Fifty-three cents out of every dollar spent on car repairs is wasted on needless work, according to the results of an undercover survey of auto-repair shops in seven U.S. cities released yesterday by the Department of Transportation.

Transportation Secretary Brok Adams told reporters that "when we took test cars into repair shops at random, we found we had only about a 50-50 chance of getting a car fixed right and for the right price. We found it was almost a sure thing that the shop would do something wrong on the engine."

The survey involved cars brought into 62 repair shops earlier this year in Atlanta, Philadelphia, Miami, Nashville, Houston, White Plains, N.Y., and Brooklyn, N.Y.

Adams said the names of the repair shops were being withheld because the information on their overcharges has been forwarded to local law enforcement authorities for possible prosecution. But, he added, "outright fraud is only a small part of the problem.

"What we have instead," he said, "is a variety of wastful practices, including habitual overrepair or 'package deals,' such as replacing points, condensers and spark plug wires, when only one spark plug is needed; replacing parts until the problem goes away-even through one part may have been needed, you wind up buying the whole batch; and finally, just plain old lack of competence to do the job right."

The survey was carried out for the DOT by the University of Alabama Johnson Environmental and Energy Center, Huntsville, at a cost of $66,000, and with the cooperation of local authorities.

The survey showed that 27 percent of all auto repairs are unneeded, "confirming what many people already knew about not getting a fair shake when it come to car repairs," Adams said. He added, however, that this study indicates the problem "is worse than we thought. Our earlier studies showed less of a problem."

Adams said the worst repair record was with engines, where either unneeded repairs were done, or the needed repairs were not done-or both-in almost 90 percent of the cases.

The combined chances of either unnecessary work being done, or necessary work not being done in areas other than engines, turned out to be 32 percent for brakes, 44 percent for suspensions, and 51 percent overall, Adams said.

In one of the more dramatic examples of overcharging, Adams told of five cars that were brought into five different repair shops with only one defect, a single spark plug that had been altered to cause the engine to miss. The only repair needed was replacement of the one plug and possible tuning.

The total repair bill from the five repair shops for the five cars were $596.22, at least $426.91 of which DOT said was unnecessary.

The repair bill also included a little more than$100 in "optional costs," which project officials said included tests and billed time that may or may not have been justified.

The repair shops replaced everything from ignition wiring to distributor caps, condensers, several valves and hoses. Individual shop charges ranged from $75 to $180.

Adams took several steps to combat the problem.

He called for district attorneys around the country to initiate programs in their areas to uncover deceptive practices described in the study.

He ordered National Highway Traffic Safety Administrator Joan Claybrook to hold a three-day conference later this month in Washington to investigate the problems consumers face in auto repair and maintenance, and bring together consumer advocates, industry representatives and congressional committee staff to come up with solutions, including possible methods of certifying competent repair shops.

He called for another look at the possibility of establishing diagnostic centers in many areas so a consumer could get a fair estimate of neededwork from someone who would not be performing the work on the car.

One of the local officials who cooperated with the DOT study, Dr. Tim Ryles, administrator of the Georgia Governor's Officer of Consumer Affairs, said the average two-car family in the United States "pays a $300 hidden tax in auto repair costs every year."

He added that fraud prosecutions in this area would be difficult because "often it is merely a question of incompetency." CAPTION: Chart, no caption, By Robin Jareaux-The Washington Post; Picture, SECRETARY BROCK ADAMS . . . criticizes "habitual overrepair"