Ford Motor Co. is extending the warranties on nearly 1 million rear-wheel-drive cars built since 1978 and offering repairs on the cars to solve a problem that could impair their power steering during cold-weather starts.

The company said that the problem did not pose a safety hazard and that it is not recalling the cars but merely offering free repairs to owners who have experienced the problem.

Ford also said it would reimburse owners of the cars who already have paid an average of about $675 each to correct the problem. The company has received some 200 complaints about the troubles.

The offer, however, covers only cars in parts of 17 Northern tier and mountain states and in Canada, Ford said. It will cover 1978- to 1983-model Fairmonts, Zephyrs, Mustangs, Capris, Thunderbirds, XR7s, Granadas, Cougars, LTDs, Marquis and Continentals equipped with rack-and-pinion power steering.

Ford said the cars can suffer a short-lived failure of power steering when started in cold weather because of a breakdown of seals in the steering gear as the cars get older. A new kind of seal material is being installed in new Fords to end the problem.

The steering problem, which apparently goes away after the car is driven a few yards and the engine warms up, has been corrected in the past by a $675 replacement of much of the steering system.

Ford said yesterday, however, that it had developed a way to fix the problem for about $200, and would do the repair for free for any owner in Canada or the affected states whose car is experiencing the power-steering loss.

In addition, the company said it was extending the warranty on the cars' steering assemblies to five years or 50,000 miles, whichever comes first. And it said it would reimburse any owner who has already had the repair done.

"We're doing this because of the high cost of repairs on these things," a Ford spokesman said. "The repair costs have been high, and we believe that the number of customers experiencing these problems could increase as the mileage of these cars increases."