I made this very simple tester so that I could determine if my condenser fan and oil cooler fan was in either low or high speed mode in my car while I was driving. Actually, it can be used for just about any test.
Having just read this month's Bench Wrenching article "Troubleshooting - The Art of Critical Thinking, Paul wasn't easily able to check if there was a signal going to the injectors.
Well I just happened to check the injectors on a local's Porsche 911 the other week. The owner installed an Electromotive TecGT engine control system and it wasn't running right; injector #5 wasn't getting a signal at all and injector #3 was intermittent. The problem...the AMPSEAL connector...brand new and faulty right off the bat. The LED's flashed nicely on all other cylinders. And this simple tester avoided having to drag my oscilloscope over to his place.
Anyway, this or better yet, a version of it, can be stashed away in your took kit. A simple LED, resistor, some short leads and two pins can be put into a 1"x 2" zip lock bag. The pins are used to slide down the injector connector to make contact with them. My unit has banana jack receptacles so that I can use my test leads with it.
A bright LED should be chosen along with an appropriate series resistor.