The success of the new Renault Alliance helped American Motors Corp. to cut fourth-quarter losses sharply from 1981 levels, but the No. 4 automaker still lost more for all of 1982 than the year before.

AMC lost $153.5 million last year compared with $136.6 million in 1981, as sales rose to $2.87 billion from $2.58 billion, the company announced Monday.

AMC lost $2.86 million in the fourth quarter compared with a $47.1 million loss in the final quarter of 1981. Sales were $856.8 million compared with $621.1 million.

Meanwhile, Conrail reported yesterday that its earnings increased for the fourth quarter and for all of 1982, but said it would have posted a quarterly loss had it not been for unusual items.

AMC Chairman W. Paul Tippett Jr. and President Jose Dedeurwaerder blamed the year's loss on the "difficult economic situation" in the United States and Canada. Mexico's economic problems and the devaluation of the yen also had an adverse effect on AMC's results.

Tippett and Dedeurwaerder noted that the Alliance led AMC to a 49 percent increase in fourth-quarter sales.

Conrail's fourth-quarter earnings were $54.6 million ($2.14) on revenue of $853 million compared with net income of $26.4 million ($1.03) on revenue of $1 billion for the same period in 1981.

The fourth-quarter 1982 earnings included unusual items of $63.5 million--$19.4 million from the sale of tax benefits and $44.1 million from the settlement of accounts with local commuter agencies and Amtrak.

Conrail relinquished operation of commuter rail lines on Jan. 1 as required by the Northeast Rail Service Act of 1981.

Conrail earned $174.2 million ($6.82) last year on revenue of $3.6 billion compared with $39.2 million ($1.45) on revenue of $4.2 billion in 1981. The 1982 net income includes $91 million from the sale of tax benefits as well as the $44.1 million item.