Auto Electical Articles #1

[N*A*M]

DISCLAIMER: outerlimits4x4.com accepts no liability for any damages incurred due to any actions taken on the basis of the information provided in this article.

i.e. If you destroy your electrical system while trying to diagnose a suspect alternator, it's not our fault - do so at your own risk.

Electrical Short

Ever have an electrical short that was so bad that it blew fuses instantly? Here’s a way to diagnose that short circuit without emptying your wallet buying fuses. Some shorts may not only be in wiring, but in components. when a fuse blows it is because the load is bypassed and moved to the fuse.

Take a old headlight and two headlight electrical connectors and put a set of alligator clips on one connector (for round glass fuses) and a set of spade connectors on the other plug (for newer blade style fuses.)

Place the wired headlight across the blown fuse and it will glow at full brightness. Start disconnecting components on that circuit until the light either dims or goes out. When it does, it will tell you whether the
component or wire is bad.

[N*A*M]

JUMP-STARTING YOUR BATTERY THE SAFE WAY

Once you have made arrangements with a volunteer get the cars as close as necessary to comfortably connect them together with the cables. Open both hoods and locate the batteries. If it is dark get a flashlight so that you can make a definite identification of the battery terminals. The identification of the polarity of the terminals is critical - don't guess else you can do SERIOUS damage to yourself and/or the car's electrical system. Batteries all have one characteristic in common. They all have one positive and one negative terminal. This is where the car's internal cables connect to the battery. On older cars you may find that the terminals are on top of the battery, however most newer cars have their terminals on the side of the battery, making them nearly impossible to see, much more difficult to clamp an alligator clip in place.

OK, so you have the hoods open and good lighting available. What now. Look for one or both of the following identification marks near each of the terminals; find either a plus (+) sign or a minus (-) sign. Or, look for a red or black marking on the battery cables or terminations. Red is positive and black is negative. This is a universally common standard in the automotive industry.

A note here about one of the dangers concerning what you are about to do. Batteries are charged by the alternator on your engine. When they are charged they internally generate both hydrogen and oxygen gas from the electrolysis of the water - remember your high school chemistry class, H2O? Well, if you make a spark in the vicinity of the battery the re is a pretty good chance that you will ignite this highly combustible mixture with a resulting explosion and very rapid distribution of battery acid!!! Bad Stuff!!

Also be aware that even if you do connect the batteries together correctly there is a slight chance that one of the batteries could explode due to an internal short circuit in either battery. Never place your head directly over the batteries during these procedures!!

So, what's next? We've need to connect the good and bad batteries together - positive to positive and negative to negative terminals. But there is a hitch. You see when batteries are charged they generate both hydrogen and oxygen gasses internally. The combination is highly explosive, so we don't want to risk touching off an explosion by making sparks in the vicinity of the batteries. The secret is to make the connection between the cars somewhere else besides the batteries.

Virtually every car made today has its negative terminal connected to the engine. The positive battery cable is connected to a terminal on the starter solenoid. So, when we connect the batteries we first connect the red jumper cable to the positive terminal of both batteries. Be extremely careful that you don't accidently connect the positive cable to the battery and then let the alligator clip hit some nearby metal - it will make a large spark and may cause an explosion. Then we connect the black jumper cable to the negative terminal on the good battery. Next, find a heavy bracket or other metal part of the engine block on the other car and connect the black cable to it. This will probably cause a spark but if there is a spark it isn't close to the battery so there is no danger of an explosion.

With the batteries connected we now start the good car's engine. Make sure that the jumper cables are not interfering with the fan belts or the serpentine belts and pulleys - dress them neatly over the fender of both cars and be careful of not tripping over them especially in the dark. I usually leave the batteries connected with the engine running for a few minutes to charge the dead battery. Now start the car with the dead battery. If it doesn't start right away make certain that the jumper cable connections are tight and that the jumper's alligator clips are on clean, rust free surfaces. Wiggle the clips and try it again.

Long jumper cables, especially the cheaper ones, can't carry the 300 amps or more required to jumpstart a car so letting the good engine run at a fast idle for five or more minutes will charge the dead battery and take some of the load from the jumper cables when you try to start the car with the dead battery.

Once the car has started run it at fast idle for a few minutes. Assuming that its charging system is up to snuff, this will charge the battery sufficiently to restart the car after the next step. To make sure that you aren't injured I recommend that you now turn off both engines so that you can safely remove the cables without getting tangled in the moving parts like fan blades and belts. Remove the cables in the reverse order that you used to hook them up, taking the ground cable clamp from the engine metal first. After removing the cables immediately re-start the car. If you feel comfortable removing the cables with the engine running have a care about loose clothing and fingers and keep the cables away from moving parts.

Next step is to close the hoods, profusely thank the volunteer who helped you and go home. If the battery had been run down completely or if the weather is below freezing it is a good idea to charge the battery overnight. If you don't have a battery charger you can purchase one at an auto supply store for around twenty five bucks. Connect it using the color codes discussed previously. Most chargers can be left connected indefinitely since they are voltage regulated and won't overcharge your battery.

[N*A*M]

Diagnosing starting problems

You get into the car and try to start it. It cranks over but sounds like it is cranking slowly. After a few seconds of cranking it still turns over, but slowly. Before you go out and buy a new battery, read this. Maybe you will eventually buy a new battery, but you will have diagnosed the problem and will not be wasting your hard earned cash...

The things we are going to check out here are a bad battery, corroded cables or a bad starter motor. A bad battery will be detected by a low voltage at the battery terminals (not the cables) while cranking. Corroded cable connections will be detected by low voltage at the cable ends. A bad starter motor will draw a high current and the battery cables will get warm to the touch.

You have to get inside the ends of these new fangled side terminal battery cable ends to make sure they are free of corrosion. Just brushing them with a wire brush is not sufficient!! Believe me!! Remove the battery cables from the battery and clean them thoroughly by dipping the ends in a paper cup full of water with two tablespoons of bicarbonate of soda mixed in. Brush the battery terminals and cable ends with a wire brush and keep dipping them in the cup until all the green corrosion is gone. Rinse the cable ends with clean water and reassemble them, positive first.

If you want to do a little bit of diagnostics either before or after cleaning the cables (assuming that cleaning them didn't fix the problem) then do the following. The first thing you have to do is to make sure that the battery is fully charged. Put a charger on it for at least three hours. Now, get yourself a voltmeter and start measuring voltages. First across the battery terminals. Should be a good solid 12 volts from your battery, 14.5 if you have the jumpers connected to a running car. A battery that is three or four years old will probably cause you problems, either now or some night when it's 5 degrees!! Replace it!

Now try to start it. While it's cranking measure the voltages across the battery terminals. It should be somewhere around 12 volts or just below. If it is much lower than that, say 10 volts, then the battery is bad and should be replaced. If it is around 12 volts then leave the negative voltmeter lead on the battery and probe the starter. The voltage there should be 12 volts or so, not much lower. If it still hasn't started and the battery runs down again then feel the cables to the starter - are they hot? If so then the starter motor may be shot and is drawing too much current. If you can get a clamp-on ammeter then use it to measure the current draw of the starter motor. 300 amps is typical - 400 means that the armature is dragging on the stator and drawing too much current.

[N*A*M]

CHECKING OUT A STARTER MOTOR AND SOLENOID

Starter motor problems usually are indicated by the following symptom: Turn the key to the START position and you hear a loud click, or sometimes you hear nothing. The headlights are bright and don't dim when you turn the key to START, and everything else electrical seems to work fine. It could be a bad starter neutral switch or a bad key switch but about 99% of the time it's a bad starter or starter solenoid.

Here is the procedure for checking out a starter motor and its solenoid.

Problems in a starter motor normally involve a "bad spot" on the commutator, the electrical section of the armature that contacts the brushes. They get dirty and worn down. The brushes sometimes wear out but not normally. Open circuits can occur in the armature or in stator windings. You could fix these problems but the normal procedure is to replace the starter with a rebuilt. If the starter motor armature just happens to stop on a "bad spot" the circuit is open and the starter won't turn. Sometimes you can "rock" the engine by hand (be careful - make sure the ignition switch is off) or in a standard transmission car you can put it in gear and "rock" the car by pushing it forward or backward a few inches - this can move the starter motor off the "bad spot" and get you on your way, but it's a crap shoot as to when it will happen again. Sometimes rapping the starter with a hammer can make temporary contact where the contact was flaky, but you can do more harm to the starter than good if ya hit it too hard!!

You can diagnose the starter by measuring the current draw. You can purchase a small "clamp on" ammeter that you simply lay on the cable to the starter - ya don't have to disconnect anything. Crank it and watch the little needle tell you what the current is. If there is a high current draw then you know that the starter is at fault. There is one main reason for a starter to fail when it is hot - worn bearings, especially in the tailshaft. The heat generated in the starter by the engine and the exhaust pipes (sometimes) causes the armature to expand. If the bearings are worn then the armature drags (actually contacts) on the stator causing a short circuit and a high friction drag. Sometimes just replacing the bearings can fix the problem.

When checking out a starter motor it is a good idea to remove it from the car and lock it firmly in a vice. If you don't hold it down securely, like in the jaws of a vice, and it turns out to be good, it will twist rather violently when it spins and possibly fall off the bench onto your big toe - could really ruin your weekend. You can do the following test with the starter in the car but it makes it a bit more difficult and there is a chance of shorting out the test jumper cables to ground. Referring to the diagram, the "big terminal" on the starter solenoid is where the battery + cable goes. There are one or two smaller terminals on the solenoid, one going to the "start" position on the ignition switch.

On a bench test, the negative battery jumper cable goes to the vice that is holding the starter by the frame. The Positive goes to the "big terminal" on the solenoid. Jump from the big terminal to one of the smaller ones with a jumper wire or a screwdriver blade to actuate the solenoid. It should click and the starter should whirrrrr. If it does, don't get carried away and let it spin freely for a long time - it's not good to run a starter with no load for extended periods of time, especially an old and tired one.

If it doesn't spin, look for another "big" wire going into the starter. On GMs you can usually see it at the other end of the solenoid - it goes into the body of the starter. I'm not sure about other makes and models. Look around. Carefully touch the + jumper cable to it and the starter should immediately whirrrr and you should get a good sized spark - that is normal - the starter is a heavy current eater.

If you get no whir from that test then the starter motor is fried inside. You can take it apart and see if it is fixable (new brushes, a clean-up of the commutator and possibly new windings, but at that point I would suggest a rebuilt starter/solenoid assy.

If the starter did whir on the last test then you can remove the solenoid and either rebuild it or replace it, the later being a good idea. The new starter will come with a new or rebuilt Bendix drive which is probably next in line for failure.

When going for a new starter make sure to bring the old one along with you. First, you can match it up to make sure the computer picked the right one for your vehicle and second, they charge a "core charge" for the old one - they want it back to be rebuilt and sold again. Just to make sure you got a good one you should bench test the new starter - it wouldn't be the first time a bad rebuilt was shipped.

[N*A*M]

ALTERNATORS

If you have been around cars for a while you might have heard the term generator. Well, those were the old days and the good old generator is history. What a generator did for the old cars, an alternator now does. You see, a car has and electrical system that carries power to such essential things as headlights, the ignition coils, engine cooling fans and other non-essential things as the radio (my son would argue that one), air conditioning fans (my wife would argue that one), and all of those other things upon which we have become accustomed to depend. All of that power has to come from somewhere!! A lot of people might think that power comes from the battery, and that is true to some extent. But the real answer is that the power to run all of those electrical things comes from Saudi Arabia! Huh? Saudi Arabia? Well, maybe Dallas, or Oklahoma. But the point is that the source for all of that energy is the gas tank. Yep. And the link from the gas tank to the battery is that mysterious thing called the alternator. It takes mechanical power from the crankshaft, transmits it via a "fan" belt, (it used to run the cooling fan as well) or serpentine belt as it is called in most of the newer vintage cars, and turns the alternator. So, the main function of the alternator is to convert power from the gasoline engine that drives you along the road, to electrical energy to keep the battery in tip-top condition.

So, what happens when an alternator goes bad? Well, at first, nothing. That is because the battery has some reserve power in it, enough to keep the engine running for quite some time, many many miles in fact. So a bad alternator doesn't necessarily mean a tow truck should be called right away. As long as energy is conserved elsewhere, like turning off the blower motor, the rear window de -fogger, the stereo and the headlights (if possible), you could make it for some distance on just the battery reserve alone. One major problem which will finally occur as the battery loses its charge is that there will not be sufficient voltage to keep the engine running well. Many years ago I was in California and saw a car coming down the street with its catalytic converter glowing white hot and flames coming from beneath the car. What had happened is the alternator quit, the battery ran down, the engine was not firing on all eight cylinders and the unburned fuel was being burned in the catalytic converter! It had been long overdue for the driver to call a tow truck!

So, how do you know when your alternator is going bad? Most of the time the alternator fails in stages. A little techie talk here. The alternator gets its name from the fact that it generates alternating current (AC). The old generators I mentioned before generated direct current (DC). Well the battery can't use alternating current so the alternator output is fed into what are called diodes, which convert the AC into DC. The alternator has a unique feature in that it is able to generate a relatively high voltage while the engine is at idle. The old generators needed to be running at a fast pace before they got up to 13 or 14 volts. The alternator can do this since it is really three alternators in one body. Each of the three sections
of the alternator generates its voltage out of phase with the other two sections. Since the complete cycle (one revolution) of the alternator is 360 degrees, each phase is shifted by 120 degrees from the next phase. So in one revolution of the alternator it puts out three separate voltages.

OK, back to the failure mode. Each of the three phases has its own windings in the alternator and each of the windings has its own pair of diodes. Each of these windings and/or diodes can fail, one set at a time. If this happens the alternator can still charge the battery, but only with a limited current, approximately 2/3 of its original capacity if one system fails. If two systems fail, then it puts out only 1/3 of its rated capacity. What that means to you is that you can go a long time on a limping alternator. Chances are if you don't need headlights or air conditioning or other high current using accessories, you would never know that the alternator was in the process of failing! The time you will find out is when it is 10 below zero and you wear down the battery by cranking the starter, then put the fan on high for heat, and then drive in the dark.

So, how can you tell if the alternator is failing without taking it apart and doing some measuring inside the alternator? It's really pretty simple. You will need a simple voltmeter. You can get one at Radio Shack for under ten dollars. Here's what you do - start the car, make sure all the accessories are off and rev up the motor to a fast idle. Set the Voltmeter to the DC scale (not AC or Ohms). Measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative (ground in most cars). The voltage should, and probably will, read around 14 volts. If it reads less than 12 volts you may indeed have a failed alternator and you can skip the next step. Next, turn on the heater, the rear window de-fogger, the radio, the headlights and anything else that draws power. Now rev up the motor and watch the voltmeter. It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that your alternator is not up to snuff.

One last failure mode is of course noise. The rotor inside the alternator rotates on bearings, normally very high precision needle bearings, and these can fail. When they do you will hear a loud grinding noise associated with the alternator. To isolate the noise take a length of tubing, heater hose will do fine, put one end to your ear and move the other around in the vicinity of the alternator. The noise will be much louder when you point it at the alternator if that is the culprit. Other possibilities are the water pump and the power steering pump which are also driven by the engine belt. To further isolate the noise disconnect the drive belt and spin the alternator by hand. If you hear a rumble or grinding noise then the bearings
are shot. If you don't hear a noise the problem may still be in the alternator since the be aring might be quiet without the loading of the drive belt tension. Check for side play in the pulley. If you are pretty certain the noise came from the alternator it is a relatively simple task to take it apart and visually inspect the bearings, else swap it in for a rebuilt. Your auto supply store will normally bench test the alternator free of charge and can tell you at that time if the bearings are noisy.

Before you go running down to the parts store for a new alternator make sure to check the connections at the battery terminals and also check to see that the voltage is the same at the alternator terminal (the big fat one with the heavy wire attached). Check to make sure the belts are tight and not slipping. Replace them if they are cracked or shiny on the side that faces the alternator pulley.

One final thing to check - the field voltage. In order for the alternator to generate electricity it must be supplied with a field voltage. If you know which wire is the one that supplies the field (normally labeled 'F') then simply check with a voltmeter to see if there is 12 volts at the field. Another check is to use a hacksaw blade or a lightweight screwdriver , anything magnetic, and hold it near the side of the alternator with the ignition switch turned in the on position. If there is a field voltage present then the metal will be attracted magnetically to the side of the alternator, not very strongly, but you will feel it pull the metal to the side of the alternator.

So, what are you going to ask the mechanic when he tells you that you need a new alternator?
1. Did you perform a load test on the alternator? If you did, what were the voltage readings? Were they all below specification?? (mechanics will use a load testing machine instead of turning on all the accessories.)
2. Did you check to see if the belts were old and cracked or possibly slipping?
3. Did you measure the voltage at the alternator connector or at the battery? Were the readings the same at both places or is there a voltage drop somewhere in the system. You can tell him the "Dead Battery" story if you want to.
4. Finally, did you check the price on a rebuilt as well as a new alternator? (rebuilt alternators are just as good as new if they are done correctly and usually cost about 1/3 as much)

Now that you know all about alternators you can feel confident that you can discuss the failure modes with a mechanic and not get shafted. It is also fun to watch the faces of a mechanic when you ask questions like those above. He will soon figure out that you know more about the electrical system of your car than how to turn the lights on!

[DAZZ]

Electrical Short

Ever have an electrical short that was so bad that it blew fuses instantly? Here’s a way to diagnose that short circuit without emptying your wallet buying fuses. Some shorts may not only be in wiring, but in components. when a fuse blows it is because the load is bypassed and moved to the fuse.

Take a old headlight and two headlight electrical connectors and put a set of alligator clips on one connector (for round glass fuses) and a set of spade connectors on the other plug (for newer blade style fuses.)

Place the wired headlight across the blown fuse and it will glow at full brightness. Start disconnecting components on that circuit until the light either dims or goes out. When it does, it will tell you whether the
component or wire is bad.

Be carefull using what we call a short light. As this may fry a low current carrying component that the fuse was protecting. But as for high current circuits this method can be very useful..

[glen1n]

thanks for the advice, my mums alternator recently died and she said it was overheating. I couldn't explain how the alternator failing could have caused the car to overheat, but now i do. Cheers, BTW i think this new forum is a really great idea.

Cheers
Glen

[robbie]

great work nam... added a short disclaimer & made it sticky..

[chimpboy]

It would be good manners to acknowledge the source of these articles.

Jason

[murcod]

Electrical Short

Ever have an electrical short that was so bad that it blew fuses instantly? Here’s a way to diagnose that short circuit without emptying your wallet buying fuses. Some shorts may not only be in wiring, but in components. when a fuse blows it is because the load is bypassed and moved to the fuse.

Take a old headlight and two headlight electrical connectors and put a set of alligator clips on one connector (for round glass fuses) and a set of spade connectors on the other plug (for newer blade style fuses.)

Place the wired headlight across the blown fuse and it will glow at full brightness. Start disconnecting components on that circuit until the light either dims or goes out. When it does, it will tell you whether the
component or wire is bad.

Be carefull using what we call a short light. As this may fry a low current carrying component that the fuse was protecting. But as for high current circuits this method can be very useful..

I'm glad someone has pointed that out!!! I have visions now of melted wiring and electronic devices going up in smoke.

A better method is to completely disconnect the battery and locate the short circuit by using a multimeter. Simply measure the resistance to the chassis from the fused wire while disconnecting components that are hanging off that supply line. As soon as the resistance reading jumps high you've found the fault.

Digital multimeters are available for under $15 these days. No smoke, no fires.

[murcod]

Here's another handy one:

How to determine the polarity of a speaker's terminals:

You can determine the polarity of a speaker's terminals that aren't marked + or - very simply by using a 1.5v battery (eg AA, C or D cell). Simply get two short lengths of wire and temporarily connect them to the speaker's terminals, then hold one of the wires on one of the battery's terminals while shorting out the other intermittently. The speaker cone will move everytime you complete the circuit.

When the battery positive (+) terminal is connected to the speaker positive (+) and the battery negative (-) to the speaker negative (-) the speaker cone will push forwards. If the cone is pulling into the speaker basket, then the battery is connected the wrong way (eg battery + is connected to the speaker - ; battery - to speaker +.)

DO NOT USE A BATTERY GREATER THAN 1.5V TO PERFORM THIS TEST, OR YOU WILL RELEASE THE SMOKE FROM YOUR SPEAKERS!

[Bluey]

DO NOT RELEASE THE SMOKE FROM YOUR SPEAKERS!

and when you release smoke from any electrical componet, it dont work any more.

btw, good work on the tips nam. are these threads printer friendly? dont know about other people, but i find it easier to print things out then go out to shed to see about fixing prob.

[robbie]

to print..

select text with mouse.. CTRL+C to copy.. Start > Programs > Accessories > Notepad.. CTRL+V to paste.. CTRL+P to print

Not sure if it will copy the whole thing in one hit, depends on how many characters there is

[chimpboy]

DO NOT RELEASE THE SMOKE FROM YOUR SPEAKERS!

and when you release smoke from any electrical componet, it dont work any more.

btw, good work on the tips nam. are these threads printer friendly? dont know about other people, but i find it easier to print things out then go out to shed to see about fixing prob.

The trick here would be to prevail upon our illustrious hosts to install this mod on their very cool PHPBB2 server:

http://www.phpbb.com/phpBB/viewtopic.php?t=141443

Cheers,

Jason

[robbie]

send a PM to Carl suggesting it

[N*A*M]

jason, i would've quoted reference except i do not have them. these articles were in a massive pdf file i have on a cd from a friend. i suspect someone collected them from various bb's.

[N*A*M]

GQ Operative Voltages: http://www.outerlimits4x4.com/PHP_Modul ... hp?t=34749

[joel HJ60]

Glow Plug Discussion Talk on how to test them and what resistance and continuity is.

[Dooley]

In regard to jumping it's also an idea to actually check where the wires from the battery go.

I was tired and tried to jump start in the dark and my positive terminal, the previous owner had put a black cover on it, the negative had no cover. Thought naturally it was the negative terminal and tried to jump it the wrong way luckily no serious damage. The fusible link dissapeared in a puff of smoke though.

Later during the day I had a look and the negative terminal had wires going to the chassis which I couldn't see in the dark and I also ignored the fact that accessories are normally fused off the + terminal and just went by the colour.

Mines a negative ground of course.

Not likely but hey it caught me out.

[macca81]

So, what happens when an alternator goes bad? Well, at first, nothing. That is because the battery has some reserve power in it, enough to keep the engine running for quite some time, many many miles in fact. So a bad alternator doesn't necessarily mean a tow truck should be called right away. As long as energy is conserved elsewhere, like turning off the blower motor, the rear window de -fogger, the stereo and the headlights (if possible), you could make it for some distance on just the battery reserve alone. One major problem which will finally occur as the battery loses its charge is that there will not be sufficient voltage to keep the engine running well.

unless your running an old diesel that doesnt need a spark to keep running, once turned over it just keeps going untill either you tell it to stop, the empty fuel tank tells it to stop, or the water that just came into your air intake told it to go KABOOM!!!.
so in ya old buss' its when ya cant turn it over that a know somethings wrong

[PJ.zook]

unless your running an old diesel that doesnt need a spark to keep running, once turned over it just keeps going untill either you tell it to stop, the empty fuel tank tells it to stop, or the water that just came into your air intake told it to go KABOOM!!!.
so in ya old buss' its when ya cant turn it over that a know somethings wrong

Haha a mate at work discovered the hard way that a jacket over the intake was the only way to stop his old Discovery from revving out of control and bellowing smoke out the exhaust after his turbo seal blew and it started running on its own oil. Luckily the snorkel was sealed well so he could just quickly ram things down it to stop airflow, which just happened to be the jacket off his back.

[Winger]

Here's another handy one:

How to determine the polarity of a speaker's terminals:

You can determine the polarity of a speaker's terminals that aren't marked + or - very simply by using a 1.5v battery (eg AA, C or D cell). Simply get two short lengths of wire and temporarily connect them to the speaker's terminals, then hold one of the wires on one of the battery's terminals while shorting out the other intermittently. The speaker cone will move everytime you complete the circuit.

When the battery positive (+) terminal is connected to the speaker positive (+) and the battery negative (-) to the speaker negative (-) the speaker cone will push forwards. If the cone is pulling into the speaker basket, then the battery is connected the wrong way (eg battery + is connected to the speaker - ; battery - to speaker +.)

DO NOT USE A BATTERY GREATER THAN 1.5V TO PERFORM THIS TEST, OR YOU WILL RELEASE THE SMOKE FROM YOUR SPEAKERS!

when I had to install a stereo in my mates car and he had already cut the wires from the old radio. To identify the speakers I useed the continuity on my multimeter and when I had the right two wires a sound could be heard from the speaker.

[bazzle]

when I had to install a stereo in my mates car and he had already cut the wires from the old radio. To identify the speakers I useed the continuity on my multimeter and when I had the right two wires a sound could be heard from the speaker.[/quote]

The purpose of the DC battery is that speakers have to be phased. ie. all cones either going in or out at the same time or the music looses clarity and frequency response.

Baz