Electronic Diagnostics Reminder
Dec 28, 2017
Back in the day, it was common when testing voltage on farm equipment to find an unpainted bolt head or scrape the paint off nearby metal and use them as grounds while testing. That technique still works on older equipment, but if you're working on newer machinery, that strategy can lead to mis-diagnosis.
Most electrical systems on modern machines, especially if there's any sort of computerization involved, have a separate ground circuit that goes all the way back to the tractor battery. That mean a control valve on the marker arm for a 24 row planter, or for a boom shutoff on a 120-foot spray boom, ultimately gets its ground from the tractor battery ground post. That means there are TWO circuits you have to worry about during testing: the power circuit (12 volts, 8 volts, 4 volts, maybe even as low as 2.5 bolts on a CAN-bus circuit), and the ground circuit.
If you ground to a nearby piece of metal during testing, and the actual broken wire or problem child is somewhere in the ground wire going back to the battery, your testing might show the power wire is in good shape. But since you've bypassed the ground circuit you're missing any short circuits or problems in the actual ground circuit.
Long story short--get in the habit of always using the machine's ground circuits when testing, otherwise you're testing only half of the circuitry that could be the cause of your problem.