Ford says the traditional two-week summer break is being cut to a single week and General Motors isn't doing any shutdowns. Chrysler plans a two-week break at just four of its ten North American assembly plants.

Both GM and Chrysler are rolling out critical new models.

The three Detroit carmakers don't have the luxury this year of a two-week window to do maintenance and change the machinery for new models.

U.S. demand for new cars and trucks has been strong, up 7 percent through April, led by soaring demand for full-size pickup trucks as home construction rebounds.

And after closing more than two dozen factories during the recession, U.S. automakers need to use their remaining capacity to its fullest.