The push comes after the pension fund failed late last year in efforts to allow shareholders to vote on requiring companies to rotate or switch audit firms every seven years.

Activist investors often try to persuade companies to change practices by submitting resolutions for shareholder votes at companies' annual meetings. These proposals seldom succeed unless they win the support of corporate management.

Auditor rotation became a hot-button issue last year as regulators considered ways to make auditors more independent and skeptical when checking companies' financial statements. Many companies have had the same audit firm for decades, raising questions about whether auditors are too chummy with clients and over-reliant on them for a steady stream of revenue.

The Public Company Accounting Oversight Board, which polices the corporate auditing industry, stirred up the furor in August when it said it would consider mandatory audit firm rotation as one way of improving independence.

Many investors have no idea that companies keep the same auditor for decades without putting the work out for competitive bids, said Ed Durkin, director of corporate affairs for the carpenters' union pension fund.

"There needs to be an independence report provided every year that gets into this relationship," he said. "Let's do this before we have issues that affect the value of our portfolio."

Requests for shareholder ballots on better disclosure of auditor tenure have been filed at three U.S. companies and will be sent to 12 more by the end of next week, Durkin said.

The shareholder resolution will ask companies to disclose whether they periodically consider switching auditors and if not, why; how much has been paid to the audit firm over the years; and whether the board's audit committee looks at potential risks from having a long-tenured auditor.

The carpenter's union suffered a setback late last year when the U.S. Securities and Exchange Commission ruled that companies would not be punished for leaving auditor rotation off their ballots because it was ordinary business that did not have to be put to a shareholder vote. The union had tried to place rotation on the ballot at about 40 companies.

Durkin said he believes the SEC ruling was erroneous, because rotation is a public interest issue that goes beyond selection of an audit firm.

"It's about auditor independence; it's not about the annual selection of an audit firm," he said. "We're not trying to micromanage that."

Durkin is not alone in wanting more disclosure on the auditor's tenure.

"I'm baffled why that has never been suggested," John Biggs, retired chief executive of mutual fund giant TIAA-CREF, said in an interview last week.

In a letter late last year to the PCAOB, Biggs recommended that firms change auditors after 10 years, but if that is not required, at a minimum shareholders should be told how many years a company has had the same auditor.

(Reporting By Dena Aubin; Editing by Kevin Drawbaugh and Matthew Lewis)

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