Effective Voltage Drop Testing
— written and illustrated by Ralph Birnbaum
We are about to provide you with a simple, step-by-step process that will let you divide and conquer common automotive circuit problems. It is called a Voltage Drop Test, and it works! Voltage drop tests are an effective way to test an electrical circuit’s ability to deliver power to electrical loads.
The Nature of Circuits
Circuits and power supplies exist for one reason: to supply electrical power to operate loads. Remember this: we’ll return to this concept often.
This circuit includes a power supply, a load (light bulb), wiring, and a switch. Note that the circuit is circular, creating a current path from the power supply to the load--and back again. For the circuit to operate, the circle must be complete.
A load is a device that uses electrical energy to do work, or to make heat, spark, or light.
When loads fail to work, a call goes out for an automotive electrician! That’s where we come in.
Voltage Drops-Good or Bad?
Voltage drop tests are usually performed to test loads and locate circuit problems. As a result, we may have a tendency to think of voltage drops as bad things. But voltage drops can be good or bad; it all depends on where they occur in the circuit, and whether they operate loads, or simply waste energy.
Good voltage drops are essential. Loads won’t work without them. Available voltage must be dropped across the load, or it cannot work.
Bad voltage drops allow available voltage to be “dropped” at a high resistance elsewhere in the circuit; this steals electrical energy from the load. A bad voltage drop in a circuit converts electrical energy into heat.
Rules of Circuits
Let’s agree on the following rules:
1) Circuit current is determined by the load(s). A 75 watt bulb demands less current than a 100 watt bulb screwed into the same socket. Installing a higher wattage bulb increases the circuit load. Adding a second light to a circuit also increases the circuit load. Adding loads increases circuit current. Circuits are designed to supply a specified amount of current. Fuses are designed to protect wiring from excessive current.
2) Everything has resistance. The wires, connectors, and switch contacts that make up a circuit all have some resistance and, as its name implies, resistance opposes (resists) current. In an undamaged, properly designed circuit, normal resistance is small enough that it doesn’t keep the load from working.
Unwanted resistance in the circuit reduces the amount of electrical energy delivered to the load.
Causes of unwanted resistance include: loose connections; corroded connections; broken wire strands; pitted relay contacts; and other physical damage that resists current.
Some circuit problems can be spotted quickly with the naked eye. Some won’t be so easy to spot. Even if you cannot see visible causes for unwanted circuit resistance, the voltage drop test will find them. Symptoms of unwanted resistance are just as familiar, including problems like a light bulb that glows dimly instead of shining brightly, a fuel injector that doesn’t deliver enough fuel, or a starter or wiper motor that turns too slowly.
Current through resistance releases heat. (Remember this the next time you toast a slice of bread!) If we want heat to defrost a piece of glass or warm our leather seats with an electrical heater, resistance is a good thing, a desirable voltage drop. On the other hand, a hot battery cable and slow turning starter motor indicate wasted electrical energy.
Voltage Drop Test Equipment
Voltage drops are tested with a voltmeter or scope. (A test light indicates whether voltage is present, but it doesn’t measure the amount of voltage.
• Voltmeters measure and display the difference in voltage levels between the voltmeter test leads.
• The black test lead connected to the meter COM port should be considered “the reference.” This is the baseline, the starting point for the measurement: voltage at the red probe tip connected to the meter VOLT port is compared to this starting point and displayed on the meter as a number. (Reversing the test leads won’t hurt a digital meter, but we may se e a negative voltage value displayed.)
Basics of Voltage Drop Testing
We can test available voltage with the load turned off. But available voltage will not tell us if the load will work in the circuit, any more than a full tank of gas will guarantee that an engine will start and run. To properly test a circuit, we need to turn it on and then test it.
Will this load work? Even though we know how much voltage is available at the light socket, we won’t know if the light actually works until we turn the circuit on by closing the switch.
Current in the circuit changes everything. We can measure voltage drops only when there is current. This tests the circuit’s ability to do its job when it matters—each time it tries to power the load. Remember, that is where we started, by creating a circuit that delivers power to a load!
Rules of the Game
For the voltage drop test to work, it must be done properly. It must be done the same way, each and every time, or we will not get the same results. Understanding the rules of voltage drop testing is like understanding the rules of a football game. Changing the rules changes the outcome of both a football game and of a voltage drop test. We can’t move the goal posts whenever it suits us!
Good and Bad Voltage Drops
Let’s divide voltage drop tests into two groups:
Step 1 — Test at the Load
Set your meter to measure volts DC; then connect your test leads directly across the load and turn it on.
Take your reading. Ideally, the voltage dropped across the load should be the same as the voltage available at the load. If this is the case, the voltage drop is a good one. Real world? Voltage dropped at the load will often be lower than available voltage. This is not a problem, as long as enough voltage is dropped to operate the load.
You are done testing. Everything works as it should.
If voltage dropped across the load is a lot lower than available voltage, then the load won’t work properly. There is a voltage drop in the circuit somewhere denying the load the power it needs.
Is it always practical to test right at the load? No. You won’t always have direct access to the load. For example, you cannot connect your meter leads across the terminals of an in-tank fuel pump. You can voltage drop test only those parts of the circuit accessible to your meter leads.
We can’t always get close enough to test right at the load.
Step 2 — Test the Circuit
To test the circuit for voltage drops, connect your meter leads to two points in the circuit that have the same polarity. For example:
Any voltage displayed on the meter indicates a voltage drop, and tells us exactly how much of the available voltage never reaches the load. Assuming the meter leads are good, lower readings are better.
Testing the connection between the battery terminal end and battery post: Operate the starter with the leads connected like this, and take your reading. Use the same process to test the other battery post connection.
Know Your Circuit
We need to offer a word or two of caution here. In some vehicle circuits, a resistor is intentionally inserted to reduce voltage and current available to the load. Examples include the rheostat that dims the dashboard lights, ballast resistors in some fuel injector circuits, and motor resistors used to limit blower fan and electric fuel pump speeds. Be sure you identify an “intentional” voltage drop by checking circuit construction in a wiring diagram.
Only One Way to Learn
There is only one way to learn how to perform a voltage drop test, and that is by doing it. We can throw out all sorts of hypotheticals and tell you how things ought to work, but until you actually perform voltage drops tests, you won’t understand how to do them, and won’t know what to do with the readings.
To get you started, we listed some rule-of-thumb standards indicating the maximum allowable voltage drops in common circuits. Generally speaking, a voltage drop of 0.10 volt or less at any connection is good. Really good connections will display voltage drop readings closer to zero volts. Circuits carrying a lot of current, like alternators and starters, are allowed slightly higher voltage drop test limits.
As you perform voltage drop tests on a regular basis, you’ll learn to spot good and bad voltage drops at a glance!
Typical voltage drop maximums:
Here are some exercises, all performed with the voltmeter set to volts DC.
There is no better way to learn these skills than by doing them. Test on live circuits in real cars to learn by experience.
1) Connect your voltmeter leads across the terminals of a headlight bulb, and turn on the headlights. What does your meter read? How does your reading compare to battery post voltage?
2) Connect the red voltmeter lead to bright metal on the engine block, and touch the black test probe tip to the negative battery post. Turn the ignition switch to the ON position. What does your voltmeter read?
Without moving the test leads, have someone crank and start the engine. Watch the meter. Does the meter reading increase briefly during cranking? Do you notice something interesting about the meter reading polarity after the engine starts?
3) Connect the red test lead tip to the battery positive post and backprobe the positive terminal of the ignition coil. Turn the ignition switch to the ON position. Note the reading. Crank and/or start the engine and measure the reading again. What should it be? Does your reading indicate a good connection?