Caterpillar Tractor Co. said yesterday it earned a profit of $198 million in fiscal 1985, halting three straight years of deficits that totaled nearly $1 billion.

Hughes Tool Co. and Olin Corp. each posted decreased net income for the fourth quarter of 1985 compared with the same period in 1984, but Singer Co. said its net income rose in the period.

Caterpillar, the world's leading manufacturer of heavy construction equipment, said its profit ($2.02 per share) came on gross sales of $6.73 billion -- an increase of 2 percent, or $149 million, over 1984, when Caterpillar lost a record $428 million.

Caterpillar's previous annual losses were of $345 million in 1983 and $180 million in 1982 -- its second-ever red-ink year and first one since 1932.

In the fourth quarter, the Peoria, Ill.-based company said it earned $87 million in pre-tax profits, compared with a $251 million loss in the final quarter of a year earlier.

Caterpillar said fourth quarter's sales of $1.8 billion were 9 percent higher than 1984's fourth quarter.

*Hughes Tool Co. reported a drop in net income for the fourth quarter of 1985 compared with the 1984 period, but also reported an increase in sales for the entire year.

For the three months ended Dec. 31, sales were $313.6 million, down 7.2 percent from $337.8 million a year ago. The company had net income of $560,000 (1 cent a share) compared with $11.4 million (20 cents) for the fourth quarter of 1984.

For the year ended Dec. 31, Hughes reported sales of $1.26 billion, up 3.7 percent over sales of $1.22 billion in 1984. Net income for the 12 months was $4.1 million (7 cents) compared with a net loss of $133.8 million ($2.40) in 1984.

The 1984 loss included pre-tax write-offs of $174.3 million. The effective tax rate was 67.7 in 1985 and 29 percent in 1984.

*Olin Corp. reported a 31.1 percent drop in net income for the fourth quarter of 1985, compared with the same period in the previous year, and said a restructuring caused a $165.3 million loss for the year.

The Stamford, Conn.-based conglomerate said its fourth-quarter net income fell to $8.4 million (37 cents a share) from $12.2 million (54 cents) in the corresponding 1984 period.

The latest period's income, however, was up 6.3 percent from income on continuing operations in the fourth quarter of 1984, which was $7.9 million (35 cents a share). The company recorded $4.3 million of fourth-quarter 1984 income from its Ecusta paper business, which it sold in the third quarter of 1985.

Revenue for the fourth quarter was $390.6 million, down from $431.7 million in the same period in 1984.

The 1985 loss of $165.3 million is a deficit of $7.19 a share. In 1984, Olin had net income of $88.7 million ($3.81). Operating income from continuing operations was $39.7 million ($1.73), down 46 percent from 1984's $73.5 million ($3.15).

Revenue in 1985 was $1.75 billion, down from $1.8 billion in 1984.

*Singer Co. reported its net income rose 38 percent in the fourth quarter of 1985, compared with the same period in 1984.

The Stamford, Conn.-based company also reported a 26 percent increase in net income for all of 1985, compared with 1984. For the fourth quarter of 1985, the aerospace electronics and consumer products maker earned $27.9 million ($1.48 a share), compared with $20.2 million ($1.07) in the last quarter of 1984.

Fourth-quarter revenue was $659.4 million, down from $680.2 million in the previous period.

For 1985, Singer said it earned $82.3 million ($4.30), up from $65.1 million ($3.41) in 1984. Revenue for the year was $2.4 billion, down from $2.5 billion in 1984.