Let’s say you attempt to start your tractor and your starter does not engage. No click of the solenoid, nothing happens at all. Before assuming your starter is bad, grab your DMM and set it to DC Volts. Remove the wire going to the small terminal of your starter solenoid and connect the red probe of your DMM to the wire. Now connect your black probe to a ground. Turn your tractor’s key. Your DMM reads no voltage. This tells you that power is not going from your battery to the solenoid. Next remove the wires from the back of your ignition switch that are connected to the “B” and “S” terminals on your switch. Set your DMM to Continuity and connect your probes to the “B” and “S” terminals of the switch. Turn your tractor’s key and if there is no “beep” you know your ignition switch has a short internally and should be replaced.
Think of this as electrical volume. Amps can be determined by taking your voltage (12 volts) and dividing it by the resistance of a load. This is illustrated with the following formula: A = V/R
The ability to detect, observe, and measure electricity is crucial when trying to understand how a tractor works and learning what you can do to fix it. The digital multi-meter is like a doctor’s stethoscope. It allows us to see the invisible force of electricity and measure how it interacts with the components in our engines and tractors. A digital multi-meter measures current (amps), voltage (volts), and resistance (ohms).
The accessory circuit consists of your battery, ignition switch, a STSP switch and accessories such as an electric PTO, Headlights, Taillights, etc. When your ignition switch is in the “run” position your accessory terminal is energized by the battery. Power then runs from the “A” or accessory terminal to a STSP (Single Throw Single Position) switch. This switch is wired in-series and is a way to connect and disconnect power to your accessory such as your headlights. Your headlights are then wired to the STSP light switch and are wired in parallel to each other. This means two wires will come from your light switch and connect to each headlight individually. Two ground wires will also come off each headlight individually and connect to the common ground. If you were to wire your headlights in series each headlight will only use 6 volts rather than 12 and the brightness will be diminished.