How to check the charging/power circuit.

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My Name: Ozvtr

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Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
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From time to time you may have reason to check the condition of the charging circuit of your car.
Note that I didn't say battery check...but I will get to that!
Generally speaking, the weakest link in the charging/power circuit is the battery.
Typical life span of a battery is about 4 years. However, while the battery might be the most likely cause of failures, it is not the only cause of failures.
About the only way to really check the status of a battery is with a load tester. This is a specialized and expensive piece of kit and not practical for DIYers to own.
While you may not be able to definitively test the battery, you can test other parts of the power circuit and eliminate anything else as a possible failure.

I will talk a lot about "voltage drop" around the circuit. So what does that mean?
As current flows around a circuit it will encounter 'resistance'. This resistance can be a lot or a little. For example, corrosion between two contacts (joining points of a wire) causes a lot of resistance whereas clean contacts have little resistance.
As current flows and hits this resistance it creates voltage across that resistance. This creates a 'drop' in voltage when measuring it.
So if you can measure voltage across an electrical connection, then there is resistance there. The greater the voltage the greater the resistance. Now this is with current flowing around the circuit, I.E. the circuit is doing some thing. Not if the circuit is off! In this case the alternator is supposed to be supplying charge current to the battery.
A classic example here would be between the terminal posts of the battery and the clamps that hold the cables to the battery. Corrosion here will cause resistance and therefore will generate a voltage. But I will get to show you that in a moment.
Resistance to current flow is not good, for a number of reasons but I wont go into why, just know its not good.
Ok, the weapon of choice here is the multimeter. It measures Volts, ohms and Amps, basically, and may measure other things. All we are interested in today, is Volts.
Volts is the 'potential difference' between two points in a circuit.
It's very important to remember your multimeter is reading the voltage difference between the red lead and the black lead, where ever that may be!! Today, that might be anything between 0 and 14.8 and the value is in Volts. Don't worry if there is a '-' minus sign in front of the reading, just note the voltage value.
If you don't understand any of that stuff, don't worry, I'll go step by step in the checking phase.
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The multimeter.

Any multimeter will do even a very cheap one...as long as it works. I recommend a digital meter they are easier to use and read.
The meters may have a dial or push buttons to select modes and they may be 'auto ranging' or manual range select.
Plug the black lead into the 'common' terminal, plug the red lead into the volts/ohms terminal. We will NOT be using any other terminals on the multimeter.
Using whatever form of selector the meter has, select DC (direct current) volts. If the meter has a manual range selector, select the next range above 15V (15V is about the highest voltage we are expecting to read today). In this case, on my meter, its the 20VDC range. If the meter has an on/off button...turn it on.
Lastly, you may notice the ends of the red and black leads have metal probes. These probes need to be clean and sharp. You can clean and and sharpen them with a nail file, points file, sandpaper...what ever. Some surfaces that you might measure voltage from may have a very thin layer of corrosion and this may effect our readings. So the probes need to be clean and sharp to penetrate this potential barrier.
The first thing you need to do is to remove the terminal covers for our battery and alternator.
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Next have a look at the overall condition of the battery and terminals. Take a quick glance over the alternator terminal and check that the nut is tight and the joint generally looks serviceable.
The "dolls eye". A lot of batteries have an indicator on them to show the charge status. Typically if it shows bad, then the battery is usually bad but if it shows good well...it might be good. The point is, take these indicators with a grain of salt...they are NOT reliable!
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As you can see in this photo, the negative terminal has some corrosion forming on it. This particular corrosion is weeping out from inside the hole that the retaining bolt goes through. This means that the electrolyte is wicking up the terminal and out the hole. This is poor production control but It doesn't mean the battery is no-good. This terminal can be cleaned up. This is an indicator that the battery may not reach it's full life span, but then, how long is a piece of string? Who knows?

Next, without the engine running, take a measurement of the actual battery voltage. Place the probes in the center of the battery terminals and read the voltage. At this point the voltage of a battery that has not been used for only a short while should be higher than 12V.
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If the voltage is less than 12V that isn't definite proof your battery is bad but it's not good. It's possible your battery has been sitting around doing nothing for a while.

Now start your car. If the car wont start (and you believe the problem is the battery) try charging the battery on a mains battery charger at least over night. I would recommend removing the positive and negative battery clamps and attaching the charger alligator clips directly to the battery terminals. Not because the car ECU's will blow up, but because then you know the charger is DIRECTLY connected to the battery.
Want to know when your battery has taken on as much charge as it can (note my deliberate wording there, I'll get back to that)? If you don't have an auto charger, you will hear the battery "boiling".
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Measure the voltage (with the engine running) at the same spot, on top of the battery terminals and make a note, we will come back to that reading.
The voltage should now be just over 14V. Any more than about 14.8V should be investigated. Anything over 15V will begin to boil your battery and may cause further electrical problems. If it's around 14V, your charging circuit is fine. The battery however...
During winter the days are shorter and so you may use your head lights more. Cold starting the engine draws more out of the battery. ETC, ECT.
A lot of people think that the alternator will recharge the battery "straight away". Nope! It may take hours to replace the charge taken out of a battery (maybe a bit of an exaggeration but possible)!!! So some car trips (short) may take more charge out of the battery than the alternator put's back in. If you make short, bumper-to-bumper trips in your car under certain circumstances your battery will go flat, even if the system is perfectly fine!!

Ok, if you don't get at least 14V, move over to the alternator.
Put you red probe on the center post and your black lead on the body of the alternator. You may notice some corrosion on the aluminium castings. This is why we want our probes clean and sharp, to get through that crap.
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With the engine running (the engine wont be stopped through out the tests). You should get very close to 14.6V. If not there is a problem with the alternator (either a low or high voltage can be a problem). You would think that before this point the little "battery" light on the dash might be on, or something. To be honest with you I have no idea what goes on the minds of engineers. If your light is on...investigate. If it's not...well...it's not! That doesn''t mean there ISN'T a problem!

OK so you have the alternator working but not enough voltage at the battery. What next?
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Place your red probe on the alternator terminal and your black probe on the battery positive terminal. If the meter shows more than half a volt or it shows the DIFFERENCE between the battery voltage and the alternator voltage, your problem is in the positive line.
OK, say you get 14.6V at the alternator terminals and noted the battery voltage was 12.4V (remember that battery voltage I told you to note?). If the meter indicates about 2.2V from the alternator to the battery, as I said above, this is voltage drop and indicates resistance or even an open circuit. As they say in the classics "there's your problem"! At this point there is more to do but I'll get to that in a minute.

Right, no voltage drop on the positive side. You get point-something of a volt...insignificant. What next?
Move your red probe to the metal case of the alternator and your black probe to the center of the negative battery terminal.
If you get a voltage drop, as above, then your problem is in the negative line of the charging circuit. Again there is more to do but I'll get to that in a moment.

If you get to this point and there seems to be nothing wrong...Well then you must have stuffed something up!!! Just kidding. The most likely cause now is the battery. The terminal voltage of a bad battery will go up and down just like a good battery. It simply does not take on a charge and looses its capacity. This is why the battery really needs to be load tested to check its ability to source current. If you prove it's not your charging circuit, then it has to be the battery. Even though the battery is pretending to be good and you don't have a way to independently check the battery.

Ok, we haven't finished yet. If you DO get a voltage drop across your positive or negative side, what do you do next?
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Going to either the positive or negative battery terminals (depending what "side" you are having troubles) put your probes on the center of the terminal and then onto the terminal clamp. Check for a voltage reading on the meter. If you get a "big" reading (1 to 2 volts) then there is corrosion under the terminals. Undo the clamps and clean the terminal(s) and/or clamps.
If you don't get a voltage drop across the clamp then your trouble is in either the positive lead, going from the alternator, around under the accessory drive belt (fan belt) to the starter motor, where there is a nut and stud. Then from the starter to the battery. Inspect all those bits. Check the nut on the starter for tightness.
Or if your fault is in the negative lead, then that goes under the battery box and is connected to the gearbox and chassis. Check the attaching bolts and cable.

A quick way to check the status of the battery is to turn the head lights on and use your multimeter to measure the battery voltage, without the engine running. If the voltage drops significantly from the voltage you noted way back in the beginning and the battery voltage continues to fall at a significant rate, your battery is not serviceable (bad or poorly charged). I cant give you actual voltages or rates of voltage drop, that will vary from battery to battery. However a voltage drop of even a few volts and then a steady fall of a few volts per minute shows a bad or poorly charged battery.

Well I think that about covers it. If you have any questions or something is unclear, ask away.

Oh! P.S. one of our members had a great idea to quickly and cheaply check the alternator. Buy one of those (these) USB chargers that plug into the cigarette lighter and displays the input voltage from the lighter socket (not 5V from the USB). Turn the ignition key to on and note the voltage reading. Then start the car. If the voltage goes up then the alternator is working.
Also good for monitoring the supply voltage if things intermittently go wrong while you are driving along.
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