Sketch, it's a little hard to learn at the very begging but LTspice is a great learning tool. You can plug (albiet ideal and not necessarily real world devices) into a virtual schematic, simulate it for a length of time and look at the voltage current or any equation you calculate from those at any point in the circuit. Great for basic understanding of what's going on in real circuits.
If you decide to download and play with it. I've attached the .asc file which is the same circuit (I changed a few resistors) that ROFF had you make for the AC to ECU fix you did with your car. Just click simulate and it should pop up a graphics window, clicking on any wire in the schematic will show you the voltage, clicking on the right spot on the components will show you current.