Fuel solenoid failure on Dodge 12 valve trucks almost always begins with worn starter contacts. When a starter sticks on it is usually caused by the worn contacts arcing and becoming jammed or welded together. When this happens the starter continues to run and back-feeds 12v up to the 70amp fuel solenoid relay. The relay then thinks it getting the power from the ignition switch so closes the relay and energizes the fuel solenoid via the "pull" coil. This is why the engine may not shut off by the key. This "pull" coil is very strong but low resistance so will overheat very quickly if allowed to stay energized. This is what causes the fuel solenoids to overheat and burn up. So, by installing this one-way diode between the starter and the starter trigger wire we prevent the starter from back feeding the fuel solenoid circuit and save the fuel solenoid from burning up if the starter sticks on and also allow the engine to be turned off. Again, this is the primary cause of fuel solenoid failure from overheating. A diode lets current flow one direction but not the other.