Dealers have been so cautious in ordering new cars that Ford Motor Co. more than doubled its expected down time for assembly plants in the first quarter.

But dealer orders, which have been slow for more than a year, are beginning to pick up, and showroom traffic is holding up well, indicating the current auto slump may have reached bottom, said Alexander Trotman, executive vice president for Ford's North American operations.In January, Ford Chairman Harold Poling predicted the company would lose 36 weeks of production due to temporary plant closings in the first three months of 1991. With one week of production left in the three-month period, shutdowns at Ford's 18 assembly plants have cut out 80 weeks of production.

Trotman said there would be "considerably fewer" weeks of shutdown in the second quarter.

For more than a year, dealers have cut their orders to Ford and other manufacturers as they try to reduce the number of cars on their lots, cut costs and maintain profitability.