Battery warning light doesn’t go out. On older cars this is normally caused by an alternator fault, although it can also be caused by the wiring going to earth before reaching it. To check – simply remove the warning light lead. If the light then goes out the fault is in the alternator. If it doesn’t the problem is in the wiring. N.B. There are some exceptions to this rule as some vehicles are fitted with an item called a charge relay but the test applies in about 95% of cases. However in recent years things have changed, so if your car is fairly new much of this next section may not apply and you should have a look at the bottom section of this page.
Battery warning light doesn’t come on – Remove the warning light lead and earth it. If the light comes on, its an alternator problem if it doesn’t it’s a wiring/bulb fault. (The same exceptions apply as in the previous paragraph).
Battery light goes out but battery does not charge.
This is normally NOT a alternator fault. The cause is frequently a slipping fan belt (they don’t always screech) or a bad connection either to earth or to be positive side of the battery. To check the fan belt inspect it for tightness, wear, and also inspect the pulleys. The belt should drive on it’s sides – not its bottom. If the belt is too deep in the pulley groove inspect closely and suspect wrong/worn belt or pulleys. Secondly if you connect a multi- meter between the positive and negative terminals and then move the positive to the alternator outlet with the engine running the voltage should be the same i.e. 13.5 – 14.5 volts if it is charging.
The following grid gives some more common faults.
Because the same symptoms can have a number of causes but explaining the tests to find faults can be extremely difficult I have not attempted to give them all .Look in alternator testing for more detailed information
Modern vehicles continue to evolve, and technicians the world over are struggling to understand the march of technology and this applies to charging systems as well, but first a bit of background.
Alternators replaced dynamos during the 1960's because electrical loads were increasing, over the years the electrical content of vehicles has continued to grow, and this shows very little sign of slowing down. For larger electrical loads require larger alternators and manufacturers have had to evolve new systems to stay ahead of the game. Modern alternators can be controlled by the vehicles computer, and in this case the symptoms and tests described earlier in this section will not work. Your only choice is to take the car to a specialist.
To help the more technical amongst you a new section has been added under the title of Teccie Talk. This section will help shed light on the ever more complicated world of vehicle electrics and electronics.
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