Digital Multimeter Basics, Checking Fuses, Speakers, Power Options

[TUN35]

Ok there seems to be quite a few questions surrounding my amp doesn't seem to be powering up or my headunit. So i thought i'd post up some basics that you can apply yourself to check if you have a DMM (Digital Multimeter) handy.

Sometimes it is hard to tell if a fuse is blown and once again a DMM can come to the rescue. First you must pull the fuse to remove it from the circuit. With the leads still plugged into the same position, turn the dial to ·))) and then connect the red lead to one side of the fuse and the black lead to the other. If the fuse is good then the DMM will emit a beeping noise and if it is bad it will make about as much noise as a dud Tijuana-firecracker.

You can also check to see if you have blown a speaker in the audio system by using the DMM to measure the speaker's impedance, or resistance. The dial should still be set to Ω and the red lead will be connected to the positive terminal of the speaker and the black lead goes to the negative side of the speaker. Most speakers should have a resistance of 4Ω . and the impedance is often stamped on the back of the speaker's magnet. A badly blown speaker will usually read under 1Ω .

One of the most common uses for the DMM is to measure battery voltage. Cars operate on a direct current (DC) system. On our DMM we connected the black test lead to the COM jack and the red lead to the V/W/F jack and set the function dial to V-. The red lead goes to the positive side of the battery or circuit being tested and the black lead goes to the negative post or ground. If the leads are connected backwards the DMM will read the proper number, but there will be a negative sign in front of it. A healthy battery will read around 12.5 volts with the engine off and when the car is running it should read close to 13.8 volts. This can quickly diagnose a bad battery or a faulty charging system. The voltmeter function on a DMM can also be useful when determining if accessories are getting the full power they need to function properly. Accessories like amplifiers or fog lights will still work if they are receiving less than 12 volts, but they will not be as bright or as loud as they could. To see how much voltage the accessory is receiving, connect the red lead to the positive wire running it and the black lead needs to be grounded. If there is a significant voltage drop between the battery and the accessory than the wire might be too small of a gauge or a connection might be corroded.

There we go guys just something that's nice and simple to follow and help you diagnose problems a little more yourself.

This post has been edited by TUN35: Aug 3 2005, 12:40 PM

[MJG]

Ctrl-C + Ctrl-V

Nice work Nedge bro

TRD Rolla

[Smoky60Sleeka]

[TUN35]

Thanks for that mate Was late at night and I was bored so what can i say mistakes were bound to happen!

[PreludeBoi]

You know what, a test light will also help you test to see if a fuse is blown. DMM espically cheap D/smith ones are not that accurate... In fact it cost quite a bit of money to get a decent one.

My (fluke)meters are worth over $2000.... a $35 meter will show you some things but if u drop it or don't take care of the leads, it may be misleading.

Don't forget meters are calibrated and before purchasing one you should check the box to see what standard it's been calibrated to, if u want a half accurate reading.

If your just going to be checking for blown fuses then use a test light, espically if it's a blade fuse, just simply connect your earth and put your test light prob on the top of the fuse where u can see the metal on each side of the fuse... (kinda hard to explain in words)

[CSX 16]

turn the dial to ·))) [right][snapback]4934193[/snapback][/right]

It's called a continuity test. A lot of the time it's the same setting as the diode test you just have to hit the select button to switch between the two.

Don't bag out cheap Digital Multi-Meters, a $30 meter will most of the time give a more accurate voltage reading then a $2000 Analogue Volt Meter. From what I've seen if you get a meter around $60 - $80 it will be very accurate.

This post has been edited by CSX 16: Mar 3 2005, 12:09 PM

[chkn]

mine dont have a ))) thingy... one side is OHMs one side is Volts, you can dial to 2000, 200,20, or 2... lost the instruction manual, i just basically use it as a circuit tester and to check voltages, i would like to use it as more, but i dunno how, i think its a dick smith cheapo one.. but its useful!

[Mcleod]

mine dont have a ))) thingy... one side is OHMs one side is Volts, you can dial to 2000, 200,20, or 2... lost the instruction manual, i just basically use it as a circuit tester and to check voltages, i would like to use it as more, but i dunno how, i think its a dick smith cheapo one.. but its useful!
[right][snapback]5144440[/snapback][/right]

The ))) or the Ohms thing they are referring to :-)

-
Mcleod

[dodgeboy440]

The 4Ω impedance of your speaker. Is not actually a resistance. Impedance is comprised of 2 or 3 components depending on the circuit. In the case of a speaker, one components is the Resistance of your coil windings. This opposes DC. Where the other component, is the inductive Reactance of the windings. This opposes AC (audio signal) and is completly invisible to DC. The inductive Reactance is also proportional to the frequency you put through the inductor (coil). So if you measure the resistance of your speaker with a DMM, all you are going to get is the resistance of the copper windings, as the DMM uses a DC current to check resistance, so as far as it's concerned your measuring a straight piece of copper wire. Even if your speaker is 20Ω impedance, unless the windings are extremely thin, you shouldn't measure more than a couple of ohms. A more accurate method is to supply you speaker with an AC signal of known voltage. And of a known frequency, 1Khz works well. This can be done by making a cd with a constant tone, playing it and measuring the voltage at your speaker. And measure the current the speaker draws. Divide the voltage in rms. By the current draw rms. And you will have the impedance of you speaker.
I hope this helps a few people understand the basic difference between impedance and resistance.

[PreludeBoi]

csx 16 my meters I was refering to wasn't an anolog meter... they are digital meters and one I refer to is a hand scope which is good for checking your wave forms and checking speakers imput while operating. If u have a clean imput but still a crackle in your speaker then it's obviously your speaker.

I don't however normally use my scope for that it's used mainly in checking engine sensors imputs and outputs to help fault find.

[Heated Up]

Thanks mate.... now i know why my lights wernt working... well they were working.... but the interior ones wernt.

~heated Up~

[Vindog]

hey, i got a mutlimeter from supercheap for 10 bucks...(down from 12 of course)...
and its served me fine for everything I've needed it for...
Might not be the most accurate, or it might be, cant exactly compare it with anything else can I...
Long story short is...A 10 dollar multimeter will work just as good as an expensive one for certain things (continueity tester for one)...
Im upgrading very soon though, as Im using it alot more than my first thought, checking more than just my fuses and wiring...

My 2 cents...