Control Data Corp., troubled by slow sales in several of its principal lines of computer-related businesses, is facing some bad accounting news as well.

The Securities and Exchange Commission asked the company to revise its 1984 earnings downward because it disagreed with the use of some tax deductions and accounting techniques, and the company itself lowered its earnings for the second quarter of 1985, increasing its loss for the first half of the year, because of previously unrecorded losses at two subsidiaries.

The change brought earnings for 1984 down to $5.1 million (12 cents per share). Previously, 1984 earnings had been reported as $31.6 million (81 cents). The company had reported a $5.4 million loss for the first half of 1985; the change increased it to $14 million (36 cents).

Total revenue for 1984 was $5 billion, and revenue for the first half of 1985 was $2.5 billion.

A spokesman for the Minneapolis-based company, which plans to announce a public offering with an unspecified purpose in the next few days, said the request by the SEC "in hindsight makes sense. But we believed we were on the right accounting track at the time."

However, Control Data spokesman Dick Reid said the failure to include the losses of two partly owned subsidiaries in Control Data's statement of earnings for the first half of 1985 "was a mistake."

The largest reduction for 1984 had to do with a $16.8 million tax benefit taken because the company was closing down one of its computer-peripheral businesses. The company wanted to use that write-off in future years but record it on its books for 1984 as money owed to the company. That would have had the effect of increasing the company's reported income. The SEC auditors said that approach was inappropriate.

"Our position is that the accounting literature is clear. It would be a very unusual circumstance in which you record a receivable from the government," said Howard Hodges, chief accountant in the SEC's division of corporate finance. "Our position is, don't count your chickens yet."

The company also incorrectly accounted for 1984 losses at one of its subsidiaries, Earth Energy Systems Inc., the SEC suggested. Because Control Data's stake in Earth Energy has increased over the years, the firm has changed the accounting method it uses to figure the results of the wind-energy distribution firm. The SEC wanted the effect of that change taken as a $9.7 million charge against income in the fourth quarter of 1984, and the company complied.

Earth Energy also accounted for one of the reductions to income in 1985. As Reid explained it, Control Data's accountants simply failed to include any of Earth Energy's operating losses in either the first- or second-quarter financial statements. Control Data owns about 76 percent of Earth Energy.

"It doesn't happen often that they make a mistake like that," said analyst George J. Podrasky Jr. of Duff & Phelps in Chicago. "They should have had more control."

The other reduction in 1985 came from another subsidiary, Inter-Regional Financial Group. It didn't announce its second-quarter loss of $16.7 million until July 30, after Control Data had published its results for the first half of 1985. Reid said that some employes at the company knew the figures were coming, but that the word didn't get to the people who were preparing the financial statements.

"The correct word didn't get to the right place at the right time to reflect that loss," Reid said. "We just have to say it has to do with a lapse of communication and timing."

Control Data's accounting firm, Peat, Marwick, Mitchell & Co., could not be reached for comment.

The lapses came at a time when the company's computer-peripheral business still is suffering from the general downturn in the computer industry. The firm is reducing its work force -- it has shrunk by 3,800 full-time employes since June 1984 and total "people costs" are down by $100 million -- and getting out of such businesses as regional data centers.

Company executives would not comment on the purpose of the upcoming public offering, but analysts suggested the company wants to obtain more long-term debt to refinance and reduce its short-term debt load.

Paradoxically, one of Control Data's few bright spots is a Baltimore-based financial-services subsidiary it tried to sell and gave up on when it couldn't get a price high enough. The company said in June that it was taking Commercial Credit Corp. off the market. It originally had asked $1 billion for the firm last November, but reduced that to $844 million in the spring.

Now, according to analyst Podrasky, lower interest rates and reductions in overhead costs mean that Commercial Credit is contributing favorably to Control Data's bottom line. Spokesman Reid said Commercial Credit is doing "very well."