rookies

Do Not apply a voltage to anything, test the subject cable with a DMM to see what it is doing. No what that wire should be doing, is it neg going, is it pos going? Etc Etc. Do not use power probes. If you must (and only the wires you suspect should be ground going, use a snap-on bulb tester, this will act as a buffer, never use this on a suspected Can or similar data wire. Regards and good luck.

As you pointed out, there's a time and place for everything.. I'm not sure why you think it's odd that ASE mentions test lights; they are a very handy tool.. much better than DMMs in some respects.

Take a 12 battery and a 100 ohm resistor (You can probably use a larger resistor also). Attach the resistor to the positive terminal of the battery. Use a DMM and measure voltage between the free end of the resistor and the negative terminal of the battery. Using an incandescent test light (NOT led), hook the clamp to the negative terminal and touch the free end of the resistor.. does the test light come on? If you've still got the meter hooked up, what happened to the voltage?

So, the meter measures 12v, but the test light doesn't come on.. explain why.

Application - lighting wiring.. just got a call on this last week. Bad ground between the rear of the truck and the negative terminal of the battery.

Jim

Mad Scientists wrote:

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Take a 12 battery and a 100 ohm resistor (You can probably use a larger resistor also). Attach the resistor to the positive terminal of the battery. Use a DMM and measure voltage between the free end of the resistor and the negative terminal of the battery. Using an incandescent test light (NOT led), hook the clamp to the negative terminal and touch the free end of the resistor.. does the test light come on? If you've still got the meter hooked up, what happened to the voltage?

So, the meter measures 12v, but the test light doesn't come on.. explain why.

Jim

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If you put a resister in series with a test light, the resistor will drop most of the voltage across it. Why? Because the resistance across the bulb is much lower than the resistor. Ohms law. And it doesnt matter if the resistor is before or after the test light, the result will be the same. The resistor will drop a majority of the voltage. There will not be enough left to light the bulb.

Lets look at whats going on. A 12-volt circuit with a 100 ohm resistor will have 0.12 amps of current going through it. Even less current if a test light is in circuit. Light bulbs are current hungry and in most cases it won't light at 0.12 amps. Becuase the resistor is dropping a great majority of the voltage.

Now lets move to 10 Ohms. Current through that circuit is only 1.2 amps. If we assume the bulb has 2 ohms resistance then we have only 1 amp of current. That means the resistor is dropping 10 Volts and there are only 2 volts left to light a 12 volt bulb. It won't light.

Basic Ohms law and it dont lie

You are correct Ranger.. now take the test light out and use the DMM.. do you see that it's going to measure 12v on the resistor? But it's not 12v when you try to load the circuit. Test lights load the circuit, meter's don't.

fkelsey - lighting, injector wiring, HVAC and radiator fan, electric antenna, power windows, door lock actuators, and anywhere motors and powered actuators are used is a place to use a test light.. but, as was noted, you'd better know what you're doing. More than one airbag has been deployed by backprobing the airbag harness with a powered test light.

Jim

Mad Scientists wrote:

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You are correct Ranger.. now take the test light out and use the DMM.. do you see that it's going to measure 12v on the resistor? But it's not 12v when you try to load the circuit. Test lights load the circuit, meter's don't.

Jim

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Do you mean measuring a resistor in an open circuit? If so it doesnt matter what value the resistor is. If you apply 12 volts to one side you will read 12 volts on the other side, regardless of value. We need to complete the circuit inorder to have a voltage drop.

Test lights load circuits because of the low resistance of the bulb. Look at Ohms law. Current and Resistance are Inversely Proportional. For a given voltage, as resistance gets lower, current goes higher. Thats what makes test lights in modern cars such a bad idea

ranger svo wrote:

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Mad Scientists wrote:

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You are correct Ranger.. now take the test light out and use the DMM.. do you see that it's going to measure 12v on the resistor? But it's not 12v when you try to load the circuit. Test lights load the circuit, meter's don't.

Jim

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Do you mean measuring a resistor in an open circuit? If so it doesnt matter what value the resistor is. If you apply 12 volts to one side you will read 12 volts on the other side, regardless of value. We need to complete the circuit inorder to have a voltage drop.

Test lights load circuits because of the low resistance of the bulb. Look at Ohms law. Current and Resistance are Inversely Proportional. For a given voltage, as resistance gets lower, current goes higher. Thats what makes test lights in modern cars such a bad idea

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I've heard of Ohm's law in the past.. and I agree with every you say, up to that test lights are bad ideas in modern cars. Sure, for some things they are, like data lines, but for situations where you're checking wiring that carries current, it's better to use something that'll load the circuit. For low current stuff, use an LED testlight. For higher current, a test light works better than a meter when checking for power (if you know what you're doing). Use a measuring device that most closely replicates how the circuit normally works.

Occasionally, I'll see individuals here direct people to measure resistance on the ground path back to the battery. On a wire that might be carrying 200A or more, what would be considered a bad resistance, and how would you measure it?.. Short answer, you can't directly measure resistance to see if it's too much or not.. but you can measure voltage drops.

use the right tool for the job.

Jim  

I have, and use a power probe....only where, and when, it can be done with certainty! You cannot put a tool like this into the hands of a "rookie" (I.M.O.)

It's a good to when used for properly. It's NOT the best, nor is it used the most....but, it's nice to have.

I only started this thread because reading through the queries and questions it seems a lot of people mainly rookies know nothing about testing. Remember I'm an old non PC mustard who started in this game when Ungo were the only US alarm maker and the Blaupunkt Berlin was the Bees Knees! Yes I installed 8-tracks and remember valve radios!

Nearly all the replies to this thread have been very helpfull and wise, the point is I tend to use a test light first, on circuits I'm sure are current draws such as motor wires etc., second a RED / green high impedance LED device and thirdly a DMM. BUT we are talking 30 years experience in my case and I usually know what to look for. NEVER do anything until you've tested the car first, then the cable you suspect, KNOW what that cable is doing i.e. does it sit on ground or pos or open circuit does it go neg or pos etc. Then test the car! Then box everything up.

Thank you all for your highly professional comments HOWARD.

I probably shouldnt bring this up, but I work with an "experienced installer" (I would not let him within 50 ft of my car). He was working on an 06 Dodge with an Infinity system. He was looking for the remote turn on wire with 12 volts (not a test light, but a stupid idea). Applying 12 volts to the data wire (he thought it was the remote turn on wire) that turns on the amp was a bad idea. Amp no longer works.

Some people should not be anywhere near a modern car. I am also a dinosaur. I installed a lot of older Pioneer. Remember the component stuff. But I do try to keep up with whats going on. I almost never use a test light. Better safe than sorry.

Thats my 10-cents worth

I agree upon using a multi meter for nearly everything. sure a test light is extremely convenient, but it draws current and can cause problems if hooked to the wrong wire. it's a matter of fact that testing wires using a multi meter will cause less or no problems compared to a test light, so it's better for those who aren't pros. I did a grand am a while ago it I swear the entire car had only like 4 colors of wires. every bundle had like five wires of the same gauge and color. tap into that with a test light? hell no, it's got data wires to open the doors. now on my honda there were very few duplicate wires, so I was pretty sure what wires were what when there's only one burgundy/fuschia in the bundle.

we wired up a push button in my buddie's doors, he had solenoids already in there to pop the latch, but the buttons were under his seat or via remote, which was less convenient than on the doors themselves. so we cut a wire leading to the door to use, it was for factory power windows which he doesn't have. passenger door went no prob, would have been a bitch to go through the 2 sets of plastic plugs (gotta take the door off) and rubber boot to get to the door, so we cut one unused factory wire. the driver door however, we kinda rushed. it was getting late, there was an unused blue wire in the door, and it had continuity to a blue wire in the car, but to the side of the wire leading to the back of the car. so we hooked it up to that wire and it worked great. a few days later he calls me and tells me when he changes his stereo from cd to radio, his door pops open on the highway. every time. so we tap the wire into another blue wire in the same bundle (didn't see it the first time!) and it works properly this time. after the fact he mentions that his power antenna wasn't working earlier either! I was just glad we didn't damage anything, I'm sure the radio wasn't expecting to get a ground on its power antenna wire.

only after the wire has been verified to the best of my ability (wire matches diagram, voltage readings are as expected) would I apply voltage to check the circuit such as door unlock and such. cigarette plugs make good power sources, multi meter leads can plug into the factory plug most times.

again everybody has their own ways of doing things, in fact I bet the only thing everyone can agree on is nobody wants to fry electronics and have to replace them. if you want to be the safest and not blow up peoples' electronics, disconnect the negative battery terminal, use a multi meter, and leave your fuses out till they're needed. I also leave the horn/siren for last so I don't annoy all the neighbors.