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Nissan Enthusiast
Join Date: Mar 2005
Location: New Cumberland, PA
Posts: 217
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Charging System FAQs
Here it is, my one and only full write-up on the Sentra charging system, all about it, and how you can diagnose and isolate your problems.
1. Why are my "charge" and "brake" warning lights both on at the same time?
2. How does the charge indicator light work?
3. How does the alternator and charging system work?
4. How come my alternator doesn't charge until I rev it to 3000 RPMs?
5. Why do my airbag and/or ABS warning lights come on with a bad alternator?
not finished, more to come. I'm just tired now.
1. Why are my "charge" and "brake" warning lights both on at the same time?
Answer: Actually, your charge indicator light is on, and the brake light slaves it. It's a flaw that nissan designed into the warning light system. Basically, there's a "bulb check relay" somewhere in the Sentra that is wired into the circuit that controls the alternator light. that relay connects the BRAKE light to ground. So any time the alternator light is on, that relay is activated and it turns on the BRAKE warning light too. WHY? Nissan thought it was a good bulb-check feature so that you know the brake light works when you start the car, even if the parking brake isn't set, since the alternator light is always on whenever the ignition is on and the engine isn't turning. They didn't stop and think about what would happen when a legitimate alternator defect while you're driving would kick the charge indicator light on. Most other car manufacturers have this bulb check feature built into the ignition switch. There's actually two contacts at the "start" position, one set activates the starter solenoid and the other set grounds the BRAKE (and temperature) warning lights whenever you are cranking the engine. Start any Chevy or older Ford and you'll see what I'm talking about.
2. How does the charge indicator light work?
Answer: It's just a simple light bulb in the cluster, behind a red lens that has a battery symbol printed on it. The positive side of this light bulb goes to the ignition switch, and receives +12 volts any time the ignition switch is turned on. The negative side of the light goes to the alternator through the "L" terminal on the connector.
When the ignition is turned on, but before the engine has been started, electricity is fed into the alternator to energize the field coil and produce the electromagnetism needed to generate electricity. That small amount of electrical power flows from the ignition switch, through the bulb for the alternator light, and into the alternator through the L terminal. Since that circuit is complete, the warning light is on (we call this the bulb check). Once the engine starts, the alternator begins to rotate and generate its own power. Once it's up to full RPM and is generating its own power, some of the power generated is tapped and re-directed, and now that is used to energize the field coil instead of the power coming in through the ignition and alternator light. Since the alternator is generating its own power, and power coming in through the warning light isn't used anymore, that warning light circuit is essentially "broken" or no longer connected to ground, and the bulb shuts off. This happens a second or two after the engine starts.
When the alternator isn't generating power, no power is re-directed back into the field coil any more, so it uses the power available from the alternator light circuit again. When this happens, current flows through the bulb filament, and the warning light turns on to warn the driver that the alternator isn't producing electricity.
It's VERY important to realize that this warning light won't always come on when the alternator goes bad. For instance, a bad voltage regulator (common problem here, too) would never complete the indicator light/filed circuit, and therefore power would never be generated. abroken field coil would do the same thing. When I bought my second 94 sentra, I knew before-hand there was an alternator problem, but it was confirmed when I turned the ignition on and the alternator light stayed off. Sure enough, I was right, because two minutes later the airbag light came on and the car died. When you turn the ignition on before you start the car, the warning light MUST be on -- that's your assurance that the field circuit is OK. if that light isn't on, you have a bad alternator.
Then, of course, it should go out when you start the engine.
3. How does the alternator and charging system work?
The battery, ignition switch, charge indicator light, AC generator (alternator), and wiring compose the charging system.
The alternator is composed of a rotor coil that spins with the engine, three stator coils that are fixed in place and never move, graphite brushes that transfer power to the rotor coil while it's spinning, copper slip rings on the rotor shaft that touch the brushes and make the connection, a voltage regulator, a rectifier, and a diode plate or diode trio. Bearings at the front and rear allow the rotor shaft to spin in place.
There are four electrical connections made to the alternator. The B terminal is the main output and provides the electricity generated. The S terminal is the "sense" terminal used by the voltage regulator, and is connected to the battery, and always has battery voltage. The L terminal (for "lamp") goes to the charge indicator light on the instrument cluster. And a ground connection is needed somewhere on the alternator's case.
Initially, the voltage regulator uses the battery voltage at the sense terminal as a reference point. This current isn't actually used for anything, the regulator just uses it to determine the battery's state of charge and the electrical load. Whenever the voltage at the S terminal hits a preset voltage (usually 14.4 volts), the regulator essentially "shuts off" the alternator until the voltage drops back down, then it "turns on" the alternator. This cycling of turning on and off the alternator happens several hundred times a second. The voltage sensing is done with a zener diode, which allows current flow at certain voltages. The regulator controls the alternator by turning on and off the -12 volts going to the field coil.
The +12 volts to the field coil is always there whenever the ignition is on, regardless of the regulator. So essentially, whenever the system voltage is below 14.4 volts, the field coil is energized and is producing electromagnetism.
Three fixed coils, called the stator coils, induce this electromagnetism when the alternator begins to spin. The power generated comes out in three legs of a three-phase AC power system. These three legs travel to the rectifier bridge, which converts this three-phase AC power to single-phase DC power. The "B" terminal on the back of the alternator comes straight from the rectifier.
Between the coils and the rectifier, there's a diode trio that taps into each leg and acts as a small rectifier to convert it to single-phase. This is the diode trio, or diode plate. Once the output power is tapped and converted, it's fed into the positive side of the field coil now, and that's how the alternator stays energized.
In order to get power from an alternator, you need to put power into the alternator. This is done through the "L" (lamp) terminal. There first needs to be a source of electricity to energize the field coil before the engine starts and spins the alternator. This is done through the L terminal which is connected to the ignition switch. When the ignition is on, current flows from the battery, through the alternator warning light bulb, and to the positive side of the field coil. Since the voltage is below 14.4 volts (remember, the engine isn't started yet), the voltage regulator turns the field coil "on" by connecting the negative side to ground. Now we have a complete circuit through the battey's positive terminal through the alternator light, through the coil, and to ground. We've got electromagnetism at the coil, and since the circuit is complete, the alternator warning light is ON. (bulb check.)
When the engine starts, the alternator RPMs get up to speed. Now the field coil is spinning inside the stator coils, and the stator coils begin to induce this electromagnetic field and three-phase AC power is generated. The power travels to the rectifier where it's converted to DC, and used to charge the battery. The three-phase power is also tapped and fed through the diode plate, converted to single-phase, and fed to the positive side of the field coil. Now, the alternator is generating its own power, and a very small fraction of that power generated is re-directed back to the field coil for the electromagnetism. Essentially, the alternator is now powering itself. When this happens, the initial current flowing through the alternator light is no longer needed to initially excite the coil (since the alternator is doing that for itself through the diode plate), so that circuit is no longer active. With the circuit not active, the light goes out. This is usually a few seconds after the engine starts. The B-terminal is connected via a thick 8-gauge wire straight to the positive battery terminal for charging.
Now the alternator is self-sustaining. If there's some problem where the alternator isn't producing electricity, there won't be any power generated at the stator coils. When this happens, the alternator won't be able to tap the power for the field coil, so it'll take it from the warning light circuit. Since the warning light circuit is now active again, the warning light comes on. The field coil will still be energized with the warning light on, even though the alternator isn't producing power, indicating battery discharge (the whole purpose of that light).
5. How come my airbag and/or ABS warning lights come on with a bad alternator?
__________________
Dan
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"if you aren't really shifting, you aren't really driving."
Last edited by blackmaxima88 : Oct 26th, 2005 at 10:43 PM.
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