You may have studied circuits, but you should troubleshoot current paths. Circuits get complicated, and when you troubleshoot current paths, you simplify the complicated. There are only two current paths that you will work with regardless of the circuit load(s) when you look at circuits from a current path viewpoint. The first current path is always between the battery positive (+) terminal and the input(s) to any load on the vehicle.
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The second current path in any circuit is always between the output of any load and the battery negative terminal (Figure 3). The current path from the output of the two different circuits provides ground from the output side of each load all the way back to the battery negative (-) terminal. The parallel loads could be two backup lights, or two brake lights or two ground outputs on a transmission control module.
Engineers hook all kinds of loads between the battery terminals, place protective fuses and circuit breakers in for safety in the event of a short circuit, and provide some means of control which could be a mechanical switch, electromagnetic relay, or solid state driver, whatever engineering can dream up. The bottom line to all of this is that nearly all of the potential difference (battery voltage) should be used to push (or pull) electrons from one side of the battery to the other passing through some useful load to get some work accomplished.
Once the value of your source voltage is recorded, you have a basis for knowing what value to expect at the input terminal(s) of any load being tested. You can look for this value in two different ways; how much of the total voltage measured across the battery positive and negative terminals is available at the input to any load tested, or how much voltage has dropped between that measured at the battery positive terminal and the input to any load on the vehicle.
The drawing (Figure 6) represents the voltmeter probe positions you’ll need to find out how much of our source voltage (available across the battery) is available at any load tested. This probe position requires us to do some math if we want to know how much of the voltage potential across the battery has been lost, or has dropped on its way to present itself at the input to the load you are testing.
Let’s look at that last statement before we go on. I test for voltage drop in current paths — current paths on the voltage feed (input) side and current paths on the load ground (output) side. When there is a difference between the voltage I measured across the battery (either open circuit voltage with the engine off, or charging voltage with the engine running) and the voltage available to the input pin of any load being tested, I know this amount of difference is simply the sum of all voltage maintenance pressures required to push current through any resistance encountered. Ohm’s law states that it will always take one volt to push one amp through one ohm of resistance regardless of where that resistance shows up.
If current is flowing, voltage will drop at each encountered location of resistance. This is the reason you will never see exactly the same value of voltage measured at the battery available at any load input. Again, if you do, no current is flowing. You are measuring an open circuit.
You don’t repair a voltage drop; you repair the cause of the resistance that caused the voltage to drop at that point in the current path where excessive resistance is present. Once the root cause of the excessive resistance is gone, the excessive voltage drop will be gone.
Down To Earth
If your testing finds that there is no excessive voltage dropping on the voltage feed side, that is never a guarantee that the load will get to use all of the value that is sitting at the input pin. You could have an excessive amount of voltage dropping on the ground side of any load between the output pin of a load and the battery negative terminal (Figure 8).
Notice that the voltmeter is set to mV (millivolts) because the maximum allowable values on the ground side for all loads except the starter are within the mV range of most meters.
Voltage drop (voltage maintenance, voltage positioned to overcome resistance, the amount of source voltage needed to push electrons through resistance) testing is not difficult. It really is easy once you get the hang of it.
But when you work from the battery (positive or negative) terminal, you include all conductors, connectors, and connections that are providing a current path to the load you are testing.
If your voltmeter leads are not long enough, just use a long jumper wire to get you from the battery to your voltmeter probes, whether testing the load input or output. You will find the cause of excessive voltage drop using this method.
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