Using a Digital Multi Meter

Basic Electrical Troubleshooting

By Vince Illi

In my ongoing quest to make people less scared of automotive electrical systems, I'd like to talk a little bit about two common electrical troubleshooting tools and how to use them.  First, though, we'll need to quickly review electrical basics:


Current Diagram

Remember that current flows through an object, while voltage is measured across an object.  Ohm's law tells us that voltage (V) is the current (I) multiplied by the resistance (R):

V = I R

Before we get into discussing troubleshooting tips, a word of caution is in order:  If you are troubleshooting something in an engine's sensor system (such as fuel injectors, MAF sensors, IAC valves, or the like), first DISCONNECT THE COMPUTER AND REMOVE IT FROM THE VEHICLE.  Electronic control units are very sensitive, and their sensor inputs operate on very small voltages and currents.  If you accidentally put a higher current or voltage onto an ECU pin than it is designed for, you can easily damage the ECU.

Test Lights

One of the most common tools used to look at a car's electrical system is the test light.  A test light is simply a light bulb with two leads.  One lead (the ground) is usually an alligator clip, and the other lead (hot) is usually a sharp spike (or probe).

Electrical test light

The test light, while simple to use, really isn't very useful.  Then again, that probe spike can probably be used to kill a vampire or something.

A test light is the simplest tool around.  You connect one end to the chassis (ground) of a vehicle.  Then you touch the sharp probe end to a wire or connector pin.  If the test light illuminates, the wire or pin is "hot," or has 12 volts on it.

Test light to Battery

The positive battery terminal is obviously "hot," so the test light illuminates.

Really, though, the test light doesn't tell you much beyond whether or not a wire is "hot."  It doesn't tell you voltage, current, or resistance.  And it can't really help you troubleshoot much of anything.  For that, we need something more versatile.

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Wednesday, May 18, 2011 8:14 AM
I have a buddy whose minivan is constantly draining its battery if not driven for a few days. I'll have to pass this article along his way but I'm pretty sure I'll still be over there helping him track down the source of the drain.
Wednesday, May 18, 2011 8:39 AM
If I'm testing whether or not I'm getting voltage to the fuel pump, MAF, or some other ECU controlled sensor or component, how would I test this with the ECU removed? On every Nissan I've worked on, the ignition powers on the ECU (well, the ECU's relay), which then signals a number of other relays (ignition, fuel pump, etc) to turn on. No ECU, no power to these items.
Der Bruce
Der Brucelink
Wednesday, May 18, 2011 8:51 AM
Ah the test light, quite literally a light in the darkness of many electrical mess. Most trailers you might see that have been "fixed" with multile accesory wires are from people who didn't know how to use a simple test light to do it right. Great article!
Dusty Duster
Dusty Dusterlink
Wednesday, May 18, 2011 9:00 AM

The fuel pump is almost invariably connected via a relay, so disconnecting the ECU isn't necessary. Working on the sensor subsystem can be done with the ECU still connected, but you should NOT use test lights in that case, as the current pulled by them would be too high. If you have to work on the sensor subsystem with the engine on, use only a voltmeter.
Dusty Duster
Dusty Dusterlink
Wednesday, May 18, 2011 9:02 AM
Additionally, it may be better to pull the ECU anyway and apply 12V directly to the pins on the ECU's plug to test voltage at sensors.
Wednesday, May 18, 2011 9:13 AM
@ Dusty: Ah, word. I've never, ever, used a test light to troubleshoot anything, so your statement seemed a bit overkill to me. DMM is a superior tool in nearly every way, except a manly one, in your fuel pump case.

As for applying voltage to individual pins, that'd take some reading of electrical diagrams in the FSM, something I am loathe to do... :)

Either way, electrical gremlins are not a good time. I once accidentally touched my intake manifold to the positive battery terminal with the grounds still attached. Fried pretty much the whole harness. Whoops. Oddly, the ECU survived.
Dusty Duster
Dusty Dusterlink
Wednesday, May 18, 2011 9:22 AM
Applying voltage directly to the pins also rules out the ECU as being the problem. Always strive to eliminate double variables.
Wednesday, May 18, 2011 12:00 PM
OMG Its Weasel
OMG Its Weasellink
Wednesday, May 18, 2011 3:32 PM
wait... dusty is vince? mind blown.

give me a grinder, impact wrench, sawzall, anything but a multimeter and im golden. im scared shitless of electrical stuff, its the only thing ill pay someone else to do to my cars.

thanks for some insight though, your vocabulary and phrasing are simple enough that you dont need to have a sparky friend handy to translate electricity into english.
Dusty Duster
Dusty Dusterlink
Wednesday, May 18, 2011 5:36 PM
Yep. I'm Vince. Or Dusty Duster is me.
Thursday, May 19, 2011 3:48 AM
I really have to disagree with how you diagnosed the fuel pump there Vince. You could have easily done it with the DMM and without your manly test light. What you forgot to do is measure the voltage on the ground side of the circuit. All voltage will be "consumed" by the time it reached the source (battery). You did measure the "hot" side of the fuel pump which showed that there was 12V applied to the pump. If you measure after the pump, it should read close to zero because ideally the pump should "consume" all 12V. I bet in your case you would have most likely measured something high meaning that that voltage was still being consumed elsewhere in the circuit such as high resistance at a corroded ground or faulty relay.

DMM is by far my favorite tool in my box. Call me weird, but I love electrical work on cars.
Thursday, May 19, 2011 6:18 AM
I've got a theory that for every 10 mechanical guys there is 1 true electrical guy.

I'm with you # MxExUx. I'd rather rewire a car than do any thing else.

Of course a BS in electronics doesn't hurt. ;-)
Dusty Duster
Dusty Dusterlink
Thursday, May 19, 2011 7:20 AM

It was near impossible to measure after the pump in this case because of the way the vehicle was wired. The fuel pump wiring had a single two-prong plug that ran to the fuel tank, and the wiring was very difficult to get to. The only way to really measure the voltage at the fuel pump was to disconnect the pump and measure it across the connector's terminals.

It's much easier to hook up a high-amperage test light to mimic the fuel pump's draw than to take several different voltage measurements at different portions of the circuit, especially when most of the circuit is inaccessible.

In this case, measuring the ground side of the circuit would have found the problem, because it was a bad ground. But if the bad connection was on the "hot" side, measuring the ground would not have found the issue. A third measurement would have needed to be taken between the ASD relay (under the hood) and the actual fuel pump connector (under the rear axle). So, you see, it is easier to simply hook up the test light (which is guaranteed to find the problem) than to take three separate voltage measurements, several of which would be very difficult to make.
Thursday, May 19, 2011 4:57 PM
...you could have back probed the ground wire at the connector for the fuel pump.

I guess I just prefer to look at numerical readings than trying to decipher something as subjective as the visible brightness of a light bulb.
Thursday, May 19, 2011 7:51 PM
I always have fun when this stuff comes in at work.

My favorite tool I own is a Snap-On Vantage graphing DMM. Best money I ever spent.
Friday, May 20, 2011 11:38 AM
This is a good artical aboute basic diag i just wanted to add that the test light and DVOM are great tools one thing that i noticed no one said anthing about power probe three that is the best tool ever to do basic diag its good for beginners and provides fast and easy diag you can get one cheeper than a DVOM tooltopia has them for abote 150$ best tool ever
Dusty Duster
Dusty Dusterlink
Friday, May 20, 2011 7:22 PM

Reread the article. I said if you can't tell the brightness of the test light, you can measure the voltage across it, which is an objective measurement.
Saturday, May 21, 2011 6:58 PM
I can't believe your teaching people to use a DMM on a test light... That's just silly. If you know how to use a DMM there is no need for a test light.

Whats the point of hooking up the test light then measuring the voltage across it when you could have just hooked up the DMM to begin with?
Dusty Duster
Dusty Dusterlink
Monday, May 23, 2011 7:15 AM
The point of this article is to make electrical troubleshooing easy for beginners. Now whats easier to explain: using a test light, or taking multiple ambiguous readings with a DMM?

Now go troll someone elses article.
Thursday, May 26, 2011 2:41 AM
I am sensing that I may have offended you... If this is true, please accept my apology. I thought we were simply debating over your choice of how you diagnosed your fuel pump circuit.

Honestly, I think the article is great up until the last page; I think its a good thing what you are trying to do. A lot of people can benefit from learning electrical. But, electrical diagnosis is not an easy concept to grasp (at least it wasn't for me).

Maybe diagnosing a fuel pump is not a good example to use for a beginner level. One of the main reasons I like electrical work is because most of the diagnosing happens in your brain or on a diagram. Hopefully, by the time you get to taking readings with a DMM you know what you are looking at or know what the readings mean; so there is nothing ambiguous about it.
joel flores
joel floreslink
Tuesday, January 21, 2014 2:03 PM
I have an 88 lincoln town car and for some reason the lights turn on and off....dont know what it could be I was thinking of getting the whole car rewired any suggestions of what it might be
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