the decision to close the plant.

"Mahwah had a less favorable quality history than another plant which we had considered, and the cost was high," said Harold A. Poling, executive vice president for Ford's North American operations.

John A. Manoogian, Ford's director of product interview: "There was some evidence of improvement, but if you have to choose, you select the plant that's historically given you a little more heartburn." Mahwah rated "consistently poorer than other Ford pllants, he said.

Although Ford has not documented comparisons publicly, it did give several congressmen a look at the ratings at a mid-April meeting April after the announcement. Rep. Benjamin Gilman (D-N.Y.) said Mmahwah was seventh out of eight plants in a comparison of manufacturing costs in recent years. It was equally low on direct labor costs and the expense of repairs the company had to pay for under warranty programs.

At Ford, trying to combat Japanese imports whose buyers praise their workmanship, quality has become the No. 1 priority, Manoogian said.

A swarm of protests followed Ford's criticism of the Mahwah plant.

"We're not there to jeopardize our own jobs," said auto worker Jim Berke, who now faces unemployment in June, along with 5,000 other Mahwah workers.

"This is a rough business," said Gerry Felker, a production line supervisor. "It takes years to get accustomed to what you have to do. But 95 percent of our work force is sincere about their jobs."

"Sure yoou have problems," said Richard Dragotta, chairman of Mahwah's United Auto Workers local. "It's the most boring monotonous job there is."

But he and other plant employes said that Mahwah's problems were in the past and in recent years, quality has been good.

Joseph Reilly, former president influx of minority group workers in the late 1960s created tensions within the plant.

"You've got a real melting pot here." said Reilly. "It took a long time to get all those relations worked out. But the last few years have been pretty good."

The company and the union clashed in 1976, when the local led a nationwide strike against Ford.

"They were trying to run the place like a military installation," said Dragotta, the local chairman. Following a change in management, the atmosphere improved, he and other Mahwah employes say. "We haven't had an arbitrator in here in six years," Dragotta said, and the number of disputes over work grievances now is almost nil.

But Mahwah, it appears, could not escape its history.

Ironically, Tarrytown's reputation was even worse nine years ago.

It was known as having one of the poorest labor relations and production records in GM," according to Robert H. Guest, a Dartmouth University professor whose study of the plant was published in the Harvard Business Review. "Workers were mad at everyone. They dislike the job itself."

Tarrytown, however, was in from the start on the GM-UAW experiment in teamwork that began in 1973, when the company and the national union agreed to seek ways of improving the quality of work life at the plant.

That led to a series of voluntary training adn problem-solving meetings in 1974 for plant employes. Although the program took several years to develop fully, its roots were planted. The company and the union had begun to listen to one another and put themselves in one another's place.

The direct benefits from closer cooperation have come steadily -- fewer window leaks and breakages, improved welding operations and better morale.Union records show that absenteeism dropped from 7 1/2 percent of the work force when the program began to between 2 and 3 percent in December 1978. By then, the number of union-company grievance cases had dropped to 32 from an average of 2,000 in the early 1970s.

GM officials say over and over again, however, that such measurements of increased productivity were not -- and are not -- the goal of the "quality of work life" program now in place throughout GM.

"At GM, quality of work life is both a goal and a process," said D. L. "Dutch" Landen, who directs the program for the company.

The direct goal is not improving profitability or productivity, he said. Instead, the program aims at improving customer satisfaction and employe morale and effectiveness, he added.

At newer GM plants like one in Brookhaven, Miss., the entire philosophy and organization of the assembly line has been altered, and along with it, the traditional lines of authority.

"It is a very egalitarian approach," said Landen. "There is no dining room for executives, no reserved parking places nearest the front door for the management. It's first come, first served." The point, he says, is that many of the things companies have done to distinguish between groups of employes are no longer appropriate.

"The job is still putting nuts on bolts. The basic elements remain the same. But the role of the person in the organization becomes a fundamentally different one," he said.

GM clearly had a better idea, Mahwah workers say.

But Ford Mahwah plant was headed in that direction, too. About five months ago, Ford management and the local UAW established a committee of electricians, machinists and other skilled workers to talk through plant maintenance and repair problems handled by the skilled trades.

It was not a honeymoon, said local union official Arthur Kolb, "but we were becoming partners with the company" on maintenance problems.

Both sides found that was an inprovement on working by the book said electrician Henry Herman. "They were getting cooperation."

"It was on the way to being a success," said Kolb.

But not in time.