HOW DO I TEST……?
I answer a lot of posts on here with “have you tested it?” or “what reading do you get?”,as I take it for granted that everyone knows how to use a meter to take a reading to find out just what is going on,but from the number of PMs I get it looks like I need to assume less and explain more.
There are two basic tools that I use everyday at work that I’d be lost without,a digital multimeter as shown below.You can pick these up now for as little as £10,but look at spending £30 or more for a half decent one.These are better than the old fashioned analogue meters which are a lot more fragile and can be difficult to read in a hurry,leading to possible costly mistakes.
Look for the following minimum specs if you buy one:-
DC voltage…………..20 volts or more
DC current………….10 amps or more (20 amps is better)
Resistance…………..200 ohms minimum and audible continuity
These power probes are also very useful as they give a much easier to interpret indication of power or earth conditions.
HOW TO USE A MULTIMETER
Unless it’s a very cheap one,most modern multimeters will “autorange” to some extent,but unless you know exactly what you’re going to be measuring,then you should always start on the highest scale and work your way downwards.As we’ll be measuring in the region of 12 volts,then the next higher range which is normally 20 volts full scale will be the one to use.
When we want to measure voltage we have to connect our probes to the meter as shown,the black will go to the “COMMON” socket,and the red into the “V/OHMs” socket,and put the probes across where we need to measure the voltage,it doesn’t matter if we get the positive and negative probes round the wrong way as it’ll just give a negative reading instead of a positive one,but the number of volts will still be the same.
METER SET TO MEASURE VOLTAGE
The graph below shows the sort of reading we should get from a healthy charging system measured across the battery terminals when we start the engine,let it idle for a short time,and then stop the engine,this graph was compiled from my car which has a known good charging system,and probably an “average” age battery.I’ve had the car for nearly two years on the same battery,and it must have been at least a year old when I first got the car,I would expect a new or nearly new battery to have a higher off-charge voltage in the region of about 12.7 or 12.8 volts instead of my 12.2 volts,and a heavy duty or newer battery won’t drop as low as 10.5 volts when the starter is spinning.The car had purposely not been started for several hours before I did this test.
This test was done with no extra load on the charging system,if you turn everything possible on that will load the alternator such as the lights on mainbeam with fogs,the air con,the wipers,heated rear screen etc,you should get no more than a 1 volt drop when the engine is running.
If you don’t get significantly more voltage when you start the engine than with it off,then the alternator is faulty and will need replacing,and on newish cars such as the mk.4 platform range you should always get more than 14 volts.If you get slightly less it could be as simple as a loose or corroded connection on the alternator or battery,the earth to the case of the alternator can be just as likely a cause of low voltage as the thick black charging cable that is connected to the fuse nearest the middle of the car as shown below.
If you get significantly more voltage drop when you do the load test (2 volts or more),then it could again be a bad connection,but will more likely be the alternator on its way out.
If the voltage is as above with the engine running,but drops off quickly as soon as you stop the engine to less than 12 volts,then the battery will almost certainly need to be replaced,use the heaviest duty one that will start your engine especially if you have a diesel,not always so important for a petrol engine.Don’t forget that the battery only has one real job,and that is to start the engine,once it’s running the alternator supplies ALL the power to run the car.
If both of the above tests check out ok,but you come out the next morning to find the battery is flat,then something in the car is faulty and drawing more current than it should do,this is not all that difficult to find if we follow a few easy steps.
To measure current we need to break the circuit where the suspected problem lies,and the easiest way to do this is by taking a fuse out and putting the test probes across the vacant terminals,the meter should be set as shown below.
METER SET TO MEASURE HIGH CURRENT
Always start on the highest range like this until you get a reading,then if it’s off the bottom of the scale,go to the next one down as below,this normally involves moving one of the probes as well.
METER SET TO MEASURE LOW CURRENT
We are looking for the lowest reading possible,a typical car with maybe one or two extra accessories (after market head unit etc) should read less than 50 mA (0.05A),and preferably as low as 30mA (0.03A),but don’t get paranoid if you get 55mA.Also if your mate has got an apparently identical car to yours,and his car gives 5mA less draw for the same test as yours,it’s a fact of life that can be down to a combination of different factors and is nothing to worry about.There is no carved in stone absolute right and wrong reading,so please note that these are nothing more than guidelines.
The first test should be through the fuse shown below that powers everything inside the car.
When you do this test you should have the car in as dormant a state as possible,i.e. ignition turned off and boot and doors shut so that no courtesy lights are on.
If this test proves negative then try the other fuses,it is possible for the alternator to show as putting out enough voltage to charge the battery,but can draw up to about 5 amps,which will be enough to flatten even a healthy battery in a couple of hours or less.
If the problem is found to be inside the car,then you can do the same test through the smaller blade fuses in the fusebox at the drivers end of the dashboard until you find the guilty one.
VERY IMPORTANT…….WHEN SWITCHING BETWEEN MEASURING VOLTS AND AMPS,DON’T FORGET TO MAKE THE APPROPRIATE CHANGES TO THE METER(especially the probes),OTHERWISE THEY’LL BE AN EXPENSIVE BURNING SMELL.
METER SET TO READ RESISTANCE
We can use the resistance range of our meter for a number of different things.
In the above example where there’s a large voltage drop under load,we can measure the resistance between the case of the alternator and the negative terminal of the battery,it should be as low as possible,ideally under 1 ohm (you will never actually see zero ohms as there will always be a slight contact resistance),the same with the engine block to car chassis and chassis to battery.If it’s a bit higher i.e. 2 ohms or so this is still nothing to worry about,but you can try scratching the tip of the probe against the metal case of the alternator to get a better contact.
Got a subwoofer and don’t know what impedance it is? Then set your meter again to the lowest ohms range,the resistance will read slightly lower (about 20%) than the impedance,so for a 4 ohm sub we should see about 3.5 ohms or thereabouts.
The audible continuity range can be used for testing switches,bulbs,fuses (out of their holder) ,wires suspected of having a break somewhere,or even sending Morse code.
HOW TO USE A POWER PROBE
The type of powerprobe shown above works by attaching the large positive and negative crocodile clips to the cars battery,and when the tip of the probe is touched onto a wire,it shows a +ve voltage by the LED turning red,and a –ve or gound condition by turning green,and we can also put a +ve or –ve pulse down the tip by flicking the rocker switch up or down.
The latter feature is useful for checking things like central locking (-ve pulse on mk.4s),and by attaching the short flying lead to one of the terminals on a loudspeaker,and putting a +ve pulse into the other side by flicking the rocker switch,we can test speakers not only to see if they work (but please NOT tweeters,they must always be used through a crossover),but also to check phasing,handy if the terminals aren’t marked +ve or –ve.The terminal that makes the cone move outwards when we pulse it will be the +ve one.
Lights and indicators are a lot quicker to check with one of these probes to find out if it is the just the bulb that is blown,or a wiring fault elsewhere if the fuse tests as being ok.
We can also use it to find any blown fuses if we don’t have access to the fuse diagram.Turn the ignition on and touch the probe to the tiny bit of exposed metal at each end of the top of a blade fuse,and you should see the same at both ends,i.e. preferably a red for + 12 volts,if you get red at one end,and a green at the other,it means that fuse is blown.
Earth contact switches such as the doors,boot and bonnet can also be checked,on the bonnet it should only show earth when open,the boot when closed a +ve changing to ground when opened,and the same with the doors at the puddle light,always on brown wires with different coloured tracers.
It can also be used to check continuity by attaching the flying lead (this is already connected to ground) to one end of whatever we are testing,and touching the probe onto the other end,if it is working ok then we should see the LED light up green.
PLEASE HOWEVER BE VERY CAREFUL WHAT YOU TEST WITH ONE OF THESE PROBES AS THEY DO HAVE A SMALL POSITIVE VOLTAGE LEAKING OUT OF THE TIP,WHICH CAN CAUSE VERY EXPENSIVE DAMAGE TO SENSITIVE CIRCUITS.NORMALLY ANY WIRING LOOMS OR CONNECTORS WRAPPED WITH YELLOW OR ORANGE TAPE WILL BE FOR THE AIRBAG,AVOID THESE LIKE THE PLAGUE,ALSO IT IS BEST TO AVOID ANY ECU CONNECTIONS WITH ONE OF THESE,AND AS BEFORE IF YOU ARE IN ANY DOUBT WHATSOEVER,GET THE HELP OF SOMEONE THAT IS QUALIFIED TO KNOW WHAT THEY ARE DOING.