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

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

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