General Motors Corp. announced two separate recalls yesterday involving 19,500 passenger cars and trucks produced by its Buick, Chevrolet, Oldsmobile, Pontiac and GMC Truck & Coach divisions.

GM said the largest recall involves 15,300 1979-model full-size, mid-size and small passenger cars and light duty trucks equipped with cruise control. All were assembled during January 1979.

On some of the vehicles, the company said, the brake stoplight and cruise control switch, which are operated by the brake pedal, may have been assembled with an incorrect lubricant on the actuating plunger.

GM said the defect could result in adverse wear on some of the switch contacts, causing the rear brake stoplights to become inoperative.

In addition, it said, the cruise control, if activated, would resume its original set speed when the brake pedal is released if the car has not been brought to a complete halt.

The second recall involves 4,200 1979-model Chevrolet Monzas, Pontiac Sunbirds, Oldsmobile Starfires and Buick Skyhawks assembled in February 1979 and equipped with the 231-cubic-inch, V-6 engine.

GM said the engine oil pressure switch on the vehicles may have a random internal electrical short that could result in the engine starting by itself when the ignition switch is turned to the "on" position. If the transmission selector lever is in drive or reverse when the engine starts, the vehicle may move unexpectedly, GM said.

The company said it has received no reports of accidents or injuries as a result of either defects GM said letters will be mailed to owners of the affected vehicles and necessary corrections will be made at no charge to owners.

Western Auto Supply Co. has recalled about 3,000 single-arm auto floor jacks because they could present a potential safety hazard.

The company took the jacks off store shelves in mid-May after a store reported that one had failed. There have been no injuries reported from failure of the jack, however, the company said.

The recall was ordered last week. Western Auto says it put about 3,000 jacks on the market in its 4,000 retail and dealer stores nationwide in February 1978. not all of those jacks had been sold, however.