Only three days after settling differences with its U.S. workers, General Motors Corp. is threatened with a strike that could close all 13 of its Canadian plants and seriously disrupt vehicle production in the United States.

The new danger comes from the Canadian branch of the United Auto Workers union, which was moving toward a strike deadline in contract talks that affect 36,800 GM employes.

The Canadian union is emphasizing pay increases over job security, and has rejected settlement proposals that reversed those priorities in the UAW's U.S. negotiations with GM and Ford Motor Co.

A contract offering novel job security provisions and relatively moderate pay increases was ratified Sunday by union-represented GM workers in this country. That same day, UAW negotiators at Ford tentatively approved a practically identical agreement for their U.S. members.

But the U.S. agreements do not reflect Canadian economic realities, a UAW-Canada spokesman said yesterday. GM over the past two years has been producing cars, trucks and parts in Canada at an average hourly labor rate that is about $7.50 less than its U.S. production costs, the spokesman said.

"There is enough flexibility within that differential for GM to give us better pay," the spokesman said. "We're not trying to do something better than what was done by the UAW in the United States. We're simply trying to do something different to reflect the realities as they exist in Canada," the spokesman said.

Whether or not the union walks out at noon today, one of those realities is that GM is largely dependent on its Canadian production system, and as a result, vulnerable to any interruptions in it.

Another reality is that a Canadian strike would surely scrap GM's calendar-year 1984 manufacturing plans, according to analysts at Detroit-based Ward's Automotive Research. Those plans already have been set askew by parts foul-ups and a seven-day strike last month by UAW members at 17 plants in the United States.

The manufacturing errors and work stoppage wiped out the production of an estimated 100,000 GM cars and trucks -- a loss that probably will not be made up by overtime, because most of the company's plants already are running at full capacity, the Ward's analysts said.

A Canadian strike of 30 days duration would knock out another 75,500 vehicles -- including popular passenger-car models such as the Pontiac 6000, Grand Prix and Bonneville, which now are made exclusively in Canada, Ward's analysts said yesterday.

But Canada also is an important parts center for GM -- which the Canadian UAW, like its U.S. counterpart, chose as a strike target over Ford.

GM's iron foundry in St. Catharines, Ontario, produces blocks and castings for engines. Vehicle axles, fuel pumps, disc brakes and spark plugs are also made there.

Metal stampings and plastic parts -- tail light covers and instrument panels, are examples -- are made at GM facilities in Oshawa. Buses and heavy trucks also roll out of GM's Canadian plants.

All of the Canadian parts plants -- even though some of them turn out components made elsewhere in the GM empire -- are vital to the company's North American manufacturing operations, according to Nick M. Hall, a GM of Canada spokesman.

"We're so completely integrated" with production in the United States, "it's hard to separate us" into two different groups, Hall said.