From a discussion on the Makergear 3D printer forums …
A Makergear M2 user with an older printer (dating back to 2012) had a bed heater failure:
all of a sudden I noticed that my bed temps had started dropping
With a 12 V heater, the most likely problem is at the power input connector on the RAMBo board. The wires from the 12 V power brick generally work loose inside their screw terminals, whereupon the absurdly high current heats up the weak joint and destroys the connector. You can find some hideous pictures somewhere on the forum.
Next most likely is a broken wire between the RAMBo and the heater, caused by repetitive stress injury from all that back-and-forth motion. You may be able to find this with a ohmmeter and some wiggle-jiggle action on the cable, but if even one strand remains intact, the resistance will remain very low at the meter’s trivial test current. Pulling the wires out of the braided sheath will be more definitive; the insulation will be wrecked at the break.
Least likely seems to be the connector where the cable from the heater terminates on the RAMBo.
Start by inspecting the connectors; you may find some seriously charred plastic.
Depending on what you find, you may have a zero-dollar repair.
It’s like you’re psychic.
Suffice it to say you’re not the first person to see charred plastic … [grin]
My solution was to move the high-current switching off the RAMBo board to a solid state relay, with the heater power from a separate 40 VDC supply: