Arduino reading different ground levels?

Ok so hi everyone, this is my first post so sorry if its not like a big deal. Im planning to use an arduino as a glowplug controller in my truck, basic work is ok, ignition turns on and it cycles the glowplugs for 15 secs and then shuts them down till ignition is turned off and on again, the problem is, the truck doesnt always need 15 secs, when cold it could need more and when hot it needs only 5 secs. the engine has a temp sensor but it changes ground resistance so, is there a way to make the arduino read these ground resistances?? thanks in advance!

If there is a mechanical temp instrument connected to this sensor, then the sensor will have a varying voltage on it. Measure that voltage with a DMM when the engine is cold and when the engine is hot (and not running). That voltage can be used to make the Arduino do what you want. Leo..