Why is my battery light on?

Failing voltage regulators are extremely rare. It's much more common to have a corroded connector terminal. You would find that by measuring the two voltages on the two small wires on the back of the alternator. You must take those measurements with the engine running. One will have full battery voltage. The other one must have less than battery voltage but not 0 volts.

Those two voltages will tell us which direction to go next. Usually the 12 volt feed will be there because it is spliced in with the injectors and coil pack so if that entire circuit is dead, the engine wouldn't run.

The second wire should have roughly 4 - 11 volts. The lower the voltage, the greater the DIFFERENCE in voltage between them meaning the greater the electromagnet it's creating and the more output you'll get. The voltage regulator is incapable of drawing that voltage all the way down to 0 volts, so if that's what you find, either the brushes are open from wear and there will be no output from the alternator, or that wire is shorted to ground and you'll have way too much output. Worn brushes are real common and on many older models can be replaced without even removing the alternator from the engine.

If you find exactly the same voltage on both wires, there is an open circuit after that point. It's possible it could be an open regulator circuit but it's much more likely you'll find a break in the wire going to it. You can find that by measuring the voltage at the connector terminals on that wire, or by grounding that wire at various accessible test points. If you ground that wire at a connector terminal and the output goes way up, you know everything is good up to that point.

Sounds like the battery might have been disconnected or run dead recently. You probably did that when you replaced the alternator. If so, the Engine Computer lost its memory and has to relearn "minimum throttle" before it will know when it must be in control of idle speed. It also might not give you the normal "idle flare-up" to 1500 rpm when you start the engine. To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals.

This is like asking a "yes" or "no" question and getting "maybe" as an answer. That voltage you did get could be full battery voltage which would be fluctuating if the charging system was working intermittently. It also could be the control side which should be lower than full battery voltage but will rise to battery voltage if there is an open circuit after it, typically from a corroded connector terminal or possibly from an open voltage regulator in the computer.

By they way, you didn't unplug that connector to take the readings, did you? If you did, you'd still get the full battery voltage on one wire but the lower voltage on the control wire has to come through the alternator so that has to stay plugged in. You can push the meter probe through the back of the connector to get the readings.

If you find 0 volts on the second wire, there is either a bad connection in that plug or there's a worn brush inside the alternator. Worn brushes are pretty common and easy to replace but that shouldn't be a problem if your alternator was recently rebuilt.

That code is for the throttle position sensor. Any chance you unplugged it while the ignition switch was turned on? That code will also prevent the relearn of minimum throttle. You have to coast for at least seven seconds because the computer wants to see high manifold vacuum for that long. You can snap the throttle and get high vacuum for an instant, but not long enough. The only way to get it for seven seconds is to be coasting and that implies you're moving. At the same time the computer sees a low and steady voltage from the TPS. THAT'S when it takes a voltage reading and puts it in memory. If there's any fault code related to that sensor, the computer won't trust the reading and won't put it in memory. Once that voltage is memorized, anytime after that when it sees it again, it will know your foot is off the gas pedal and it has to be in control of idle speed.

If the battery light is off, measure the battery voltage with the engine running. If it's at least 13.75 volts, the system is working.

If the Check Engine light is staying on while driving, measure the three voltages on the throttle position sensor. You must do that with it plugged in. Back-probe through the rubber seals alongside the wires.

One wire must have 5.0 volts; one will have close to 0.2 volts. The third one should have near 0.5 volts at idle and up to around 4.2 or 4.5 volts at wide-open-throttle. If that one is reading correctly, the first two have to be right.

If you do get that 0.5 to 4.5 volt sweep on the signal wire, either there is a momentary glitch or "dropout" of the signal or that voltage signal is not making it back to the Engine Computer due to a break in that wire. You would figure that out by connecting a scanner that displays live data to see what signal voltage the computer is seeing.

If the sensor wasn't unplugged, there must be a momentary dropout in its signal for it to set a code, or the signal wire is broken going to the computer. You would find that by measuring the signal voltage right at that wire at the computer connector or by looking at live data on a scanner.

That's why you have to test things, to see if it's the sensor or wiring. The wire color at the computer is the same as the one at the sensor that varies in voltage as you move the throttle. I have to search for the right service manual. Will get back to you shortly.

The signal wire is orange with a dark blue stripe. If you got that nice 0.7 to 3.7 volts when opening the throttle, the 5.0 volt feed (violet / white), and the ground return, (black / light blue) have to be okay.

The signal wire is pin 35 in connector C1. Even if you back-probe that wire at the computer, there is still the rare chance the terminal there is making a bad connection. That would only show up when viewing the voltage on a scanner.

Under the "sensors" menu all of them will be listed along with their voltages. All you have to do is watch the TPS voltage as you press and release the gas pedal. The voltage should change smoothly and never pop up to 5.0 volts.