CITY OF DAYTON FIRE-RESCUE FIREFIGHTERS WERE RETURNING FROM A FIRE ALARM WHEN THEY NOTICED A "BURNING WIRES" SMELL IN THE PASSENGER COMPARTMENT OF THE 2016 PIERCE SABER THEY WERE DRIVING. AS FIREFIGHTERS WERE ATTEMPTING TO FIND A SAFE LOCATION TO STOP THE TRUCK, ALL OF THE CAB AND DASH LIGHTS BEGAN TO FLICKER ON AND OFF REPEATEDLY. THE PASSENGER COMPARTMENT OF THE TRUCK RAPIDLY FILLED WITH SMOKE AND THE DRIVER/OPERATOR HAD TO MAKE AN EMERGENCY STOP WITH HIS VISION IMPAIRED BY SMOKE. AS FIREFIGHTERS WERE EXITING THE APPARATUS, THE DRIVER/OPERATOR WAS ABLE TO SHUT THE ENGINE DOWN AND TURN OFF THE ELECTRICAL POWER. FIREFIGHTERS HAD TO REMOVE TWO PANELS IN THE PASSENGER SIDE DASH/FLOORBOARD TO EXPOSE THE FLAMES. THE FIRE WAS EXTINGUISHED WITH AN ABC DRY CHEMICAL EXTINGUISHER. TWO FIREFIGHTERS WERE EXPOSED TO SMOKE AND TOXIC GASES PRODUCED BY FLAMING COMBUSTION. BOTH WERE EVALUATED ON SCENE AND REFUSED FURTHER MEDICAL TREATMENT AT THAT TIME. THE FLAMES WERE SEEN BEHIND THE CIRCUIT BOARD/FUSE PANEL, BUT THE HEAT WAS CONDUCTED THROUGH THE PANEL CAUSING DAMAGE AND DISCOLORATION TO THE FRONT OF THE BOARD. SUBSEQUENT INSPECTION BY A PIERCE CERTIFIED MECHANIC (EVS FIRE APPARATUS) INDICATED THAT THE FIRE WAS CAUSED BY ELECTRICAL ARCING CAUSED BY A COOLANT LEAK IN THE TRUCK'S HEATER CORE. HE REPORTED THAT THE FLUID HAD PENETRATED THE RELAY CONNECTED TO THE BACK OF THE BOARD THAT CONTROLLED THE ABS BRAKING SYSTEM. THIS CASE APPEARS TO BE CONNECTED TO THE NATIONAL HIGHWAY TRANS