Battery and brake light on

When are you getting that twelve volts at the battery; with the engine off or running? Where are you measuring the fifteen volts; at the generator's output stud or at the battery?

The fifteen volts proves the generator is working, but you need to measure that again, right at the battery. If you have considerably less at the battery, there is a break in the wire between the battery's positive post and the generator's output stud. There should be a fairly large fuse bolted into the under-hood fuse box for that circuit.

If your car does not use a fuse for that circuit, there will be a fuse link wire spliced into the circuit.

If you do find that fifteen volts at the battery, see if it drops off as the battery charges up. If it does not, I will have to dig up a wiring diagram to see if some computer controls are involved, but it suggests there is a problem with the voltage regulator. The clue would be the battery voltage increases even more when engine speed is increased.

Dandy. I would not panic just yet. The acceptable range for charging voltage is 13.75 to 14.75 volts. You are a half a volt high which is significant, but not alarming, as long as it does not increase much with higher engine speed.

One thing to consider is the condition of the battery. For voltage regulators to be so small and lightweight, they use "pulse-width modulation" which turns the generator's electromagnet fully-on, then fully-off, around 400 times per second. Without drowning in electrical theory, neither condition produces much heat, the deadly enemy of electronics. The problem is just as with an ignition coil, the generator's field coil develops a huge voltage spike when current flow through it is turned off abruptly. That is most desirable in the ignition coil, but those spikes from the generator are harmful to the rest of the electrical system. Just ask any GM owner about the four to six replacement generators they went through in the life of their car.

The battery is the key component in damping and absorbing those harmful spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. On GM products those spikes will damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals. Other car brands have much fewer problems, but you have to consider how the regulator is responding to those spikes. In some cases the spikes cause an increase in the strength of the electromagnetic field, which in turn increases the generator's output current and voltage. In some cases it reacts to those spikes by cutting back on the strength of the magnetic field to reduce output.

Also, related to an old battery, if it's nearing the end of its life, a large percentage of its lead will have flaked off the plates. That leaves you with, in effect, a smaller battery, but the same charging current. The voltage regulator may be unable to cut back enough to hold system voltage down.

If there is any validity to this story, you will see charging voltage drop when a second battery is connected with jumper cables. Voltage will also drop when many loads are turned on. The biggest loads are the heater fan on high, the radiator fan, head lights, and rear-window defogger.

The "Battery" light is run by the voltage regulator. It means the charging system is not working, but on some models the light also gets turned on for low and for high charging voltage.

Also, on some models, once the generator is up and running, a tiny amount of output current goes right back in to run that field winding. GM's very nice 1986 and older system is a perfect example. Those are self-energizing, but that current to run them has to be "rectified" through three diodes, called the "diode trio". Those are quite different than the other six large diodes used in all AC generators. ("Alternator" is the same thing, but they were developed by Chrysler for 1960 models, and they copyrighted the term). When one of those diodes in the diode trio fails, which was very common, in effect, the voltage regulator turned off one-third of the time. That resulted in the dash warning light turning on dimly. Often there were no other symptoms or problems because those cars did not use much electrical power. It is possible to have a similar problem today, but a computer circuit detects the problems, then turns the warning light on full brightness. We do not see dimly-lit lights any more.

When one of those six output diodes fails, all you will be able to get is exactly one third of the generator's maximum rated capacity. 30 amps from the common 90 amp generator is not sufficient to run the entire electrical system under all conditions. The battery may have to make up the difference until it slowly runs down over days or weeks. Also, it varies by brand and model how the voltage regulator will respond. The output is three-phase alternating current that gets rectified and turned into direct current by the diodes so it can be stored in the battery. When a diode has failed, one of those phases is missing, and when that one is supposed to be occurring, output voltage will drop quite a bit. Some voltage regulators respond to that by bumping up charging voltage. Some systems continue to operate normally, but the drop in voltage results in less current through the field winding, a weaker magnetic field, reduced output current, and therefore reduced voltage.

The point is, with a failed diode, it is quite possible for charging voltage to go up a little. The clue is since the regulator still has control of the system, voltage will not increase when engine speed is increased. I should clarify that three things are always needed to generate current mechanically. That is a wire, (coil of wire works better), a magnet, (an easily-controlled electromagnet, in this case), and most importantly, movement between the two. That's where the drive belt comes in, and is why speed is a factor.

What you should consider now is having the charging system tested at a shop. You did the first step already by measuring charging voltage, but all that means is it's okay to do the rest of the tests. You need a professional load tester to measure "full-load output current" and "ripple voltage". If one of the diodes has failed, as I already mentioned, you will only be able to get one third of the rated current. Ripple voltage is the difference between the highest voltage developed, and the lowest it drops to during the missing phase. Most testers simply show it between "low" and "high" on a relative bar graph. A few that can make printouts will show it as an actual voltage.

The red "Brake" warning light is not related to this problem. If the light is yellow, that is for the Anti-Lock Brake system. It can take a lot of current to run the numerous solenoids in the hydraulic controller. The ABS Computer knows it can't operate those solenoids reliably with low system voltage. It will turn itself off, then turn on the yellow warning light to tell you. I suspect on your car the ABS Computer gets a command from the charging system when there's a problem worthy of turning on the "Battery" light. It does not know the cause is not low system voltage. It just knows there is a problem. Worry about the charging system first. If that does not take care of the ABS light, you will need to find a mechanic with a scanner that can access the ABS computer to read and record the diagnostic fault codes. Those will indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.

Dandy. Without looking at the wiring diagram, I suspect that fuse is in the circuit the regulator uses to sense system voltage. Happy to hear you solved it.