If pictures would help aid in this question, I will take and post some. I live in Alaska where almost every car has a number of heaters on it to keep it from freezing solid. My car has 3 heaters...a battery pad, and engine block (freeze plug) heater, and likely an oil pan heater (I have not crawled under the vehicle to identify). these 3 heaters plug into a 3-way extension cord (this is within the engine compartment), and that cord that sticks out of my grill, so I can plug the car in.
Two of the heaters are two prong, the other is a 3 prong.When I plug the car into the wall outlet, the GFI trips and power shuts off. What I have found is that the 3 prong heater is causing the GFI to trip. I have partially traced the cord and the heater was dealer installed, but it travels down the engine compartment to the frame and then heads back towards the engine. I have not crawled under the truck to fully trace to determine the exact heater. So here comes my question:
Can that 3 prong heater be tested to determine if it needs to be replaced? I have a multimeter if that helps, but I need to get this figured out before it gets too cold. Thanks for your help everyone.

 

These all run on 120 volts AC?

 

they do, yes

 

the third prong (round) is the ground. power will run through the other two prongs...flat.
to test a block heater ...unplug it and measure the ohm reading across the flat prongs with a meter....most block heaters measure approx 35 ohms. not sure what a battery pad or oil heater are resistance wise but i suspect if the resistance is low....the element is shorted.

 

So I set my multimeter to resistance ohms? I will take a reading and post it here.