Bobby Duckworth, whose third-quarter fumble looked as if it would cost San Diego a victory, sprinted 88 yards with a pass from Ed Luther for a fourth-quarter touchdown that sped the Chargers toward a 20-7 triumph over the Chicago Bears tonight.

Duckworth's reception gave his team a 13-7 lead. Lee Williams' 66-yard return of an interception with 1:15 to play added insurance.

The Bears, who won their first NFC Central Division title last week, punted 11 times and never mounted an offense under third-string quarterback Rusty Lisch. He came in when Steve Fuller, himself a replacement for injured starter Jim McMahon, went down with a slightly separated shoulder in the first quarter.

The Bears (9-5) hurt their chances for home field advantage in the playoffs. They have not had a playoff game at home since 1963.

Duckworth caught three passes for 185 yards as the Chargers won without first-string quarterback Dan Fouts, out with a groin injury.

Duckworth, running alone, fumbled away a certain touchdown in the third quarter, but atoned when he slipped past defensive back Shaun Gayle and caught Luther's pass.

That came after Walter Payton had gone 10 yards for a third-quarter touchdown that gave the Bears a 7-6 lead, following a fumble by San Diego's Earnest Jackson at his 34.

On the possession after Payton's touchdown, Luther hit Duckworth on a slant pattern from San Diego's 46 and the third-year pro took off for what seemed to be the go-ahead score. But near the end zone he inexplicably lost control of the ball and Chicago recovered at the four.

The game was a defensive struggle from the outset, and the Bears got the worst of it.

They punted each of their first six possessions, and a fumble on the seventh set up Rolf Benirschke's second field goal of the first half, staking the Chargers to a 6-0 lead.