Turnbull replaces Neil Whalley, who resigned earlier in the season - the appointment marks his return to football management after being sacked as manager at Conference North club Barrow earlier in the campaign.
Turnbull combined his role as manager at the Bluebirds with his duties at Glanford Park and will do the same with this job.
Meanwhile, they have also signed towering former Iron loan striker Mat Bailey on loan from Crewe.
Bailey is yet to make a senior appearance for Dario Gradi's Championship side since signing last summer but has had spells with Northwich and Hereford.
He was on loan with United for a month at the beginning of last season following an injury to Steve Torpey on the eve of the campaign.