Quoted from Alan_L:
There is a ceramic 500 ohm resistor on the lugs of the Change Relay coil. Sometimes that relay locks on. Is that resistor there to keep the coil from burning?
Resistors do these 2 things: (1) Draw current, and (2) Dissipate that current. A 500 ohm resistor across 24 volts will draw about 0.05 amps which is negligible so (1) isn't what the resistor is for. Thinking about (2) though, the resistor could dissipate current flow.
Looking at rolf_martin_062 's typical excellent research in post #2 above, I see that in every case the resistor is installed in a circuit with a contact disk. So it must be that the resistor is installed to dissipate the voltage spike caused by suddenly switching off the flow of current to the coil in order to prevent arcing and wear at the contact disk.