For some years now engine mechanics have been using infra-red heat guns to track cooling system water flow and the like. But, these guns also have a place when working around electrical systems. You need to remember that the primary by-product of excessive electrical resistance is heat. Also, you need to remember that the things that cause excessive electrical resistance are things like wire too small for the task, and loose or corroded electrical connections.
These heat sensing guns are super sensitive and will help you find the bad spots in an electrical circuit. In the photo above I’m aiming the laser point on the gun at the primary starter motor lug on an outboard engine. I have an assistant cranking the engine so that current is flowing. I’m looking for a rise in temperature at the terminal. It was a hot day when the picture was taken at 90 degrees F. But, if while cranking I noticed a rise of about 25 degrees or more, I’d know without touching anything that there was an issue at that connection point. Keep in mind that often, especially at battery terminals you can’t always see the problem, its on the inside of the clamp or under the connection lug. In any event, a rise in temperature would tell me to remove and clean the connection surfaces and test it again.
I’ve seen these guns on sale for as little as $29.95 at places like Radio Shack, so they are not an expensive addition to your tool box. One online source for them that I can recommend is: www.professionalequipment.com