HOUSTON (AP) _ Tenneco Inc., crediting improvements in each of its divisions, Thursday reported third-quarter earnings of $46 million, compared with a $693 million loss in the same period a year ago.

Earnings per share amounted to 28 cents, compared with a loss of $5.69 for the third quarter last year. The July-September 1991 period included an after- tax restructuring charge of $427 million, or $3.49 per share, and 36 cents per share lost from discontinued operations.

''The earnings we report today show that Tenneco is on track with its aggressive turnaround,'' Mike Walsh, Tenneco chairman and chief executive said. ''This third consecutive quarter of dramatic improvement in a weak economy shows that our focus on results and consistent profitability is working.''

Revenues in the most recent period were $3.18 billion, up slightly from $3.12 billion a year ago.

Tenneco's natural gas pipelines division made $58 million, up from $17 million a year ago. Revenues were $526 million, up from $422 million. The company has been moving more gas through its Kern River Pipeline, which is operating at capacity. The company also credited cost control.

Tenneco's JI Case farm and construction equipment division lost $68 million. The performance, however, was an improvement from the $169 million loss posted a year ago.

Tenneco said a continued decline in markets for farm and construction equipment will result in additional 4 percent production cuts to keep inventories in line with demand. The company said Case also would post a fourth-quarter loss.

Tenneco Automotive, which includes Walker exhaust products, made $60 million, up from $49 million a year ago. Newport News Shipbuilding made $68 million, up from $61 million in the 1991 period.

Another division, Packaging Corp. of America, which makes boxes and containers, made $49 million, more than double the $23 million of a year ago.

For the first nine months of the year, Tenneco income was $257 million, or $1.71 per share, compared with a loss of $706 million, or $5.87 per share in the same period of 1991. The year-ago performance included an after-tax restructuring charge of $472 million, or $3.06 per share. Revenues were $9.825 billion, down from $9.832 billion a year ago.

Tenneco's stock closed off 25 cents on the New York Stock Exchange, closing at 35.37 1/2 cents.