General Motors plans to idle the plant where it assembles the Chevrolet Volt plug-in hybrid for four weeks starting next month to retool for production of the all-new 2014 Impala and a small number of 2013 Malibus.

GM will close its Detroit-Hamtramck plant from Sept. 17 until Oct. 15, one source said. Union representatives last week told the plant's roughly 1,500 workers about the scheduled downtime, the source also revealed.

GM also notified suppliers last week, according to another source in the supply chain.

GM assembles the Volt and recently began making small numbers of the redesigned 2013 Chevrolet Malibu at the Detroit-Hamtramck plant, which straddles both cities' limits. GM assembled 590 Malibus in June and July, according to the Automotive News Data Center.