Automotive Charging Systems

By Charles Evans

Electricity is the life blood of the automobile. This has always been the case as nearly all gasoline internal combustion engines use electricity to ignite the fuel. Today the total automobile is controlled and operated by electricity and electronic equipment. This include fuel control, emission control, accessories such as the heater and air conditioning system, steering control, brakes, lights and many more. It is obvious that the charging system that provide this power is a very important system.

The charging system usually include a method to generate electricity and a battery to store the power. This sound pretty simple but it is no longer a simple system. The typical system consist of an alternator that is usually belt driven by the engine. The out put of this alternator must be regulated to control the voltage with in a specific range. The usual range is 12 volts to about 14.2 volts. If the voltage gets to high or too low electronic and electrical components will not work or they may be damaged.

Every alternator is controlled by some form of electronic regulator that controls the output of the alternator. In years past that was a regulator mounted to the vehicle independent of the alternator. A few years ago the regulator was moved into the alternator and the system was called internally regulated. A few manufactures decided that the electrical system should be interfaced with the computer network. Now most cars either control the charging system by computer or monitor it and set certain levels by using the engine control computer. Many systems reduce the alternator load during periods of hard acceleration.

Can I test and diagnose problems with my alternator? That depends upon the equipment that you have. A good multi meter will give a fair indication of the state of a charging system. If all that you have is a multi meter measure the voltage at the battery terminals before starting the engine. Then measure the voltage with the engine running at a fast idle. The resting voltage should be close to 12.6 volts and the operating voltage generally ranges 13.2 to 14.2 volts at the battery. You can also measure the output voltage at the alternator and it should measure with in 0.1 volts of the battery voltage. After measuring the voltage at a fast idle, turn on all of the accessories and the lights. The voltage should remain close to the same as at a fast idle (with in about 3 or 4 tenths of a volt).

A customer came in the other day and stated that one of the parts people at a national known auto parts store had told him that the only sure way to see if an alternator was working was to disconnect the negative battery cable and see if the engine continued to run, if it does the alternator is okay. DON'T EVER DISCONNECT A BATTERY CABLE IF THE ENGINE IS RUNNING.

Disconnecting a battery cable with the engine running will most likely damage your voltage regulator, engine management computer and other components. This could end up costing you thousands of dollars. For the same reason do not reverse the battery terminal when jump starting a car.

Can I change my own alternator? Yes if you have the correct tools. Get a good repair manual before you try to diagnose or change your alternator.