Tokyo — Japanese auto engineers are baffled by the regular, lengthy close-downs by American auto plants for the costly retooling needed to handle model changes.
''With our computer we can do the whole thing in as little as three to six hours,'' says Teuro Osawa, a plant administration executive of Nissan Motor Company.
Assembly line modifications for a model change at Nissan's automated Zama plant near here, in fact, can be carried out during three-hour breaks in the early evening and early morning when the line is normally stopped for routine maintenance.
Also, the subtle working of Nissan's ''flexible assembly line operation'' allows the No. 2 automaker to produce different models on the same line with a minimum of fuss.
The Zama plant, for example, produces an average of 50 cars a month for the Danish market. In accordance with Danish law, each headlight incorporates a small ''widescreen wiper,'' a feature not seen on any other of the thousands of vehicles pouring off the assembly line.
This attention to fine detail and willingness to make modifications to suit even the tiniest foreign market is certainly a factor in the heavy inroad the Japanese have made in American domination of the world auto trade.
Zama is Nissan's showpiece. Visiting American and European auto executives are usually taken there.
The main reason for this is Zama's automated welding line. Robots handle 97 percent of the work, which is watched over by no more than half a dozen humans. It takes only one minute for each car to flow through the welding line. (An entire vehicle is constructed in 18 hours.)
The tireless robots (each given the name of a Japanese female pop singer to provide a slight human touch) allow Nissan to produce simultaneously the exact number of tailor-made cars required for widely differing foreign markets.
A visitor finds that a welding machine, bobbing and weaving like a grotesque praying mantis, isn't bothered in the least if the body shape in front of it constantly changes. The welding arm unerringly hits the prefabricated body panels at the right spot.
Assistant plant manager Teuro Osawa explains: ''The machine's sequence of movements is controlled by a central computer. Information is fed in telling the entire welding line they will be handling in order, say, a sedan, two hatchbacks , another sedan, a station wagon, and so on.
''If a car has to be hauled out of the line because a defect has been spotted (every worker has the right and even duty to do this), the computer can quickly be reprogrammed for the changed order.''
Osawa recalls how last October there was a major model change for the popular Sunny (know as the Sentra in North America).
For the first six months, however, this only applied to the domestic market. Subsequently, the changeover took place gradually in foreign market.
With the flexible assembly line system, Nissan was able to continue producing the old and new models side by side and make the gradual changeover easily.
Visiting US auto executives have gone away full of admiration at the way Nissan has eliminated costly plant shutdowns and large-scale replacement of machinery, reports Osawa, adding a prediction that ''they will eventually adopt this system, too.''
Because of this flexibility, coupled with extensive fabrication and reduction in parts per vehicle, plants like Zama don't have to operate frantically around the clock to stay profitable.
Zama, for example, operates only two shifts, separated by the three-hour maintenance breaks and a one-hour lunch break. (The 5,000 workers also halt for 10 minutes twice a day for coffee. The line is stopped and started by one worker in rotation with split-second accuracy.)
And, belying the Japanese workaholic image, the plant also closes down completely on weekends and for a 10-day summer and one-week winter vacation.