Western Union will report a large write-down.

The company said it plans to record special charges of about $300 million against fourth-quarter earnings of last year, and it expects to report a net loss of about $370 million for all of 1985. The charges are primarily for the write-down of older switching and transmission equipment. Company officials said that details of the fourth quarter and 1985 results will be announced in about two weeks. Western Union has not posted an annual profit since 1982.