Know Your Test Lights

Chris Diagnostician New York Posted   Latest   Edited  

Being in tech support, I frequently have people check powers and grounds. More often than not, I hear the telltale beep of a power probe while they are checking fuses and/or essential computer powers and grounds.

Is this a problem? The short answer is yes. When checking powers and grounds, your test equipment needs to put some load on the circuit. A circuit tester (used incorrectly) will show a circuit as good when it may be highly corroded and have only a few strands of usable copper.

While I was in my driveway working on some other stuff, I decided to do some testing and show what I mean.

I did not use anything fancy to do any of this. I ran all my test lights in series with my multi-meter in AMP mode. I then took pics of the readings while current was flowing

One of my go-to lights is this simple marker light socket and bulb that I saved from a used tail light assembly. 

Its a regular old 3156 bulb that I put some male spade connectors on. 

With this, I can use some adapters from my AESwave probe kit, and use it to simulate a load on any circuit. This type of light can be done with any bulb, My H7 bulb seems to have grown legs before doing this testing.

You can see, once lit, this bulb pulls 2.27 amps. This is great for solenoids, relay control wiring, and some computer wiring. (I prefer to use an H7 when trying to stress a computer circuit)

Now we have a standard incandescent test light. While quick for seeing if there is any power at a connector or fuse, it only pulls 51 milliamps. This isn't the greatest thing to be using when trying to verify or condemn a $1000.00 module.

Next, we have one of my personal pet peeves. The LED test light. For the life of me, I don't know why these are made. More-so, these are the standard now on tool trucks. When the apprentice at the shop I was working melted my incandescent light, the owner bought this to replace it. I couldn't even grab a regular light off the truck!! They don't stock them anymore!!! I had to special order one!! Hooked up in series, it only pulls 5 milliamps. This is useless as a circuit checker. At best, it is a pointy inaccurate volt meter.

Now we come to the bane of my existence. The Power Probe. As you can see, I have the Power Probe one. It doesn't have all that fancy BS on it, because I personally don't see the need for it. Well, the Hook has the adjustable voltage at the tip, so that's nice. Other than that, this is good for putting power or ground to something AND THAT'S IT!! If you are checking fuses or powers and grounds, you are using it wrong. Period!

You can see that it only pulls 11 milliamps. You know what else pulls 11 milliamps? An LED bulb! That is not enough to test ANY circuit!!

Hope you enjoyed my rant. Thanks for reading

Please feel free to tell me how wrong I am about the Power Probe in the comments...

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Agree
Justin Technical Support Specialist
California
Justin Default
 

I applaud this break down Martino. I too struggle daily with tech support and on the other end of the phone you hear the identifiable power probe beep. Or I get the I checked continuity in the signal line and it is less than 0.5 ohm. I love loaded circuit testing and on signals I constantly request disconnecting from both ends and jumping power in to do a voltage drop or load substitution to…

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Michael Owner/Technician
Pennsylvania
Michael Default
 

Justin, do you feel the LoadPro is good for load testing all circuits?

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Brandon Diagnostician
Pennsylvania
Brandon Default
 

Great demo Chris! I hope the lesson is well-recieved and understood among the younger viewers. This is a VERY important lesson to learn. Thanks again , “ Mahhhhhh-teee-ohhh”👍

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James Diagnostician
Georgia
James Default
 

The best part of my … probe is the long extension lead that came in the kit . It’s the only thing I use much from the kit

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Matthew Owner/Technician
Georgia
Matthew Default
 

What is also important to note is understanding the intended load on that circuit, especially when talking about circuit protection strategies and how too high a load could introduce faults that don’t truly exist.

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Chris Diagnostician
New York
Chris Default
 

Correct. It is always good to look at a diagram and see what is running the circuit.

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Michael Owner/Technician
Pennsylvania
Michael Default
 

If you're not sure what to use for load testing i.e head lamp, parking light bulb etc in terms of amperage would it be beneficial to remove the fuse for that circuit and install a fuse loop then check the amperage with an amp clamp?

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Sean Technician
Ohio
Sean Default
 

Yes. When I first started doing diagnostics about 8 years ago, got my fancy power price, had a blower motor on a Ford escort. It lit 12v to the motor, new motor still wouldn't work, finally I used a test light, would not even light corrosion in the bulkhead connector, that was when I learned the importance of loaded test.

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Bob Owner/Technician
Massachusetts
Bob Default
 

This issue is probably responsible for a high percentage of incorrect diagnoses and hours of wasted time. That being said, I have several power probes and I use them all the time. I use the features that make sense. Applying power and ground to a circuit, activating components, continuity testing, not to mention having a great extra long battery ground lead. The secret as you mentioned is…

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Brandon Diagnostician
Pennsylvania
Brandon Default
 

Agreed... ALWAYS know the limitions of the tool/test being implemented 👍

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Orlando Technician
Ontario
Orlando Default
 

Great write up. I will champion the power probe tho. It's another tool in your toolbox that has its uses if you know how and when to use it.

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Glen Owner/Technician
Illinois
Glen Default
   

Chris, I have Snap-on test light probably similar to the one you posted, it is a very useful tool. eg. pulling circuits high or low to activate things such as door lock actuators, I can also momentary tap a the RPM signal circuit of say a Ford with a PWM/hydraulic fan clutch and watch the RPM signal change on a scan tool. I rarely use filament bulbs for circuit load testing, to much heat for my…

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Jim Mobile Technician
Pennsylvania
Jim Default
 

GREAT reminder of the basics, Chris. I was waiting for a tech to respond that has attended one of my Sherlock Holmes Diagnostic presentations, I've been known to ask every tech that owns a test light to raise their hand, I then ask them to keep their hand up if they know the EXACT amount of resistance of their test light !!!!!!!!! The most common result is I'm the only one with my hand still up…

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Jim Mobile Technician
Pennsylvania
Jim Default
 

Chris, As I already stated above, This is a GREAT reminder about the possible effect of "Loading a Circuit". The two waveforms I have attached were from a small 4 cylinder Waste/Spark ignition vehicle. When I was called in to diagnose the vehicle, it barely ran, I checked the secondary output on one of the coils and it was within specs of approx. 7 amps, the other coil just about had an output…

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Mike Instructor
California
Mike Default
 

Great post, Chris! We were just discussing using standard test lights for diagnosis in my Electrical Class last Saturday. The old stand by test light is nothing more than a switch with a light bulb in it, in my opinion. I always ask the class: "How many times have you heard something click when you connect the test light to a circuit?" I always tell my students that often it's better to know…

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Martin Instructor
British Columbia
Martin Default
 

Nice write-up and photographic support Chris. This is a topic of importance, that many novice and even experienced technicians may not give a second thought towards. In the last 50 years, I've accumulated around 12 or more test lights of various types for specific duties, along with the logic probes for times when they can serve a useful demonstration purpose. A simple example is for monitoring…

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Robert Engineer
North Carolina
Robert Default
   

Hello Chris, You did an excellent write up! Basic Ohms Law is a concept that the best techs get. Voltage, Current, Resistance. We struggled with this about 12 years ago. People ordering ECUs from us because the "injectors don't fire". Well the problem was not the injectors, or the ECU, but rather the upstream supply. If you used your DVOM you saw battery voltage, wonderful, but when loaded like…

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Rudy Technician
California
Rudy Default
   

So this thing can test for up to a 30A load??

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Robert Engineer
North Carolina
Robert Default
 

Hello Rudy, Yes it can. The magic is the fact that the pulse is only milliseconds and little energy is actually transferred. You can test through a small fuse without any damage. It also protects by not loading low voltage circuits like reference voltages from ECUs. Not only can you check a supply circuit but you can check a ground return. Simply connect the tip to a "known good power point"…

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Rex Technical Support Specialist
Ohio
Rex Default
 

I use this from time to time. 10 ohm 20W ceramic resistor. Especially if I am validating wiring where I suspect corrosion. Measure the voltage, load the circuit with the resistor and measure the AMPs. Was taught about it in a Jasper trans troubleshooting class years ago. Also what are your opinions on the Load Pro leads? I have a set and have been experimenting with them. No conclusions yet of…

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Rex Technical Support Specialist
Ohio
Rex Default
 

Meant to thank Chris at the start of my post. Great write up and a good topic. I keep an analog VOM around. I get confused between high and low impedance on digital meters. ( I get confused on a lot of stuff i.e. .40 vs .357 vs 10mm vs .45ACP etc.) Having been bitten, and having had some techs bitten by "ghost voltage" in the past I find it simple to just confirm with an analog meter or the…

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Geoff Diagnostician
Hawaii
Geoff Default
 

To add one more layer of complexity, there are some circuits that will shut down if the test-light you are using flows too much current. That was covered in a GM TechLink article many years ago, and was the reason GM switched from a 150mA test light to a 50mA test light for their essential tool. I think it was the same antenna circuit Martin mentioned. I use the PowerProbe 2. It has the banana…

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Martin Instructor
British Columbia
Martin Default
   

Exactly Geoff. Thanks for pointing this out. I didn't clarify it in my earlier response. Using our preferred tool to load the circuit, just because we believe that it stresses the control circuit to the desired level, just makes an ASS out of U and ME! We need to know a bit more about the circuit before bringing out the "big guns" to volt drop test a circuit. When GM introduced the…

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Samuel Technician
New Jersey
Samuel Default
 

This is an AWESOME post!!! Basic test lights are not so basic. They can be the core to all testing. I am using this as my go to reminder on basic circuit testing. Many guys love the power probe, I have the Snap on equivalent and NEVER use it. Wasted dollars. A basic test light or a bulb and socket is not only cheaper but the absolute best way to begin circuit testing. However, I am guilty of…

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Angel Technician
Massachusetts
Angel Default
 

Great write up Chris, this thread came up in an ATG intermittent Drivability Class I took last night. In it we talked about many things but the importance of stressing a circuit came up. Great read and great examples. Thanks for sharing

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Thanks