Get the meter out? Yes, absolutely, and I'd agree with 90% of that. But ask any HGV or heavy diesel mechanic working on 24V systems.

When you get a call at 2am from a driver saying the trailer lights aren't working, and it's absolutely pissing down with rain, I'll grab a test light first to check for output at the trailer socket. I'm testing a lighting circuit, so a test light is often the quickest and most practical tool for the job.

I've got four test lights, all with different current draws. They're mainly used for testing ABS/EBS power supplies to trailer modules and for loading circuits while fault-finding.

There's also the LoadPro, which works in conjunction with a multimeter and is an excellent diagnostic tool.

Dan Sullivan's Fundamentals of Electrical Troubleshooting is an excellent book.

Thats why I have a selection of home made test lights and combined with the multimeter I try to choose the same amount of current draw test light as the load on the circuit. For 5v power supply only led test light which is only 10-20 milliamp

It's about time a vehicle orientated multimeter incorporated a selection of loads that can be switched in.

The circuit test approach depends on what we are testing.

OEM, Dealer and Electronic Module supplier engineers are DMM centric and don't use bulb testers where the light or display is in the handle. They will use an LED light to act as a visual indicator to show an intermittent short, but I regard this as a special case, and what you will find is the engineering team has created their own custom low current LED indicator and also a low current audible buzzer.

For vehicle level diagnostics key on engine off the following steps are generally used:

1) A low electrical noise, high current (75 Amp), adjustable protected DC regulated power supply is used to power the vehicle and put a slight charge on the battery, like 13.5 VDC measured at the power distribution box (usually after the large bolted +12V main power fuse or Maxi-fuse depending).

2) We have six low inductance power resistors on heat sinks (the size dependent on the power dissipation) wrapped is a large diameter heat shrink tubing open at the ends to allow heat to escape having silicone test leads terminating in stacking shrouded banana jacks, of Ohmic values of: 3.0, 4.7, 12.5, 47.0, 100.0, and 470.0.

3) Select the DMM = Fluke 87V. For circuit testing a test extension (+12 V Red, GND Black) is used to connect power and ground at our reference voltage origin.

Testing examples:

In all cases a dummy load of a known resistance will be placed across the DMM Volts input jacks. So, the DMM and the dummy load are in parallel.

1) Faulty electronic module: Module +12 V power and power ground (battery ground) circuit integrity with the module disconnected. We want to draw sufficient current to verify the wiring is low impedance but not too high where we would blow a fuse or overload a wire. We select the 4.7 Ohm resistor which from 13.5 VDC to ground will draw 2.87 Amps of current. With the DMM on DCV the load resistor is placed across the meter input (they have stacking shrouded banana jacks) and to this connect the needed meter test probes. What we have done is essentially just reduced the DMMs input resistance.

For the module +12 V power feed, low side dummy load, if the DMM reads a voltage of 13.2 VDC to test ground we know that the dummy load is drawing 2.8 Amps (Ohms law), and by extension confident that the module +12 V power feed wire and fuse are intact and low impedance.

Then the module grounds are tested in the same way with the load/DMM on the high side.

2) Suspected high resistance in bolt down high current fuse feed. With the DMM on DCV, use the 3.0 Ohm dummy load across the DMM on the low side, and test both sides of the fuse. At a reference voltage of 13.5 VDC we should read 13.5 VDC at the fuse input which will draw 4.5 Amps, then if we measure 0.5 Volt at the fuse stud or output bus bar, we know that the connection is high resistance. If we measure 13.5 Volts at the fuse output we know the fuse is okay.

3) Suspect PCM 5 VREF corroded connector pin, at the PCM or an intermediate junction connector. With the DMM on VDC, without any load connected across the DMM, test the sensor VREF at the TPS for a reading of 4.9 Volts which is a little low. This tells us that the PCM should be operating and on-line. Then connect a 100 Ohm dummy load across the DMM and measure again. Let's say it reads 4.2 Volts. At 5 Volts the 100 Ohm resistor will draw 50 mA, at 4.2 Volts, 42 mA. This tells us that in the VREF feed we have wire or pin corrosion creating unacceptably high source impedance.

4) Electronic module off line due to blown fuse. Using a fuse breakout adaptor we gain access to the fuse input and output, and add a fuse saver tester at a matching current to see if the short is present. Upon key on engine off the fuse saver trips indicating there is a power short to ground. In place of the fuse saver we use the DMM on DCV with a 12.5 Ohm dummy load across the inputs and connect across the fuse. At 13.5 VDC reference voltage with a short capable of 1 Amp, the DMM will read around 13.5 VDC, we know that the power is shorted so we then add our LED indicator across the dummy turning on which gives us a visual indicator as we do a harness wiggle test. When the light flickers (LEDs are much faster than an incandescent light) we know we have found the location of the short. If we cant see the light, the buzzer is used instead (which is also very fast) and we wiggle until the sound interrupts.

With a known test reference voltage, a known low inductance resistive load value, and the voltage across the load, we know much more about the circuit than an incandescent light bulb indicator or load.

My test lights are 1157 and a head light bulb and a132 . Banana leads on them so I can run them through the DVOM . Works great

In defence of the humble test light it is a very handy test instrument when used the correct way and correct style for the circuit under test.

Umm who uses a test light to test low load resistance

I'm getting so many mixed messages about test lamps, diagnose Dan loves them...

I think Daren is correct the first choice should be to switch the circuit on and back probe for voltage drop circuit connected and if you find a fault keep it all connected and work back though each connection until you have no volt drop then zoom in on the fault. But if you have no voltage drop at the component it could be there is more than one problem you will usually see the main lives are ok but for example the wake up live is not waking up the module so it is not drawing enough current to fully load the high amp lives and grounds. Then you need to know the circuit you are testing and the wires current carrying limits and load it accordingly to test correctly before condemning a component or module. I think this is what he is getting at and a test lamps will most likely not have enough load to test the circuit fully.

Test lights don't fail at diagnosis. They are a tool and like every tool they have to be used in the right situations by a technicians that knows when to use them and what their shortcomings are.

For Incandescent test lights use for
- quickly checking fuses
- checking power and ground to low power devices (solenoids, relays, etc)
- checking for continuity (Example test light to battery positive touching on pin 87 in the relay box of the fuel pump, if the light lights you know there is at least continuity)

For LED test lights
- check for short to ground (Test light to battery positive, component or harness disconnected, if the light lights there is a short to ground)
- Check fuel injectors for power, ground side switch and continuity (test light to ground back probe negative side of fuel injector light blinks when cranking means injector is getting power, not electrically open, and is getting a pulse signal from the PCM)

In short, test lights are an inexpensive tool to quickly find common faults. Every technician should have them and know how to use them.

Good work 👍

I'm not  super confident on technical systems but I always sort the problems out......in the end ! Power probe for a quick check + powering components and the load pro for everything else. This is a great set of tests, thanks.

😊 감사합니다^

There are many ways to get if right. Just remember a comment from the trainer. If you get a job in on Monday and you got it aced by Thursday that's a hobby. Some jobs require different tools especially if you want go get through 10 or 20 a day.

well a meter can easily lead you astray as well.  a modern meter with 20k input impedence will show voltage while only passin .0006 amps. so you may have voltage thats unable to deliver the amps.  to understand the whole idea you must see and understand all the connections (pun intended).  knowing your lights amp draw and understanding the circuit under test will lead to success.  not knowing can get costly.

Great video. I think for an experienced tech who understands how electrical circuits work, a test light is still a very useful tool. However, for a beginner, they aren't the best choice and could easily lead them down the garden path.

Seems like test lights are really only good for checking ground points.

Power Probe Maestro is all you need

@10:53 Those are absolutely worthless. I have seen where a blown fuse still has voltage on both sides due to feedback caused by a shorted wire.

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