DETROIT ? The vital signs are improving for American automakers.

General Motors may reopen some shuttered factories because it can’t produce four of its vehicles fast enough to meet demand, and Chrysler is set to hire more engineers and product development workers.

While both companies still depend on government help, the moves are signs of increased confidence that the U.S. auto market bottomed out last year and will improve in 2010, even without a jolt from a Cash for Clunkers-style program.

Factories making the Chevrolet Equinox, GMC Terrain and Cadillac SRX, all SUV-like crossovers, and the Buick LaCrosse sedan are at or near capacity, GM’s North American president, Mark Reuss, said at the Detroit auto show.