....Question: On my gas furnace (DMO Industries MM series) electrical schematic diagram there are two places that have a symbol of a plus sign inside a circle.....what does it mean? (see wires 39 and 40 on photo going to symbol of plus sign inside circle on 2nd photo....see wire from GROUND on ingnition module going to same symbol on first photo ...Note: All grounds on diagram (furnace, blower, inducer motor grounds) are not denoted by this symbol but usual ground symbol)
They're ground screws
Those are screw-head symbols (stylized Philips heads) that represent where chassis ground wires are screwed to the furnace chassis/sheet metal. I cannot tell you why they're used for inside-the-control-box grounds though (vs. the use of the regular earth symbol for outside-the-control-box-grounds). Perhaps it has something to do with them being explicit wire connection points?