By Paul Lienert and Eric Beech

DETROIT/WASHINGTON, April 16 (Reuters) - General Motors engineers reported accidentally turning off ignition switches in a Cadillac SRX with their knees more than eight years ago, and they ordered a similar fix to a similar problem in smaller, cheaper cars linked to 13 deaths, according to documents from parts maker Delphi Automotive.

The documents, provided to U.S. safety regulators, show GM GM.N used the same part, from the Cadillac Catera, to make ignition switches more difficult to turn off on the 2007 Cadillac SRX and the 2007 Saturn Ion and Chevrolet Cobalt. Delphi supplied the GM-designed switches for all of those models.

The documents are the first indication that GM's luxury Cadillac brand may have experienced ignition-switch problems similar to those that triggered the recall earlier this year of 2.6 million GM compacts, including the Cobalt and Ion.

GM did not immediately respond to requests for clarification.

The Delphi DLPH.N documents, released on Wednesday by the U.S. National Highway Traffic Safety Administration, also may indicate that earlier versions of the Cadillac SRX, from model years 2004-2006, used a switch part from the 2003 Ion.

The original SRX ignition switch part also was used in the 2003-2007 Cadillac CTS sedan, according to online GM parts catalogs reviewed by Reuters.

The Delphi documents did not make clear whether earlier versions of the Cadillac SRX had ignition switch issues. The SRX has not been recalled for ignition-switch related issues. GM did not immediately respond to request for comments.

The original Cadillac ignition switch and the original Saturn ignition switch carry similar GM part numbers: 12450257 for the 2004-2006 SRX and 12450250 for the 2003-2006 Ion.

GM GM.N requested a change to the Cadillac SRX ignition switch in early 2006, months before the relatively unchanged 2007 SRX went into production, because "test drivers (were) turning off the car with their knees while driving," Delphi told the National Highway Traffic Safety Administration.

(Reporting by Paul Lienert in Detroit and Eric Beech in Washington, editing by Peter Henderson)

((Paul.Lienert@thomsonreuters.com)(313-202-6922 or mobile 313-231-0200)(Reuters Messaging: paul.lienert.thomsonreuters.com@reuters.net))

Keywords: GM RECALL/CADILLAC