When John F. Brennan went to the repair shop to pick up his 1978 Honda, he discovered a new problem: a window on the driver's side had been broken while the car was in the shop. But when Brennan pointed out the damage and asked that it be fixed, the shop refused and suggested he file an insurance claim.

Instead, Brennan, a trade association official who lives in Bethesda, telephoned the Montgomery County Office of Consumer Affairs and told investigator Marjorie Wax what had happened at the repair facility operated by Wilson Pontiac in Silver Spring.

"She called me back an hour later and said it was all taken care of, that Wilson had agreed to replace the window," said Brennan. "And they did the next day."

That is one example of how local consumer agencies can provide some practical help in resolving disputes over everything from overcharging to deceptive advertising. An outgrowth of the consumer activism inspired by Ralph Nader during the 1960s, the agencies are designed to assist consumers with legitimate grievances against local businesses and to serve as a source of useful information that can help prevent mistakes in buying.

As a general rule, consumer agencies in Maryland and Washington have more power than those in Virginia because of differences in the laws that established them. But agencies in all three jurisdictions typically address a similar range of problems, including automobile purchase and repair, home improvements, consumer fraud, credit, refunds and deceptive trade practices.

When presented with a consumer complaint, the agency typically tries to mediate a solution. In some cases, the agency also may have legal powers and can sue the company, negotiate a consent agreement or take some other legal steps to remedy the situation.

Today, consumer officials emphasize education so that consumers have the information they need before they make a purchase.

"If people would use the consumer office for advice before entering a contract, they could avoid a lot of problems," said Barbara B. Gregg, executive director of the Montgomery County consumer office.

In many jurisdictions, the consumer office will answer questions over the telephone, mail printed literature and tell consumers how many complaints have been filed against a local business. The offices also send representatives to talk to groups about consumer issues.

But agency officials urge anyone with a problem to try to work it out with the business in question. If that does not work, consumers should contact their local agency, but with these things in mind:

* The agency may require a written complaint.

* The complaint may never be resolved to the consumer's satisfaction. If the company refuses to cooperate and the agency lacks the power to force compliance, the consumer will be out of luck.

* The consumer office may refer the complaint to another agency. The Washington office refers complaints about doctors and other professionals to other agencies, such as the medical licensing board. Insurance complaints may be referred to the insurance agency.

Here is a list of area consumer offices and telephone numbers:

Maryland: Anne Arundel County, 224-7300; Frederick County, 694-1071 (open only on Thursdays); Gaithersburg, 926-8223; Howard County, 992-2176; Montgomery County, 279-1776; Prince George's County, 952-4700. The Attorney General's consumer protection division can be reached at 528-8662.

District of Columbia: 727-1158 to request a complaint form; 727-1294 for information, records of complaints against local businesses and to arrange for speakers; 727-1308 for administrative personnel, and 347-4100 for taped information on consumer topics.

Virginia: Alexandria, 838-4350; Arlington, 558-2142; Fairfax County, 691-3214; Prince William County, 221-4156, and the Northern Virginia office of the Department of Agriculture and Consumer Services, which serves the areas where there aren't local consumer offices, such as Loudoun County, Manassas, Manassas Park, Fairfax City and Falls Church, 573-1286.