"Suck-bang-go-blow,” or a version thereof, is a phrase that every tech has heard. It describes the basic four strokes of an internal combustion engine. “Suck” is the intake stroke, “bang” is the combustion event at TDC of the compression stroke, “go” is the power stroke and finally, “blow” refers to the exhaust stroke. The basic function of the engine seems to be ingrained in our subconscious — we don’t even think about it as we go about diagnosing drivability concerns.
Let’s focus on the intake stroke for the moment. Just before the intake valve opens, the piston is moving upward in the cylinder and pushing out the post-combustion gasses through the open exhaust valve. Just before the exhaust closes, the intake opens to help clean out the last remnants and to help ensure that the new charge of air is nice and clean.
As the piston changes direction, the space above it grows and that causes the pressure in the cylinder to drop. If the intake valve opened directly to the air outside of the engine — air, by the way, that is at atmospheric pressure and higher than the pressure in the cylinder — the pressure differential between the two would cause the air molecules to stream into the cylinder until the two equalized.
But it doesn’t, does it?
There’s a restriction between the high-pressure area (that is, the atmospheric pressure that lies outside the manifold) and it’s called the throttle plate. When the throttle plate is closed, the opening between the plate and throttle bore is just wide enough to allow some air in. There’s no difference in the force of the high pressure trying to get to the low — it’s just not possible to do so rapidly. In other words, fewer air molecules can get to the intake side of the throttle plate over the same given time frame.
Wait a minute — that sounds like the MAF reading in grams per second!
When the throttle plate is open, the restriction to the air molecules is reduced and that high-pressure area will stream in to fill the low pressure area at a much higher rate, won’t it? If you don’t believe me, go grab your scan tool and graph the MAF data while you operate the throttle from idle to WOT and back.
What’s that have to do with solving electrical problems?
Remember Ohm’s Law? Do you see the similarities?
The pressure differential between the atmospheric air outside the engine and the cylinder is what causes the air to flow. Similarly, the battery has a high number (pressure) of electrons gathered around the positive post and a lower number (pressure) at the negative post. When we place our voltmeter across the battery terminals, we are measuring that pressure differential. In fact, when we place our meter leads ANYWHERE in the circuit, we are measuring the pressure differential, or voltage, BETWEEN the meter’s leads.
If we provide a path that connects the two sides of the battery, current is going to flow from one to the other, isn’t it? Who hasn’t accidentally arced a tool before?
The measurement of current is “amperage” or “amps” and is the equivalent of our MAF reading in grams per second. Both are a measure of flow.
The debate is which way does current flow?
If we stick with our example, the answer would seem to be that current flows from positive to negative — that is, from the high pressure side of the battery to the low pressure side. This is called conventional current flow theory but is not entirely accurate. The electrons are negatively charged and the reality is that current flow is from negative to positive.
But who cares? Does that make any difference in how I fix the car? No, it doesn’t.
Finally, there is another factor that impacts the ability of current to flow in the circuit. That’s the electrical term “resistance” and is measured in “ohms.” Our throttle plate restriction is the image to consider when thinking of electrical resistance and Ohm’s Law confirms what we know intuitively — the more resistance there is, the lower the current flow.
One more consideration
“Current” is what does the work in an electrical circuit. The work may be illuminating a light bulb or turning a motor or creating the magnetic field that opens the fuel injector or fires the spark plug. And the device or component in the circuit that is responsible for doing the work is called “load.”
But electrons don’t move willingly, so we need to “force” that movement. That’s the role of the pressure differential or “voltage.” And, just like the throttle plate, there is some opposition to that current flow present, some “resistance.”
A question for you. If you wanted to measure how much resistance to airflow existed in the intake path, would you take that measurement statically (with the pistons stationary in their respective cylinders) or would you perform that test dynamically (with the engine running)?
Dynamically, of course! How would you be able to measure any opposition to airflow if there was no airflow at the time of measurement?
The same applies to testing an electrical circuit’s resistance. If resistance is the opposition to current flow, how can we test it accurately if there is no current flow? Did some say “with an ohmmeter?” Even that is not a true “static” measurement. The ohmmeter is performing a test that you can perform yourself and it’s called “voltage drop.”
Making sense of voltage drop
What happens to the air pressure after it overcomes the resistance caused by the throttle plate? It drops, doesn’t it? And that, in the simplest explanation is voltage drop. In the case of an automotive DC electrical circuit, the applied voltage “drops” across the resistance of the “load”, with next to nothing left over on the other side.
Remember this fundamental electrical principle.
“All voltage applied will be consumed by the resistance in the circuit.”
In a perfect world, the load would be the ONLY source of resistance. But in reality, everything in the circuit offers some resistance. The fuses, the switch, the connections — even the wire itself — offers a small, almost minute, amount of resistance.
And the electrical pressure, or voltage, applied will drop across every one of them.
Back to the rule I just shared.
“All voltage applied will be consumed by the resistance in the circuit.” is more accurately stated as “all voltage applied will be consumed by all sources of resistance in the circuit proportionally.”
Do you understand that? It is vitally important that you do!
If one resistor has 10 ohms of resistance and we measure the drop in pressure (or voltage) across it, we’ll get nearly source voltage at the positive side (Figure 1) of the resistor and almost a perfect 0.0 volts on the other side (Figure 2) of the resistor. Any minor variances can be accounted for by the wiring connecting the resistor to the battery, or voltage source.
Now add a second 10-ohm resistor in series with the first. What do you think will happen when we go down the line and take our measurements?
At the first resistor, we should still measure the source voltage we started with just as we did earlier (Figure 3). Moving to the ground side of the first resistor, we’ll now measure roughly half of the source voltage (Figure 4).
That’s because the voltage is being shared between the two just as the rule we just learned explained.
“All voltage applied will be consumed by all sources of resistance in the circuit proportionally.”
With no other major sources of resistance between the ground side of the first resistor and the positive side of the second, we should read the same amount of electrical pressure (Figure 5). And when we pass on to the ground side of the second, our reading should be the same as we saw on the ground side (Figure 6) of the first time around — nearly a perfect 0.0 volts.
Because all of the voltage has been consumed by all the sources of resistance in the circuit proportionally.
I can’t stress this concept enough. It is the key to understanding the use of voltage drop when testing a circuit that is not operating as it should.
Current makes the circuit work
The other key is something you have likely heard mentioned more than you care to admit — Ohm’s Law.
Remember what Ohm’s Law has to say about the relationship between voltage and current flow?
If voltage decreases, current flow decreases.
Think a moment about the two-resistor circuit. We had just over 12v going in — but is that how much voltage was being applied?
We can use what we learned about using our voltmeter to answer that question. Remember how the voltmeter works? It measures the electromotive potential, voltage, between its leads. If we place our leads just across the resistor, what do you think we’ll measure?
Approximately half of our source voltage (Figure 7)!
So only 6 volts or so was consumed by the first resistor. If this were a light bulb, what would expect to see?
A dim bulb, for sure!!
Is that dim bulb caused by the low voltage?
Not directly. The lower applied voltage was caused by the addition of resistance to the circuit - that second resistor wants its fair share of the total, remember? And that means lower current flow and THAT’S why the bulb is dim...or the motor spins slowly...or the injector doesn’t open...
Solving problems using voltage drop
Let’s apply this to the real world. If the customer is bringing you an electrical concern, it’s more likely in the form of SOMETHING that isn’t working as it should. A power seat won’t move, a brake light isn’t working, or something similar.
First step is to take a look at the wiring diagram and identify the load and its connections. Specifically, we want to identify where the voltage is coming in and where it’s going back out to ground.
When you’re first learning to understand voltage drop, take your first measurement directly at the battery (Figure 8). This is the source voltage you have to work with and should be taken under the same conditions that the circuit you are troubleshooting needs to operate. For example, I don’t need to have the engine running (and the battery charging) to test a power window circuit but I do if I want to test the function of the A/C compressor clutch coil.
Now attempt to operate the circuit you’re testing and move your positive meter lead to the positive side of the load as close to the load as you can get (Figure 9), leaving your negative meter lead at the battery. Voltage drop is a dynamic measurement and current must be flowing for voltage drop to occur. And this is the only way to test the entire circuit path at one time.
These two readings should be relatively the same, give or take a few tenths of a volt. Any difference is caused by everything else in the circuit path - the wire, connections, switches, fuses, and the like.
Remember the rule?
“All voltage applied will be consumed by all sources of resistance in the circuit proportionally.”
And the load in the circuit you’re testing should be the ONLY real source of resistance in the circuit — so no pressure, or voltage, should be lost until AFTER it passes through the component.
Let’s see if it does. Now move your meter lead to the ground side of the load (Figure 10).
You should read nearly 0 volts. The load should consume all of the available voltage except for the minute amount needed to overcome the small resistances left after the load — again, the wiring, connections and so forth.
Let’s make it even simpler
Here’s a way to remove one of the three measurements and make your use of voltage drop as a testing method even easier. Rather than measure the voltage directly at the battery first, move your negative meter lead from the ground side of the battery to the positive side. Now skip to the second measurement and place your positive meter lead on the positive side of the load, as close as you can get (Figure 11).
This method lets the meter do the math for you. Remember how the voltmeter works? It reads the pressure differential between the test leads. If the circuit is operating correctly (electrically), then there should be very little voltage between the leads, just the few tenths of a volt that the minor resistance sources (also located between the leads) are consuming.
If you DO read a significant voltage, what does that tell you? Think about it.
That’s your “red flag” — there is some other source of significant resistance between the test leads and it’s demanding that amount of voltage for itself. Remember the reading we got on the positive side of the second resistor (back to Figure 5)? Think of the second resistor as the primary load and the first is your source of unwanted resistance, your “thief,” the reason the load is not getting the current flow it needs to work the way it’s supposed to!
If this test is OK, then we move on to the last test. Return the negative meter lead to the negative post on the battery and move the positive lead over to the ground side of the load. We expect to see only a few tenths, correct? Because the load is the primary source of resistance and should consume the majority of the voltage applied.
What if, instead, you measured significant voltage here? Same reason as we just discussed — there is a thief between your meter leads! Think about the two-resistor example one more time, only this time the first resistor is supposed to be there and the second is the “thief!” Remember the reading we got when we placed our meter lead on the ground side of the first resistor (back to Figure 4)?
Remember, it was nearly the same when we moved the lead to the positive side of the second resistor? That measurement is no different than when we measured for voltage on the positive side of the first resistor! The only difference is that we knew in our minds that we SHOULD read voltage then and didn’t expect it when measured on the ground side!
And that, my friends, is why you can measure voltage even when your meter leads are both attached to (what you thought were) ground points!
