Where did you test for 12 volts and where. You have got to be real specific with this. Also, how long did you measure? Point is, the resistor drops the voltage to the relay but only when it has a load from the glow plugs. It also takes a few seconds for the load to drop. If you have the relay is working and is sending power to the plugs you should see a drop within a few seconds. If after a few seconds, the voltage is still 24 volts then your resistor has shorted or all of your glow plugs are burned out. Check the glow plugs with a test light lead connected to the positive of the 12 volt side and touch the probe to each glow plug terminal with the connector disconnected. If the light does not light then the plug is bad. You can also check the resistance between the terminal and the body. The resistance is should be around 3 ohms with clean connections IIRC. The resistor is replaced a lot as being bad because it is allowing 24 volts but I think it is mostly replaced or bypassed by people that do not understand the voltage from a volt meter is not enough load to drop the voltage. The full load of 8 working glow plugs is what will drop the voltage down. If one plugs fails than the the voltage to the remaining plugs goes up. This causes a failure in the next plug by over voltage and so on with each newly bad plug adding more voltage to the rest of them. This is why we see a lot of trucks with all the glow plugs bad and it is also why people bypass the resistor.
