(The glowing alternator light)
Looking through the owner’s manual for my 911, I noticed the description for the alternator warning light. You know this indicator as the little red light with the picture of a battery in it. The manual goes on to say that if this light flickers or stays on steady you should take the car in for repair. But what if it glows like Casper with a really bad sunburn? Believe it or not, but there is no reference to this Casper syndrome in the entire manual!
The first step in diagnosing a problem is in understanding how the system works. It is actually quite simple. A light bulb becomes illuminated when electricity (electrons) flow through the bulb. In order to have electron flow you need to have a difference in voltage potential on either side of the bulb. When you turn the key on there is 12 volts (battery voltage) at one side of the alternator-warning light bulb. If the alternator is not working there is no voltage at the other side of the bulb and the bulb lights up brightly. If the alternator is working and is producing battery voltage there will be 12 volts on each side of the bulb, no difference in potential and therefore no bulb illumination.
So what’s the deal when the light just glows dimly? The reason the light glows dimly is the there is only a slight difference in voltage potential. This occurs, when for instance, there are 12 volts at one side of the bulb and only 9 volts at the other side of the bulb. The 3-volt difference creates the low voltage glow.
But where did that missing 3 volts go? It got lost in a bad connection somewhere in the electrical system. Somewhere is the operative word and where a great deal of time can be lost in repairing this problem. Fortunately you do not need a big fancy (See expensive) piece of diagnostic equipment to locate the problem. All you need is a $12 Digital Multi Meter (DMM) from Radio Shack and a spool of 16-gauge wire to extend the leads as necessary.
To test the wiring from the alternator to the battery you would hook one meter test lead to the battery positive and one up to the alternator B+ connection. (This is a parallel voltage test) The electrons leaving the alternator that are being blocked at a bad connection (a burnt alternator to starter wire for instance) will see the voltmeter as a easier way to the battery and travel through the voltmeter. The reading at the voltmeter indicates how much extra resistance is in this electrical circuit.
When dealing with electricity it is best to think of it as water. Let’s use a running garden hose as an example. Take this hose and poke a small pin hole in the middle of it. The water will not run out the pin hole because it is easier to run out the large opening in the end (no resistance in the circuit). If we kink the end of the hose and restrict the volume of water flow (resistance) water will start pouring out the pin hole. The water coming out the pinhole is the reading on the voltmeter.
To locate the bad connection is best to load the circuit while testing it. When testing the charging system circuit it is best to raise the RPM to 2000 and turn the headlights on. This creates maximum alternator output and a high load condition with the lights on. After a voltage loss is detected shorten the length of the test area to isolate the high resistance spot.
If you get a reading of 3 volts from the battery positive to the alternator B+ but the reading goes down to .5 volts when you move the test lead from the alternator to the starter you could conclude that the wire or connections from the alternator to the starter are bad.
(See figure 1)
When making circuit tests like this it is said that .2 volts per connection is allowable.
With this formula let’s see how much voltage we could loose from the battery positive lead to the alternator:
Voltage at alternator B+ lead (14.2 volts)
0.2v - battery positive post to battery cable end.
0.2v - battery cable end to actual cable.
0.2v - battery cable to starter connection
0.2v - starter connection to alternator wire
0.2v - alternator wire to alternator connector.
1.0v – Total voltage loss
Voltage at battery positive post (13.2 volts)
With this 13.2 volt reading at the battery we can see that this is the maximum allowable loss of voltage and anything greater than the .2v per connection would require repair.
Another telltale sign of a poor, high resistance connection is heat. While making the high load voltage test, run you fingers along the wiring and feel for isolated heat spots. If a spot in the wire is hot there may be broken strands inside causing high resistance.
This method of voltage checks applies to all electrical circuits as well. Starter, headlights, blower motors etc.
Good Luck.
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