Starting tomorrow, the American Automobile Association will start a new diagnostic and maintenance service to tell Washington area motorists what's wrong with their cars.

The service will consist of a 50-minute check by an AAA technician using a specially equipped $40,000 AAA van that will travel to selected sites in the metropolitan area to conduct tests of cars. The cost for the service check will be $30 ($25 for AAA members). Appointments and payment must be made in advance by calling AAA-VANS (222-8267), the association said.

In addition, the van technician will perform maintenance checks for those who want only that service. The 20-minute maintenance checks, which cost $7.50, will include an examination of lights, windshield wipers, belts, hoses, pulleys, pumps, fluid levels, battery, air filters and tires.

One of the unique features of the van service, said Bob Livingstone, program manager for the AAA Potomac Division's Approved Auto Repair, is that motorists are provided with an "independent, unbiased analysis." AAA doesn't do any of the repair work that the car may need, so the club has no vested interest in the outcome of the tests, Livingstone said.

The AAA began operating mobile diagnostic vans four years ago, and now has 16 vans in California, one in Florida and one in Baltimore, in addition to the one now in this area.

Each van is outfitted with computerized equipment that checks and provides a printout of, among other things, the starting/charging system, the ignition systems, the engine performance and the carburetor efficiency.

If an engine malfunction or defective part is found by the diagnostic check and the car then is repaired, the AAA will conduct a free retest of the repairs if the retest is scheduled within 14 days of the original test, the association said.

The van will operate Tuesdays through Saturdays from 9 a.m. to 4:30 p.m., and appointments can be made during those hours. The separate maintenance checks will be every other Saturday.

From tomorrow through April 2, the van will operate from the visitors parking lot of AAA National Headquarters, 8111 Gatehouse Rd., Falls Church. From April 5-16, the van will be making tests at the AAA service center in Wheaton Plaza and from April 19-30, at the AAA service office in Alexandria, 801 N. Fairfax St.

For information only over the telephone, the AAA will try to answer automotive questions between 8:30 a.m. and 4:30 p.m. daily, the AAA said. That service is available to the public free by calling AAA-CARS (222-2277), the agency said.