Dave, a decent multimeter with ammeter function is a must-have tool for troubleshooting, but only if you are reasonably skilled in using it. For testing the voltage of a motor or determining that a ground has high resistance it is essential. But you still have to dig it out of your toolbox, have the necessary jumper cables and adapter wires, have access to and be able to read wiring diagrams, and especially have a logical and focused mind. As I get grayer, and my eyesight gets worse, reading and thinking become more important factors.
The Tekonsha tester works like the cheaper LED light testers but it also allows you to verify that your controller is working correctly, by simulating the resistance load of the brake magnets. There is no other simple way to do this other than by connecting your trailer!
If you do connect to your trailer and the controller doesn't work, you have no idea if the problem is in the truck or the trailer.
If your trailer is stored off site, it means that you have to wait until you are leaving on a trip to see if things are working right. I don't think anybody wants to hang around at a storage lot trying to figure out why the brakes are dead.....
It takes only a minute to take the tester out of the glovebox, plug it in, and have a buddy work all the lights and the brake controller, and you can see that everything is good by observing the six LEDs and the displays on the P3 screen. Using a meter would take WAY longer and frankly is a real pain in the butt.
The wiring at the back of a car or truck is exposed to extremely nasty conditions, including corrosive road chemicals, and it is very common for wiring and the 7 pin connector to fail, especially if there have been non-factory mods. If your connector has a steel body, it WILL rust out and then short out the wire terminals inside. Plastic ones will not do this. If somebody made a connection using a non-waterproof splice, it WILL corrode and fail, eventually.
The Tekonsh tester is a great tool to give your truck a spring checkup before you leave on your first trip. Buy it and you will thank me later. Also it is a great feeling to pull it out, test somebody's lights, and then be able to tell them immediately if there is a problem and which circuit is involved. It makes you look like a wizard. ;)
John Davies
Spokane WA