Choi Kang-hee's men came into the clash at the Jeonju World Cup Stadium unbeaten in six league matches and having conceded just four times in that period.
Jeonnam had also been impressive in recent weeks, collecting 10 points from their five previous top-flight outings.
However, the hosts proved too strong as first-half goals from Lee Jae-sung and Han Gyo-won wrapped up maximum points for Motors to help send them top.
Choi's side sit a point clear at the summit after defending champions Pohang Steelers slipped up against in-form Suwon Bluewings.
Bluewings ran out 4-1 winners - with Brazilian pair Santos Junior and Roger proving too difficult to handle for Hwang Sun-hong's defence.
Santos opened the scoring in the first minute, only for Ji Soo-hwang to level just before the half-hour mark.
However, Santos added a second shortly after the hour before compatriot Roger and a late Kwon Chang-Hoon effort condemned the visitors to a first league defeat in seven matches.
That result saw Bluewings leapfrog Jeju United into third, with Park Kyung-hoon's side held 1-1 by Busan I'Park on Saturday.
Meanwhile, Incheon United moved off the bottom with only their second top-flight win of the campaign as they won 2-0 to put a further dent in Ulsan's top-six hopes.
Ulsan have faltered of late and now hold just a two-point cushion over Seoul after Jin Sung-wook and Choi Jong-hwan wrapped up victory for the strugglers.
Second-half goals from Sreten Sretenovic and Everton Santos ensured that Gyeongnam and Seoul shared the spoils 1-1, while Sangju Sangmu and Seongnam remain level on points after drawing by the same scoreline.
South Korea international Kim Tae-hwan opened the scoring after just six minutes at the Sangju Stadium before Baek Jong-hwan levelled just before the hour mark to earn the hosts a point.