When checking fuses ...

When checking for blown fuses under the dash panels, side panels, and under hood.... Is using a test light safe not to cause damage to the cars sensitive equipment such as BCM .. or other .... about how many amps does a test light uses if anyone might know.

what does BCM stands for by the way ?

Think if it like this would you add a resistor to a wire you had no idea what it does ,or maybe what if if it was on an ecu(car computer)?? Probably not, i know i would not, well a test light adds a lot of resistance to a circuit compared to a dmm wich is a verry small resistance,now ifyou were testing a sensitive piece of equipment that cannot take the added draw then you will hurt something with a test light, so being that a meter adds a lot less resitance  there is less chance if any to harm anything. Bottom line use a dmm because test lights are no good in newer computerized vehicles and you cant do anything more than test voltage with a test light you can do a lot with a dmm.

Thanks guys, that makes sense.

Depends on the test light. The Snap-on test light I use only draws 40ma from a +12 volt source which is the same as 3 leds drawing current or from a 5 volt source would draw about 17ma which is the same as 1 led current draw.

Now i have a friend who HAD a test light from a auto parts store and i showed him the difference from a Snap-on one from his. He argued with me saying test lights are all the same until i showed him how much more his test light would draw current compared to the Snap-on one. Well his test light had drawn way over 1amp. Now he uses a LED test light and also bought a Snap-on comparable one.

OK, so if you were working on a ford vehicle, lets say a 94ish Explorer. There are about 5 different yellow wires for 12v in the ingnition column. How would you test with a test light to find which one was correct. All of the wires with test voltage but sometimes only 2 of them will read a true 12v. This is why it is very important to use a DMM. "We always say look twice, drill once" we should take the same care with wires and circuits. It's worth it to do it right the first time.

Brent, Car Audio Manager, Generation Sound

I think kparkk was asking about testing for fuses and not testing wires in ignition column with a test light.

 go2pac,

but aren't some of the wires in ignition column connected to some of these fuses?  If not ... then do you mean thats its ok to use a test light?

After reading what other replied .. it made sense.. now I am confused again.

Thanks for your clarification in advance.

go2pac,

but isn't some of the wires under the ignition column connected to fuses that I might be testing?  If not, then u think its ok to use a test light on fuses?

I understood what other guys have replied .. but you got me confused again

thanks in advance for your clarification.

Same here george. I am old school as well and I just use a test light most of the time.  I only use a DVM when I am not 100% positive.  Most of the time I don't use anything at all :)  But that comes from experience.  After a couple thousand remote starts you get to know the ins and outs.

As a word of caution to you snowman, that test light from snap on (plugs into cig lighter and has red and green leds) is not airbag/computer safe.  It is handy because it shows + or - easily but do not probe unknown territories with it.

Hardly ever use it anyway.. Can't believe I paid so much for something I use about 3 or 4 times...... a year...... But like you said, years of experience makes up for a lot when if comes to wiring a remote starter, alarm.. hell, even some of these radios from Gm.. they are making it harder and harder to make a living..

Remember when a deck and four with antenna in a honda was around a 28 minute install... the12volt, wanna chime in on this one! LOL

kparkk, if you are testing the fuses in the fuse box, then using a test light is ok.

Never test wires or circuits unless you are like Snoman who probably been around awhile and can say what he said about test lights. I'm sure all old school installers will say the same thing as snoman. You can't change an old school installer to stop using a test light, just won't happen. But of course he will know when not to use it.

I highly recommend the Snap-on test light over Matco or MAC. You can also use a LED test light which the tool trucks carry. They have a higher impeadance than incadecent test lights but they also can activate modules. If you really want to be safe, then get a logic probe that can use a supply voltage from 5volts up to about 15 volts. Logic probes are designed for testing computers.

Test lights have there place and LED test lights or DMM have there places also. For instance a +12volt wire that is pulled high through maybe a 10k ohm resistor or if a wire is severed somewhere and is barely touching each other will show 12 volts on a DMM or even light up an LED test light. Try using that wire for a switched or constant source, it just won't work. A test light however will indicated if its a high current source by the glow of the test light. The bad thing about test lights is that you can set off airbags, turn on a check engine light or activate an unwanted relay because it does not have a high enough impeadance. A DMM can tell you if you have a 12volt wire or a 11volt wire, a test light won't be able to tell you that. An LED test light can tell you if the wire is an open circuit, grounded or a positive circuit. A test light can activate a relay or module which is good for finding a doorlock circuit, parking/head lights or horn very quickly.

If you are new to installing and know minimal about how car electronics work, I won't recommend testing wires with a test light. Your best friend is a DMM or LED testlight.

Part of the reason people recommend DMM is they know that you don't or others don't have the experience and know that a test light is just not for you, period. The other reason is that most installers started off with a DMM because they were new to the industry and were told to only use a DMM. They have not used one and do not know the benefits of a test light and have heard horror stories of airbags going off.

Remember, i am not saying a test light, LED test light or DMM is better than the other, but telling what the good and bads things about them.

lol

Well Put Karl!

go2pac,

I really appreciated the time you spent on that response.  It was a very good explanation.

what I would like to know now is:

If I am checking for the wire that flashes the parking lights, and know the color wire as provided by other source (you know that always must check the wire before using to make sure its the right one).  So if I want to use a DMM, can you explain step by step how to acheive so? and could the parking lights be test flashed with the DMM?

Once again go2pac thanks much ... or more appropriate thanks all

Well since majority of parking light outputs of alarms are positve, you will need to set you meter to voltage and if its a manual one, you need to select the next highest voltage selector above 12volts which is usually 20 volt range. If the meter is auto then you don't have to do anything. The black test lead needs to be grounded and then test the wire with the red lead. With the parking light off, you should have zero volts and when the parking light on you should have 12volts or close to battery voltage.

Now if you don't have the wire color and are trying to find the parking light wire on your own, what ever wire you test, you must adjust the dimmer to be sure you don't have the dimmer wire instead. If the volage varies when you adjust the dimmer, that is not the parking light wire.

Lots of vehicles now have a relay for the parking light so if the alarm has a negative output then you can use the factory relay. Connect your meter's red lead to a constant 12volt source and test the wires with the black lead. When the parking light is off, you will have zero volts and when the parking light is on, you will have +12volts. This set up does not require testing the dimmer.

Also on some European  vehicles there will be two parking light wires, one for the left side and another for the right side. You will need to use a special relay which i don't know the model # , but i always called it the metal green slash relay or you will need to diode isolate the left and right side.