Some of the most common problems we see in the repair shop are electrical. Even many well trained small engine techs struggle with these problems.
One of the main causes of an electrical problem is the lack of current flow. If you can determine where the current flow is being interrupted, you should be able to fix the issue.
The starter motor that will not turn is a very good example. Many times folks will buy a new starter based only on the fact that they measure 12 volts at the motor with their Volt Ohm Meter (VOM). Then a few hours later they are back at the parts counter because they got what they think is a New Defective Motor. The new one doesn't turn either. So if they do have 12 volts at the motor what is the problem.
Current flow is a lot like water flowing thru a pipe. Think of the battery like it is the water pump pushing the water thru the pipe (the wire). Now lets look at a water wheel and try to think of it as a starter motor. It takes a certain amount of water to make the wheel turn. There are two ways to measure the water in the pipe, Pressure (voltage) and Volume (current). The pipe carrying the water to the wheel is sized so that it can deliver enough water (volume) to turn the wheel when there is enough pressure to push it thru the pipe. Let's put a cork with a small whole in it, in the end of the pipe. The pressure (voltage) will measure the same as at the pump but the volume (current) of water that can pass through the cork will not be enough to turn the wheel.
Back to our starter motor. We measure 12 volts at the motor with our VOM but due to corrosion or loose connections (resistance, like our cork above) not enough current can flow to turn the motor. Many times it is not the delivery of the voltage/current to the device, our starter motor in this case, but the return path to the battery. A bad engine ground for example. The current must make it all the way to the starter motor, through the motor and then back to the battery. When ever you are trouble-shooting an electrical problem try to use the same ground for your VOM as the device you are testing. Do not just hang the meter lead on the batteries terminal.
Now you can apply all the above to any electrical device. Be it a PTO clutch, headlights or a starter motor relay it is all the same. 9 times out of 10 the real bad part is some connection that is loose or corroded. Think current flow and you will fix every electrical problem you come across.
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