How do I use DVM to test for power at the taillight bulb socket ?

[Jacobson]

One of my car's bulbs is out. I want to test for power at the bulb socket.

When I test battery alternator for 14v, I set DVM to DC and 20V range.

When you test for power, I think there is a different method?
I once blew a fuse in the DVM. I think you set it for current, not voltage?

Amazon.com

 

[budmur]

Is that an affiliate link?

With a DVM you'll want to check for voltage, not current. A $10 incandescent test light is the best tool to use, since it'll show that there's enough voltage and current to (you know) actually light a bulb.

 

[4xdog]

...A $10 incandescent test light is the best tool to use, since it'll show that there's enough voltage and current to (you know) actually light a bulb.

This works perfectly to put a load on the circuit. In fact, it won't even take ten bucks to start using it. My brother and I made a 12V test light from some scrap wiring and an old 1156 bulb forty years ago, and I'm still using it.

 

[RPH]

To use that meter to measure current pick the appropriate dc current level. Insert red probe into the left socket if current flow is expected to be 10 amps max. Black lead into common. The two leads are in series with the load but you still need the the bulb for the load.

 

[Bad Eye Bill]

As was said, a test light for the socket. Use your DVM set to ohms to check the bulb.

 

[Jacobson]

Is that an affiliate link?
.

Whoa, that was from a Google search. I removed the tag. Thanks

 

[Jacobson]

I don't have a test light. I want to learn DVM.
fuse for light bulb is 25.

 

[Jacobson]

To use that meter to measure current pick the appropriate dc current level. Insert red probe into the left socket if current flow is expected to be 10 amps max. Black lead into common. The two leads are in series with the load but you still need the the bulb for the load.

How do you use probe with bulb in the way?

 

[RPH]

Bulb needs to be in socket. Bulb is the load. Meter leads need to be in series with the load. Unfortunately you will need to cut wires to use this style meter in the current measuring mode.

 

[wyliesdiesels]

A couple of the previous responses are confusing the OP. The OP needs to test for voltage because the bulb is out. No need to test for current/power. you would only do that if you want to know how much power the bulb draws.

Set meter for appropriate voltage level, connect one probe to center pin in socket connect other to shell or grounded frame.

If you want to test current/power draw, get a clamp meter and wrap it around one wire connected to the socket.

 

[theoldwizard1]

A couple of the previous responses are confusing the OP. The OP needs to test for voltage because the bulb is out. No need to test for current/power.

WRONG ! Voltage without adequate power is useless !

This is why pros use an old fashion 12V test light, not a meter.

 

[Wrench97]

12v test light with a incandescent bulb is the way to go(not one of the new fancy LED types that do not load the circuit), He also needs to make sure the ground side in the socket is good as well.

 

[nadogail]

I am confused as to exactly what the OP wants to determine. Is the OP trying to find out if Voltage is available to illuminate the Tail Light, you then test or voltage. If the OP wants Power, then test for Voltage and Current Flow (generally measured in Amps).

A known good bulb is a good tester.

 

[lpakiz]

As a retired Class 8 (Eighteen Wheeler) mechanic, I chased hundreds of these problems.
Use a taillight bulb and socket from an old junk car, or grab a 1156 bulb and solder on a couple leads. Adding alligator clips will make it easy to use. The poster above is right, you need some load on the circuit to ensure there is adequate current (amps) to accomplish something.
Before testing anything that might be faulty, TEST THE TEST LIGHT FIRST, preferably without changing your ground point. Make sure the test light works!
A good ground point can be attained by clamping a vise grip onto the vehicle frame, (twist it as you clamp) then attach the alligator clip to the vise grip.
If it appears you don’t have power, try an adjacent circuit, to ascertain that your test light works.
If you have a broken wire or faulty switch or bad connection, you dont have a “short circuit”, you have an “open”.
A short is when current finds its way back to ground before it gets to the load. (Hence, “short circuit”). Usually a short will blow a fuse. Then you have an open circuit.

 

[Wrench97]

I am confused as to exactly what the OP wants to determine. Is the OP trying to find out if Voltage is available to illuminate the Tail Light, you then test or voltage. If the OP wants Power, then test for Voltage and Current Flow (generally measured in Amps).

A known good bulb is a good tester.

You can have a voltage reading on a DMM and not be able to light a bulb because of a poor/corroded connection. The DMM puts no load on the circuit thus it can have a voltage reading that disappears as soon as a load is applied.

 

[Sevenhills1952]

Lots of good advice.
DVM you use voltage scale most of the time. Right now I have a motorcycle battery on a charger. Fluke meter reads 13.8 V. If I leave black lead on negative side, hold red meter probe one hand then touch battery positive it still reads that. (11 megohm/volt meter impedance) so test light loads the circuit.
Ohms best to disconnect one battery lead, never check resistance on a live circuit, use lowest ohms scale for continuity check.
Current (amps) must be in series with load.
You hardly ever check current.
It's easier if you compare it to water.
Voltage is electrical pressure.
Current is electrical volume.
Resistance is a restriction (or break) in the pipe.
Battery is like a pressurized container of water.
Alternator is like a pump.
Wires are like pipes.
Spark plug wire carrys high voltage but low current. So it's like a small thick wall pipe (high pressure). Battery to starter is the opposite, large pipe but doesn't need heavy insulation (high volume).
As others said an incandescent test light is easy and loads circuit some. A DVM has it's place, test light is better some things. Put it across battery first to test it. Clip to a good ground, it should light on bulb socket center contact.
Put a good light bulb in (you can test bulb with test light. Bulb on battery ground, test light clipped battery positive should light touched to bulb center contact).
If good bulb doesn't light in socket, be sure ground is good. Test light clipped to a good ground should not light when touched to socket ground.

Sent from my SM-S205DL using Tapatalk

 

[Sevenhills1952]

Lots of good advice.
DVM you use voltage scale most of the time. Right now I have a motorcycle battery on a charger. Fluke meter reads 13.8 V. If I leave black lead on negative side, hold red meter probe one hand then touch battery positive it still reads that. (11 megohm/volt meter impedance) so test light loads the circuit.
Ohms best to disconnect one battery lead, never check resistance on a live circuit, use lowest ohms scale for continuity check.
Current (amps) must be in series with load.
You hardly ever check current.
It's easier if you compare it to water.
Voltage is electrical pressure.
Current is electrical volume.
Resistance is a restriction (or break) in the pipe.
Battery is like a pressurized container of water.
Alternator is like a pump.
Wires are like pipes.
Spark plug wire carrys high voltage but low current. So it's like a small thick wall pipe (high pressure). Battery to starter is the opposite, large pipe but doesn't need heavy insulation (high volume).
As others said an incandescent test light is easy and loads circuit some. A DVM has it's place, test light is better some things. Put it across battery first to test it. Clip to a good ground, it should light on bulb socket center contact.
Put a good light bulb in (you can test bulb with test light. Bulb on battery ground, test light clipped battery positive should light touched to bulb center contact).
If good bulb doesn't light in socket, be sure ground is good. Test light clipped to a good ground should not light when touched to socket ground.

Sent from my SM-S205DL using Tapatalk

 

[lpakiz]

Now, there is nothing wrong with a digital meter!! They are VERY sensitive, which makes them great for a number of things.
One of their most unique but practical uses is to measure “voltage drop” which is the difference between a theoretically perfect circuit versus the actual, real world voltage at the load.
Hook the red lead on the positive post of the battery and the black on some point farther down the line, like at the inbound headlight terminal. With the lights off, (no load) reading will be virtually 0.00 volts. Now turn on the lights. Reading will show, maybe 0.25? volts for a pretty good connection, under load. A poor connection might show 2.50 volts or more, depending on how many feet of wire the current travels thru, poor connections, switches, relays, fuses, corroded, rusty, etc.

The neatest part is you can check as much or as little of the circuit that you want. Hook the red to the inbound of a switch, and the black to the outbound of the same switch, introduce the load, and you can see how much you are losing just thru the switch.
Using the meter on segments of the circuit can reveal just where the problem is, like the switch (above) or a corroded firewall connector, for example.
This also works on the return trip, more commonly called the ground. (Remember, this part of the circuit is exactly as important as the power TO the load).
Hook one meter lead to the negative battery post and the other to a screwdriver. Turn on the load and touch the screwdriver to the brass base of the illuminated tail light bulb. The reading is how much less than perfect the connection really is. If the ground is missing completely (broken ground wire, let’s say) you’ll see something like 13 volts. Pretty good connection will be 0.04 or something like that.
Think of the meter as running parallel to the circuit, like a frontage road compared to the Interstate highway running along side. Either route will get you to the same place, but one is more efficient than the other.

 

[BillK]

The first thing you need to tell us is a vehicle year, make and model. A lot of guys are answering assuming that you have an older car with good old standard light bulbs. What if it is a newer car with LED's ? Or one like my Tahoe which actually has a circuit board at each lamp ?? This question really cannot be answered without more information.

 

[lpakiz]

In the test described above as “continuity” be aware that without load, continuity can show up as PERFECT, no resistance, thru a wire as small as a mosquitos whisker. But that small conductor won’t carry enough current to light up his flashlight.
Again, we need a load.
And Bill is spot on—these are very basic circuit testing concepts and examples I’m using. I was thinking I might be helping someone get his boat trailer lights diagnosed and fixed, rather than fixing LEDS (which draw a fraction of what incandescent bulbs draw) or other specialized items.
Sorry if I ”assumed” too much here.

 

[theoldwizard1]

As a retired Class 8 (Eighteen Wheeler) mechanic, I chased hundreds of these problems.
Before testing anything that might be faulty, TEST THE TEST LIGHT FIRST, preferably without changing your ground point. Make sure the test light works!
A good ground point can be attained by clamping a vise grip onto the vehicle frame, (twist it as you clamp) then attach the alligator clip to the vise grip.

GREAT ADVICE !

Part of the Power Probe text kit is a very long set of dual leads. On end has large alligator clips to connect the the battery. The other has female banana jacks. Now when you are measuring "sensitive" voltages (like with an oscilloscope), not light bulb voltage, you have a TRUE ground and B+ right next to you !

 

[ddawg16]

Hmmm......This is almost as bad as politics.

First off.....what vintage of vehicle are we talking about? Many of the newer cars use CAN to control ****. The only real way to know for sure is to have the bulb in place while measuring voltage.

For brake lights, in most cases the bulb has 12v on one side....and it's turned on and off with the ground. When you push on the brake, it opens a NO switch that goes to closed....and applies a ground to the light.

LED lights add another issue....low current.

So....tests really need to be made with a known working bulb in place.

 

[Terry D]

I don't understand what the OP is asking here. Did you try a new bulb and it still did not work, or are you just wanting to check voltage before you try a new one. Just put a new bulb in it and see if it works and go from there. 99% its a bad bulb. Or are you just wanting to get more familiar with using a DVM

 

[Milton Shaw]

If this is a tail light or front turn signal light the socket has 2 pins. one for the running light and one for the turn/brake light. All the lights go through the turn signal switch and emergency flasher switch on just about all American cars. A DVM would test either circuit for voltage to ground. Also check the ground on the socket if you have voltage but light still doesn't work or is dim. Hope this helps.

 

[Wrench97]

If this is a tail light or front turn signal light the socket has 2 pins. one for the running light and one for the turn/brake light. All the lights go through the turn signal switch and emergency flasher switch on just about all American cars. A DVM would test either circuit for voltage to ground. Also check the ground on the socket if you have voltage but light still doesn't work or is dim. Hope this helps.

Not any more, a lot of them are now controlled by a module and relays, some actually use multiple single filament bulbs(think the ones with yellow turn signals) and the dual filament bulbs have 3 contacts not 2(Ground, tail and turn/brake ).