Just pull the radiator sensor out and measure resistance on the wires while it's cold. Should be around 2.0 to 2.5 ohms resistance. Get a cup of boiling water from the microwave and drop it in. Resistance should drop to around 0.185 ohms or so. This way you can tell if you need to order a sensor with the new radiator. Hair dryer can also tell you if the sensor is dropping but not as accurate. You're right as far as the radiator being cracked not being able to pull coolant back because it has to have a complete seal to have a vacuum.

 

Finally got the new rad, putting it in today. Once the new one goes in, am I best to just fill the rad to the top, then fill the reservoir? Currently the reservoir is full however, the last time I filled it, the plastic green piece that fits in the neck fell into the canister... I would love to just leave it in there, thinking it will float and not sink to block the hose but I'm not sure of it's significance?

 

The green cap is an inner splash cap with a vent hole along with the main vented cap. It could float block up and block the opening. What I'd do is to is just remove the two screws and dump it out and replace the whole cap system with the new style with just a tube vented cap only that goes up the frame. Could help on future overflow problems also. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse