A man's home may be his castle, but not when it's on wheels, the Supreme Court said yesterday in a ruling that affects the privacy rights of about 8 million motor-home owners.

In a 6-to-3 ruling, the court said police, who generally are not required to seek warrants before searching cars, don't need them to search motor homes, either.

The decision, written by Chief Justice Warren E. Burger, did not deal with searches of mobile homes, which, despite their name, are largely immobile, prefabricated houses placed on blocks. But the tenor of the ruling suggested that mobile homes, like fixed houses, generally would be protected from warrantless searches.

By contrast, under the ruling police need only probable cause to search mobile recreational vehicles that are more easily moved, such as motor homes, travel trailers, vans or truck campers.

The case involved Charles R. Carney, who was arrested in downtown San Diego in 1979 for allegedly trading marijuana for sex with local youths.

Carney, who was convicted of possessing drugs for sale, argued that the police did not have a warrant to search his motor home, which was in a parking lot near a courthouse.

A majority of the justices upheld the detectives' actions, saying people have "reduced expectations of privacy" when they are in vehicles, such as motor homes, that are heavily regulated for traveling on highways.

Burger said such vehicles, like automobiles, are easily moved and fit into the 60-year-old "automobile exception" to the general requirement that officers seek warrants before entering homes.

"To fail to apply the exception to vehicles such as a motor home ignores the fact that a motor home lends itself easily to use as an instrument of illicit drug traffic and other illegal activity," Burger said.

Justice John Paul Stevens, joined by Justices William J. Brennan Jr. and Thurgood Marshall, dissented. "Although it may not be a castle," Stevens wrote, "a motor home is usually the functional equivalent of a hotel room, a vacation or retirement home, or a hunting and fishing cabin."

Stevens said when such vehicles are not on the highway, the occupants have the "highest and most legitimate expectations of privacy" in those "temporary abodes." The case is California v. Carney.

In other action yesterday, the court:

* Agreed to decide whether unions must allow nonunion workers to vote in elections to change union affiliation. The justices said they would hear appeals by the National Labor Relations Board and a Seattle bank from a 9th U.S. Circuit Court of Appeals decision that prohibited nonunion members from voting.

The ruling conflicts with a 5th U.S. Circuit Court of Appeals decision in the same issue. The case is National Labor Relations Board v. Financial Institution Employees and Seattle-First National Bank v. Financial Institution Employees.

* In a death penalty case from Virginia, agreed to decide whether the Constitution requires that black defendants, in areas where the death penalty has been applied discriminatorily, be allowed to question prospective jurors about their racial prejudices.

* Willie Lloyd Turner, who is black, is appealing his conviction in the July 1978 murder of a white jewelry store owner during a robbery in rural Southampton County. Turner was scheduled to die in the electric chair May 2 but the execution was stayed by a federal appeals court. The case is Turner v. Sielaff.

* Said it would decide whether the Constitution requires that a Rhode Island man's murder conviction must be set aside because police deceived an attorney who had called the police station seeking to represent the defendant.

Although police questioned Brian K. Urbine for several hours during the evening after his arrest, a detective told the attorney that Urbine would not be questioned the next day. Urbine confessed after having been advised of his rights and waiving the right to a lawyer, the state argued in its appeal. The case is Moran v. Urbine.

* Agreed to decide whether a defendant's silence -- after being advised of his right to remain silent -- may later be used against him by prosecutors attempting to show he was not insane at the time of the incident. The case, Wainwright v. Greenfield, involves a man sentenced to life in prison for a 1975 sexual assault in Sarasota County, Fla.

* Agreed to decide whether attorneys representing civil rights plaintiffs may ethically negotiate settlements that include lump sums for damages and for attorneys' fees. The case is Evans v. Jeff D.