Sen. John Glenn, a Democratic presidential candidate, said yesterday that a satellite malfunction and the loss of U.S. bases in Iran had left the United States "blind" and unable to monitor Soviet missile testing adequately when the Carter administration was trying to convince the Senate to ratify the SALT II arms control treaty.

"We were blind for the better part of two years," Glenn said, defending his prominent role in blocking ratification of the treaty. An aide to the Ohio Democrat later amended that, saying the satellite problem was corrected in six to nine months.

A former senior Carter administration official labeled Glenn's comment "an exaggeration . . . political hyperbole." But he conceded there had been some loss of satellite capability during that period.

Glenn, speaking at a luncheon with editors and reporters of The Washington Post, also charged that President Reagan has "overstated" the communist threat in Central America.

He said he would not send U.S. troops there as president even if El Salvador was about to fall to communist-backed guerrillas. And he said he did not think the United States should send troops into the region if Nicaragua invades neighboring Honduras.

"I don't think you want to involve American troops . . . ," Glenn said. "We can't dictate what happens in every one of these governments."

"I think the rhetoric of this administration as it regards . . . the communist threat is way overblown," Glenn said. ". . . This administration has overblown it, making everything into some sinister Soviet plot."

There are from 5,000 to 7,000 guerrillas operating in the hills of El Salvador, Glenn said, "and not all of them are dedicated Marxist-Leninists." But President Reagan and his advisers, Glenn said, "conjure up visions of all of them marching across the Rio Grande and scare our people of this country half to death."

Glenn said right-wing forces in El Salvador have caused far more deaths than the communist-backed guerrillas and that he would require the government there to do something about it in order to receive further American aid.

There have been 30,000 persons killed in El Salvador since 1979, Glenn said, and two-thirds were killed by "right-wing death squads of paramilitary groups, operating maybe not under the scrutiny of the government, but with impunity, operating out of government circles."

Glenn has offered an amendment requiring the government of El Salvador to present U.S. officials with a plan by Oct. 1 on how they intend to reign in the right-wing forces. If El Salvador fails to move against the conservative death squads, he said, "the next step is cutting off aid . . . that is one option we would have."

Glenn also criticized Reagan's policy of providing covert aid to the forces fighting the leftist government of Nicaragua.

"I do not like going in under the guise of intercepting weapons flow from Nicaragua to El Salvador and trying to overthrow the government there, when we pledged we will not do that at the OAS Organization of American States ," he said. "That undermines our credibility with the other OAS members."

Glenn now supports Senate ratification of the SALT II treaty because problems with its satellite and ground station verification have been rectified. Glenn had contended publicly that the loss of the bases in Iran seriously hampered U.S. efforts to monitor Soviet testing and thus verify that they were complying with the pact. But, he said yesterday, the malfunction of the U.S. satellite meant that the American monitoring capability was essentially "blind."

At the time, top level Carter administration officials maintained that the satellite could be made to function in a matter of months, which is what eventually happened. They also said that ground monitoring stations in Turkey and Alaska--plus the establishment of a diplomatically sensitive monitoring station inside China--would provide adequate verification.

Congressional sources said that Glenn was briefed on the satellite reconnaissance issue by the CIA and was told there was a problem. They reported that the primary system had "gone out" and they were trying to activate a backup on board. But that began to "short out."

At one point, they turned the satellite off and then began to operate it "a little bit at a time," one source said. He said this took about six to nine months.

"It is true that the overall capability . . . was somewhat degraded," said the former senior Carter administration official. "We said the degradation was temporary . . . . We had some satellite monitoring capability . . . . Then we got some more back in six to eight months. We had what we needed to know. Nothing slipped by."

A top military official said, "We clearly had a diminished capability . . . . But it was not a complete failure, and we were never blind."