02A Voltage Drop Testing _ Automotive Topics

VOLTAGE DROP TESTING

Effective Voltage Drop Testing— written and illustrated by Ralph BirnbaumWe are about to provide you with a simple, step-by-step process that will letyou divide and conquer common automotive circuit problems. It is called aVoltage Drop Test, and it works! Voltage drop tests are an effective way totest an electrical circuit’s ability to deliver power to electrical loads.

The Nature of CircuitsCircuits 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 circuitis circular, creating a current path from the power supply to the load--and back again. For the circuitto 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, wemay 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 wasteenergy.

Good voltage drops are essential. Loads won’t work without them. Available voltage must bedropped across the load, or it cannot work.

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Bad voltage drops allow available voltage to be “dropped” at a high resistance elsewhere inthe circuit; this steals electrical energy from the load. A bad voltage drop in a circuit convertselectrical energy into heat.

Rules of CircuitsLet’s agree on the following rules:

1) Circuit current is determined by the load(s). A 75 watt bulb demands less current than a 100watt 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 circuitcurrent. Circuits are designed to supply a specified amount of current. Fuses are designed to protectwiring from excessive current.

2) Everything has resistance. The wires, connectors, and switch contacts that make up a circuit allhave some resistance and, as its name implies, resistance opposes (resists) current. In anundamaged, properly designed circuit, normal resistance is small enough that it doesn’t keep theload from working.

Unwanted resistance in the circuit reduces the amount of electrical energy delivered to theload.

Causes of unwanted resistance include: loose connections; corroded connections; broken wirestrands; 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 tospot. Even if you cannot see visible causes for unwanted circuit resistance, the voltage drop test willfind them. Symptoms of unwanted resistance are just as familiar, including problems like a light bulbthat glows dimly instead of shining brightly, a fuel injector that doesn’t deliver enough fuel, or astarter or wiper motor that turns too slowly.

Current through resistance releases heat. (Remember this the next time you toast a slice ofbread!) If we want heat to defrost a piece of glass or warm our leather seats with an electricalheater, resistance is a good thing, a desirable voltage drop. On the other hand, a hot battery cableand slow turning starter motor indicate wasted electrical energy.

Voltage Drop Test EquipmentVoltage 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 isthe baseline, the starting point for the measurement: voltage at the red probe tip connected to the


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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 valuedisplayed.)

Basics of Voltage Drop TestingWe can test available voltage with the load turned off. But available voltage will not tell us if the loadwill work in the circuit, any more than a full tank of gas will guarantee that an engine will start andrun. 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, wewon’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 iscurrent. This tests the circuit’s ability to do its job when it matters—each time it tries to power theload. Remember, that is where we started, by creating a circuit that delivers power to a load!

Rules of the GameFor the voltage drop test to work, it must be done properly. It must be done the same way, each andevery time, or we will not get the same results. Understanding the rules of voltage drop testing islike understanding the rules of a football game. Changing the rules changes the outcome of both afootball game and of a voltage drop test. We can’t move the goal posts whenever it suits us!

Good and Bad Voltage DropsLet’s divide voltage drop tests into two groups:

Good voltage drops at the load Bad voltage drops caused by unwanted resistance.

Step 1 — Test at the Load

Set your meter to measure volts DC; then connect your test leads directly across the load and turn iton.

Take your reading. Ideally, the voltage dropped across the load should be the same as the voltageavailable at the load. If this is the case, the voltage drop is a good one. Real world? Voltagedropped at the load will often be lower than available voltage. This is not a problem, as long asenough 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 workproperly. 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. Youcan voltage drop test only those parts of the circuit accessible to your meter leads.


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We can’t always get close enough to test right at the load.

Step 2 — Test the CircuitTo test the circuit for voltage drops, connect your meter leads to two points in the circuit that havethe same polarity. For example:

Connect one meter lead to a battery post and the other lead to the battery cable end (seeillustration below). Connect your meter lead between the positive battery post and the hot side of a fuel injector.Then turn the circuit on and read the voltage.

Any voltage displayed on the meter indicates a voltage drop, and tells us exactly how much ofthe available voltage never reaches the load. Assuming the meter leads are good, lower readingsare better.

Testing the connection between the battery terminal end and battery post: Operate the starter withthe leads connected like this, and take your reading. Use the same process to test the other batterypost connection.

Know Your CircuitWe need to offer a word or two of caution here. In some vehicle circuits, a resistor is intentionallyinserted to reduce voltage and current available to the load. Examples include the rheostat that dimsthe dashboard lights, ballast resistors in some fuel injector circuits, and motor resistors used to limitblower fan and electric fuel pump speeds. Be sure you identify an “intentional” voltage drop bychecking circuit construction in a wiring diagram. Only One Way to LearnThere is only one way to learn how to perform a voltage drop test, and that is by doing it. We canthrow out all sorts of hypotheticals and tell you how things ought to work, but until you actuallyperform voltage drops tests, you won’t understand how to do them, and won’t know what to do withthe readings.

To get you started, we listed some rule-of-thumb standards indicating the maximum allowablevoltage drops in common circuits. Generally speaking, a voltage drop of 0.10 volt or less at anyconnection 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 droptest limits.


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As you perform voltage drop tests on a regular basis, you’ll learn to spot good and bad voltagedrops at a glance! Typical voltage drop maximums:

starter circuit (including starter solenoid) = 0.60 voltbattery post to battery terminal end = zero voltsbattery main cable (measured end to end) 0.20 voltstarter solenoid = 0.20 voltnegative main cable to engine block = 0.20 voltnegative battery post to starter metal frame = 0.30battery positive post to alternator b+ stud= 0.5 volt with maximum charging load applied (allaccessories turned on)battery negative post to alternator metal frame = 0.20 volt

Exercises: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 tolearn 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 testprobe tip to the negative battery post. Turn the ignition switch to the ON position. What does yourvoltmeter read?

Without moving the test leads, have someone crank and start the engine. Watch the meter. Does themeter reading increase briefly during cranking? Do you notice something interesting about the meterreading polarity after the engine starts?

3) Connect the red test lead tip to the battery positive post and backprobe the positive terminal ofthe ignition coil. Turn the ignition switch to the ON position. Note the reading. Crank and/or start theengine and measure the reading again. What should it be? Does your reading indicate a goodconnection?

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02A Voltage Drop Testing _ Automotive Topics