A Washington consumer group is urging Chrysler Corp. to recall about 500,000 1989- and early 1990-model vehicles equipped with Ultradrive transmissions, saying the units are plagued with potentially dangerous defects.

In a six-page letter to Chrysler Chairman Lee Iacocca, Clarence Ditlow, executive director of the Center for Auto Safety (CAS), also urged Chrysler to warrant Ultradrive transmissions for life and waive all owner-paid deductible fees beyond the first year or 12,000 miles.Through November, the CAS said it and the National Highway Traffic Safety Administration had 165 owner complaints on file and that NHTSA has received more transmission complaints concerning Chrysler vehicles than any other manufacturer.

Ultradrive, Chrysler's first four-speed automatic electronic transmission, was introduced in late 1988 for its front-drive cars, where they are more precisely known as transaxles.

Chrysler has billed them as "the world's most advanced transmission." Since then, however, Chrysler has had to repair or replace early units troubled by internal leaks.

It also ordered a "customer satisfaction" recall last winter to about 50,000 owners in cold weather regions because of problems with the transaxle fluid thickening at very low temperatures.

"Ultradrive's dismal showing in its first two years on the road earns it the dubious distinction of the world's most defective transmission," the CAS told Iacocca.

Chrysler, however, called Ditlow's charges "grossly exaggerated," adding that "his allegations regarding safety hazards in connection with this transmission are totally irresponsible."

Although the safety group said there have been no injuries related to Ultradrive failures, it said "it is only a matter of time before an Ultradrive failure causes a serious, perhaps lethal, accident."

The safety group said most owner complaints have to do with the transaxle locking in second gear, often at random and without warning.

But Chris Theodore, executive engineer - Powertrain Engineering for Chrysler, said Ultradrive transaxles are programmed to shift into second gear if it detects a potential problem with the unit so more damage is not done.

Ditlow also said many owners have experienced multiple transaxle failures and lengthy repair times.