Auto industry analysts are divided over the jobs impact of the new joint production agreement between General Motors Corp. and Toyota Motor Co. to build small, front-wheel-drive cars at GM's idle assembly plant here.

GM estimates that the agreement--which is to be signed Thursday--eventually will lead to a net increase of 3,000 jobs here with another possible 9,000 at plants supplying the new operation.

But some auto industry analysts today were not as positive about the jobs impact.

"I don't think any jobs will be created by this venture," said David Healy, an analyst with New York based Drexel Brunham Lambert. "GM essentially is working with Toyota on a substitute car for the Chevrolet Chevette, which could lead to a net decrease in jobs."

Maryann Keller, an analyst with New York based Paine Webber, said today that, "If it weren't for this joint venture, the Fremont plant would never reopen as an auto plant, and those people who have been without jobs would never again find employment in the auto industry."

GM closed its Fremont plant last March 5, putting about 2,500 people out of work. The plant, which recently produced Chevrolet Cavaliers and Oldsmobile Cutlass Cierras, had a peak employment of 6,800 workers in 1979.

Keller agrees with GM's jobs prediction, saying that the company is attempting to broaden the overall domestic small-car market and to increase its share of that market. "If they are successful, that most likely would lead to a net gain in jobs," Keller said.

The rear-wheel-drive, four-cylinder, subcompact Chevette currently is GM's entry-level car. The Chevette and a similar Pontiac model are made in Wilmington, Del.

GM officials today denied that they have any plans for phasing out the Chevette and its Pontiac cousin, an estimated 287,932 of which rolled off the line in 1982. That figure compares with 512,954 Chevette and Pontiac models (T cars) produced by GM in the previous year.

In the meantime, local United Auto Worker union leaders worked today to head off possible protest demonstrations at Thursday's signing ceremony against a statement by Toyota that laid-off UAW members would not be given hiring priority when the new plant opens. Toyota will operate the plant.

"There will be no demonstration," said John P. Scrempos, vice president of Local 1364, which represents GM's Fremont plant. "We're discouraging that kind of thing. A demonstration could jeopardize the whole agreement, hold up the whole deal. That kind of thing could make Toyota back off completely and go to the South or someplace else, where people don't act that way."

The union leader, in a statement supported by the auto industry analysts, said the UAW is dealing from a position of weakness in any relationships it will have with the management of the new joint venture. However, he said he believed that Toyota's comments were part of "a bargaining ploy."

In Washington, meanwhile, House Judiciary Committee Chairman Peter W. Rodino Jr. (D-N.J.) expressed his concern in a letter to Federal Trade Commission Chairman James C. Miller III over the long-term effects of the Toyota-GM joint venture on the competitive position of other U.S. automakers. Rodino said he was bothered by statements from some officials that the joint venture had received government approval and promised that his committee would watch the issue during the coming weeks.

The agreement was praised, however, by U.S. Trade Representative William E. Brock, who said greater Japanese investment in American industry "is the kind of domestic content that works."