Charging System Warning Lights
The importance of having an operational warning light for the alternator (idiot light) is crucial to catching problems early. In most modern systems, the electrical current passing through the filament of the warning light is what energizes a circuit in the alternator to start charging.
To check the warning light circuit, turn the ignition switch to the “on” position without cranking or starting the engine; if the idiot light does not come on, remove the plug from the alternator and ground the sense wire (wire connected to the bulb). If the light comes on, the wiring is okay but the alternator is defective; if the light still does not come on, the wiring to the light circuit and the bulb should be checked.
Don’t forget to check the fuse that controls the light circuit, too. This fuse could be labeled differently in various cars. It could be labeled “charging”, “regulator”, “meters”, “gauges” or “engine”. In some cars, if the fuse is out, the idiot light will come on but may not go off. In others (like GM), a burned out fuse may make the warning light work in reverse order; that is, when the key is on, the light is off but as soon as the engine starts and the alternator starts charging, the light will come on.
Checking further into charge light indicators, in some cases it is normal for the charge indicator light to come on when nothing is wrong with the alternator.
According to information published by GM, any car may have a low voltage reading or lights that dim when electrical loads are heavy at idle. Furthermore, this condition is normal and no repairs should be attempted unless a fault has been found.
If a car idles for extended periods of time during high heat conditions, the heat within the alternator increases, the electrical resistance in the alternator also increases, which reduces the alternator’s charging capacity.
With the alternator’s capacity for charging reduced by heat and other factors, an alternator may only be able to produce up to 70% of its rated output. So an alternator rated for 100 amps may only be able to produce 70 amps when hot at idle when there is 77 or more amps of demand on it.
In the case of Delco CS series alternators, there is a tool available from Kent-Moore tools (J-41450-B), which isolates the alternator from the car’s wiring harness and shows whether the alternator is at fault, or whether there is a wiring problem elsewhere within the car’s wiring harness. The best thing about this tool, besides being compact and handheld, is that it doesn’t require any interpretation of data by the operator. The little light on the unit lights or doesn’t light, depending upon whether the alternator is good or not.