12VDC relay suitable replacement for 18VDC?
The 18VDC refers to the coil drive voltage. If you just replace an 18 volt relay with a 12 volt relay, the coil will get 50% more voltage than it is rated for, and may (will) overheat. However, you can easily fix this. The 12VDC models have a coil resistance of 275 ohms. If you wire a 137 ohm resistor (1/2 watt power rating) in series with the coil, the resistor will drop 6 volts and the coil will be driven correctly. Since finding a 137 ohm resistor may be difficult, a 120 or 150 ohm resistor will suffice.