On the right diagnostic path

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 top load (Figure 1) represents a single load switched to ground circuit. The bottom loads represent loads in parallel that are switched to voltage. When you look closely at a wiring diagram (Figure 2), you can pick out the inputs to parallel current paths. This example is a power train control module with four voltage inputs, two receiving battery voltage at all times and two hot when the key is in RUN and START. I look at these current paths no differently than I look at the current paths in my simple drawing or any switched to voltage feed (for example, to a headlight or taillight). They are dealt with in the same manner when you troubleshoot current paths, and not circuits.

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.

I hope you are getting the idea that once you begin troubleshooting current paths you will not be intimidated by complicated wiring diagrams. It is the current that gets the work done in any circuit. Current (electrons) flows, voltage does not flow. Voltage is the difference that exists between the battery positive plate material and the negative plate material. The positive plates are void of electrons, have lots of holes in their atomic structure that are seeking free electrons. The negative plate material has lots of excessive free electrons just waiting to find a mate on the positive side of the battery.

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.

In some cases, nearly all of the voltage existing between the battery terminals is used to do work. In other cases, some of this voltage is dropped intentionally to limit current by placing resistors in the current path. In the case of our power train control module shown in Figures 2 and 4, current limiting resistors are inside to do just that. Even though current is intentionally limited inside control modules, troubleshooting a module’s voltage feed side and ground side current paths is the same as troubleshooting the voltage feed and ground side of a headlight or taillight.Checking Those Current Paths You can’t do effective voltage drop testing unless you first know how much voltage you are expected to have available to each load you are concerned with. The first measurement you take is to simply measure how much voltage potential is setting across

the battery terminals. This should always be your starting point when looking for an unwanted voltage drop. The minimum you want to see in order to continue voltage drop testing is 12.5 volts. This value tells you the battery is 75 percent charged. If less than this, forget voltage drop testing and find out why the battery is not at least 75 percent up to snuff.

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.

Subtracting the value you see at the input from what you saw across the battery terminals will tell you how much of your source voltage has been dropped before whatever is left appears at the load input. In any circuit that is operating at the time of testing, you will never see exactly the same amount of voltage that you measured at the battery available at the input to any load tested. If you do, there is no current flowing. You can never have a voltage drop without current flow.

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.

Resistance to current flow can show up on either side of the load at any time due to many different factors: corrosion, loose connections, loss of conducting strands of wire, excessive heat, just to name a few. Regardless of what caused it, the end result is always the same. Some of the source voltage (voltage you measured across the battery) left there to get the work of pushing the circuit current through where resistance to current flow exists. This is simply Ohm’s law in action.

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.

Take a look again (Figure 6) for the probe placement for finding how much source voltage is available to the input pin of any load tested. By subtracting the value on your voltmeter from your measured source, you will know if you need to go looking for where the excessive resistance is on the voltage feed side. If this difference was excessive, that is beyond the recommended allowable voltage drops, you would stay in the current path of the load being tested and work your way back toward the battery positive terminal with your voltmeter negative probe until you found an acceptable value. The problem area causing the excessive voltage drop is between the point where you found that acceptable value and where you started at the load input.

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.

Now let’s look at a way to let the meter do the math (Figure 7). By simply placing the voltmeter positive probe at the battery positive terminal and placing the negative probe at the load input, the meter will display the actual amount of voltage that is lost, dropped, maintained (however you want to think about it) between the battery positive terminal and the input to any load tested. No math is required with this probe placement; what you see is what you get, and what you get is instantly knowing if there is an excessive voltage drop on the feed side of any load tested. This method is especially useful when diagnosing voltage drop in a charging system circuit. A problem here can cause source voltage to constantly change, making any manual calculations impractical.It is your choice which of the two methods of probe placement you use. You just need to know what the value means.

The maximum allowable voltage drops shown are what I call “ball park.” If you measure something higher than these values, you have found a problem on the voltage feed side.

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.

When looking for excessive voltage drops, the most effective use of your time is what I call doing the “all inclusive” test first (Figure 9). Here the voltmeter positive probe is at the output side to a fuse protecting some load. The voltmeter is designed to display the difference in voltage it sees between its probes. This is an excellent test for finding out if the switch between the fuse and the load has an excessive amount of resistance causing a voltage drop greater than 300mV (.300V), but this probe position does not include every possible connection or wire conductor that could be hiding excessive resistance.

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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