48 GEARSMarch2008

Voltage Drop Test Helps Pinpoint Bad Connection

It’s been a while since our last look at electrical theory and diagnostic procedures, but a recent look back

at some of those articles revealed a few issues that never made it to print. Hopefully you remained current with the concepts we talked about in ear-lier articles, because those points will remain an important part of these later articles, as well as your regular electri-cal diagnosis.

In this issue, we’re going to look at one of the most important diagnostic procedures you can use for identifying electrical circuit problems: the voltage drop test.

Let’s start by clearing something up right away: All voltage measure-ments are voltage drop tests. Whenever you connect a voltmeter to a circuit, you’re looking for the difference in voltage between the connection points; that is, the drop in voltage between the positive lead and the negative lead.

So when you measure the battery voltage, you are in fact performing a voltage drop test. You’re measuring the drop in voltage between the bat-tery positive terminal and the battery negative terminal (figure 1). Nothing mystical here.

The difference between a stan-dard voltage measurement and what we think of as a voltage drop test is that, with the voltage drop test, you’re usu-ally looking for a drop in voltage where there shouldn’t be one. That’s right: During most voltage drop tests, a good circuit won’t use any voltage between the points you’re measuring. If you do find a voltage drop, it indicates a prob-lem in the circuit, such as a loose con-nection or corrosion, adding unwanted

resistance to the circuit.

Why a Voltage Drop Test?

Okay, so we have unwanted resis-tance in the circuit. Why can’t we just use an ohmmeter to check for the resis-tance? Or check the current draw, and use Ohm’s Law to identify a problem?

Both valid points. But an ohmmeter checks the circuit using low voltage and amperage levels. A problem that affects a loaded circuit might not appear at all using an ohmmeter. A common example is a frayed wire, where only one or two strands of the wire are left to conduct the current. An ohmmeter wouldn’t reveal this problem, because it doesn’t apply enough of a load for the frayed wire to cause a resistance loss.

And while an ammeter will indi-

cate a problem with the circuit, it won’t let you pinpoint the problem the way a voltage drop test will. Remember, current flow is constant throughout a simple or series circuit, and on the com-mon points in a parallel circuit. That’s why you can measure amperage nearly anywhere in the circuit. So, while the problem will show up with an amper-age test, pinpointing the root cause of the problem will require a voltage drop test.

Let’s look at a common voltage drop test, and see what makes it so dif-ferent… and so valuable.

Performing a Voltage Drop Test

Probably the most important point to remember — and the one that’s most often forgotten — about a voltage drop

Pushing ElEctrons

Voltage Drop Test Helps Pinpoint BaD ConneCTions by Steve Bodofsky

Figure 1: A voltage drop test is just a term that describes measuring the difference in voltage between two points in a circuit. A battery voltage check

is a type of voltage drop test.

GEARS March 2008 49

test is the circuit must be loaded for the test to work. In fact, to get the best results, it should be loaded to the maxi-mum level.

So, if you’re checking the circuit for a transmission shift solenoid, that solenoid must be energized for the volt-

age drop test to have any meaning. In the case of a starter circuit, the starter must be cranking during the test. For a fan, the fan motor should be set to run at maximum speed. If you don’t ener-gize the circuit, the test results won’t have any meaning.

Another point to consider is which way you connect your meter leads depends on which side of the circuit you’re measuring. Always connect your positive meter lead closest to the posi-tive circuit source.

So if you’re checking the positive side of the circuit for a voltage drop, you’ll want to connect your meter’s positive lead to the closest point in the circuit to the positive lead. On a starter circuit, that’ll usually be the positive battery post. For other circuits, that may be at a power source from a fuse, relay or computer.

When measuring the negative side of the circuit, you’ll want to connect your voltmeter’s positive lead closest to the load for the circuit you’re checking; the closest point to the positive source. On a starter, that’d be the starter case itself.

On a transmission solenoid, where you connect those leads to check the ground circuit depends on how the sole-noid receives ground. If it’s a feed-con-trolled solenoid that’s grounded to the transmission case, the closest point to the positive source may be the solenoid

Figure 2: When performing a voltage drop test, always connect your meter’s positive lead to the most positive point in the circuit. To check the positive side

of the starter circuit, that’s the battery positive post.

50 GEARSMarch2008

Voltage Drop Test Helps Pinpoint Bad Connection

or transmission case. If it’s a ground-controlled solenoid, the closest point to the positive source would probably be the transmission case connector.

Let’s go over the steps for a basic load test, and see how to track down a circuit problem. For this example, we’ll use a starter motor, because it’s so easy to see the terminals and connections.

Step 1: Connect your voltmeter’s positive lead to the battery positive post (figure 2).

Diagnostic Tip: Most of the bat-teries today have two terminals; a top mount and a side mount. This is a great advantage for performing a voltage drop test. Connect your meter positive lead to the unused terminal: This lets you measure the voltage drop starting directly at the battery terminal, so you can take a good measurement from the terminal ends; a likely source for cor-rosion.

Step 2: Connect your voltmeter’s negative terminal to the point closest to the circuit load. On a starter, that’ll be the starter power terminal (figure 3).

Diagnostic Tip: On some circuits, it may be easier to use a piercing probe to reach the point closest to the load. Just remember to seal the wire when you’re done.

Step 3: Load the circuit and check the voltage. On a starter circuit, that means you’ll have to crank the engine just long enough to record a stable voltage measurement.

Caution: Always disable the fuel pump and ignition before cranking the engine to perform a voltage drop test. Disabling only the ignition will allow the fuel system to flood the engine, which will damage the catalytic con-verter. Disabling only the fuel system could allow the engine to start on gas that’s puddled in the intake, ruining your test results.

Oh, and once you’ve taken the voltage measurement, stop cranking the engine.

Step 4: Record the voltage shown on your meter while the circuit was energized.

If you see zero volts on your meter, that part of the circuit is okay, and

there’s no reason to check it further. In fact, on most circuits it’s okay to have up to 0.1 volts drop on each side of the circuit; that’s 0.1 volt on the posi-tive side, and 0.1 on the negative side. Anything more than 0.1 volts indicates a problem. If you see excess voltage drop:

Step 5: Move the meter negative

lead back to the next connection point on the circuit. In the case of the starter circuit, that’d be the pos-itive cable connector at the starter power terminal (figure 4).

Step 6: Crank the engine, and record the voltage shown on your meter.

Figure 3: Connect your meter’s negative lead as close to the circuit load as possible. To check the positive side of the starter circuit,

that’s the starter positive post.

Figure 4: If you measure a voltage drop higher than 0.1 volts, move the negative lead back to the next point in the circuit,

and measure the voltage drop again.

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2008 ATRA Technical SeminarsDates: Locations:

3/15/08 Orlando, FL3/29/08 Phoenix, AZ3/29/08 Boston, MA4/12/08 Minneapolis, MN4/12/08 Salt Lake City, UT4/19/08 Philadelphia, PA4/26/08 Coeur D’Alene, ID5/3/08 Tunica, MS5/10/08 Fremont, CA5/10/08 Denver, CO5/31/08 Dallas, TX6/7/08 Vancouver, BC6/7/08 Los Angeles, CA8/2/08 Albuquerque, NM8/16/08 San Antonio, TX8/23/08 Cincinnati, OH8/23/08 Des Moines, IA9/6/08 Billings, MT10/4/08 Portland, OR10/4/08 Chicago, IL10/11/08 New York, NY10/18/08 Atlanta, GA11/8/08 Baltimore, MD

GM:BW4484LCT 10004L60/65E/70E6T40/45E2ML70

Ford:Torqshift4R55/5R556R60/75

Chrysler:45RFE42RLE62TE42LERFENAG1-722.6

Imports:Nissan RE5RO5AMitsubishi F4A/F5AToyota A671

4x4:FordGMNew Venture

Increase your customer satisfaction with the right fix at the right time... and

“Get ’em out the Door”

Registration Costs:Prepaid, preregistered ATRA members $149Prepaid, preregistered non-ATRA members $189On-site registration $210

Register online and save $10 per person One free registration with every four paid.

ATRA Seminar Registration2400 Latigo AvenueOxnard, CA 93030

805.988.6761

8*800.428.8489

www.ATRAonline.com

Register Today!

Check ATRAonline.com for more dates and locations to come!

52 GEARSMarch2008

Voltage Drop Test Helps Pinpoint Bad Connection

If the voltage drops back to zero, you found the circuit problem: It’s in the connection between the cable con-nector and the starter power terminal. Remove the cable, clean the connec-tion, and perform steps 2 and 3 again. The voltage should be zero.

If the voltage drop still appears on the meter:

Step 7: Move the meter negative lead back to the next connection point in the circuit. In the starter cir-cuit, that’d be the battery connector clamp or terminal (figure 5).

Step 8: Crank the engine, and record the voltage shown on your meter.

If the voltage drops back to zero now, the problem is in the positive battery cable between the last two test points you checked. Replace the cable and recheck the voltage drop at the starter terminal.

The whole point of the voltage drop test is to keep moving the test lead back along the circuit, until the voltage drop disappears. The circuit problem will be somewhere between that point and the previous test point in the circuit.

The Theory Behind a Voltage Drop Test

So why are we so concerned with a little bit of voltage drop in a circuit? To answer that, we need to go back to our earlier discussions about the principles

of simple and series circuits. Probably the most significant of these principles when looking at a voltage drop test is Principle 6 for Series Circuits:

Each resistance in the circuit will use its share of the applied voltage, depending on its resistance. The sum of the voltage drops across the individual resistances will equal the applied volt-age.

So any voltage drop, anywhere in the circuit, must be caused by a resistance in that circuit. That could be a planned resistance, such as the resistances found in a series circuit. But the object of most voltage drop tests is to identify and isolate unplanned or unwanted resistances, such as a bad or corroded connection in the circuit.

These faulty connections create an additional resistance in the circuit. And because that resistance is using “its share of the applied voltage, depending on its resistance,” it’s using up some of the voltage that the other, planned resis-tances need. Which means the circuit or component may not operate the way it’s supposed to. And too often, those components end up getting replaced for no reason; there’s nothing wrong with them. They’re just not receiv-

ing enough voltage, because something else in the circuit is stealing some of the voltage away from it.

Diagnostic Tip: Another source of unplanned resistance can be inad-equate wire gauge; that is, the wire’s too thin to handle the circuit’s current requirements. Suspect this as a possible problem if someone has worked on the circuit before.

This is why a voltage drop test is so valuable. It allows you to identify faults in virtually any circuit, based on a simple electrical measurement: voltage. And it’s why a voltage drop test should be one the first tests you perform when looking for a problem in any electrical circuit.

Each resistance in the circuit will use

its share of the applied voltage, depending on its resistance. The

sum of the voltage drops across the individual

resistances will equal the applied

voltage.

Figure 5: Keep moving back along the circuit until the voltage drops back to around zero; the excess resistance is somewhere between this point

and the last point you measured.

CHALLENGE!Try your

Voltage Drop knowledge on the

test next page!

GEARS March 2008 53

TEST QUESTIONS:1. TechAsaysavoltagedroptestisverydifferent

fromastandardvoltagemeasurement.TechBsaysavoltagedroptestcanhelpyoupinpointabadconnectioninacircuit.Who’sright?A. AonlyB. BonlyC. BothAandBD. NeitherAnorB

2. Whichofthesestatementsaboutavoltagedroptestistrue?A. Avoltagereadingusuallyindicatesaproblemin

thecircuit.B. Thecircuitmustturnedoffperformavoltage

droptest.C. Avoltagedroptestisonlypossibleifthere’san

exposedconnectorinthecircuit.D. Alloftheabove.

3. Whenmeasuringavoltagedroponthepositivesideofacircuit,always:A. Connectthepositiveleadasclosetothe

positivesourceaspossible.B. Connectthenegativeleadasclosetothe

negativesourceaspossible.C. SetyourmetertomeasureVoltsDC.D. Alloftheabove.

4. Whenmeasuringavoltagedroponthenegativesideofacircuit,always:A. Connectthepositiveleadasclosetothe

positivesourceaspossible.B. Connectthenegativeleadasclosetothe

negativesourceaspossible.C. SetyourmetertomeasureVoltsDC.D. Alloftheabove.

5. Avoltagedroptestismoreeffectiveforfindingcircuitproblemsthanaresistancecheckbecause:A. thevoltagedropteststhecircuitunderactual

operatingconditions.B. thevoltagedropcanbemeasuredanywherein

thecircuit.C. thevoltagedropallowsyoutopinpointthe

failure.D. Alloftheabove.

6. Avoltagedroptestismoreeffectiveforfindingcircuitproblemsthananamperagecheckbecause:A. thevoltagedropteststhecircuitunderactual

operatingconditions.B. thevoltagedropcanbemeasuredanywherein

thecircuit.C. thevoltagedropallowsyoutopinpointthe

failure.D. Alloftheabove.

7. TechAsays,whenperformingavoltagedroptestonastartercircuit,youshoulddisabletheignitionsystembeforecrankingthestarter.TechBsays,whenperformingavoltagedroptestonastartercircuit,youshoulddisablethefuelinjectionsystembeforecrankingthestarter.Who’sright?A. AonlyB. BonlyC. BothAandBD. NeitherAnorB

8. TechAsaysthere’snoreasontoperformavoltagedroptestonshiftsolenoidswithacommonpowerfeed,unlessnoneofthesolenoidsareworkingproperly.TechBsaysyoumustenergizetheshiftsolenoidcircuittocheckitforavoltagedrop.Who’sright?A. AonlyB. BonlyC. BothAandBD. NeitherAnorB

9. Unlessotherwisespecified,howmuchvoltagedropisacceptable?A. 0.1voltsperconnectorB. 0.1voltspercircuitC. 0.1voltspersideofthecircuit(positiveand

negative)D. Novoltagedropisacceptable.

10.Whichoftheseconditionscancauseexcessivevoltagedropinacircuit?A. CorrosioninaconnectorB. LooseconnectorC. InadequatewiregaugeD. Alloftheabove

TEST QUESTIONS:Answers:1.B;2.A;3.D;4.D;5.A;6.C;7.C;8.B;9.C;10.D