Mechanics

WHY IS MY BATTERY LIGHT ON?

1999 Plymouth Voyager • 138,000 miles

I've had a lot of trouble with this can for the last few months. I had the alternator rebuild and had a new battery installed. Van would start and run fine but when you slowed down it would die. This happens every time. I had the ignition system tested and everything passed but the voltage regulator. Bought a remanufatured computer and had it flashed. Now van runs really rough and the battery light is still on. Had the system tested again and the same thing. Fail voltage regulator. What would cause the voltage regulator to keep failing? What else can I test? What would cause the van to keep firing everyone I push the brakes to slow down?
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Greenchevymalibu
June 7, 2012.



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.

Caradiodoc
Jun 7, 2012.
If the two wires have good voltage then what? And if they have bad voltage I'm assuming that the wires have a break or corrosion?

Tiny
Greenchevymalibu
Jun 7, 2012.
Would this problem cause the van to die everything I come to a stop?

Tiny
Greenchevymalibu
Jun 7, 2012.
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.

Caradiodoc
Jun 7, 2012.
I test the two wires but not sure if it's right. On one wire the readings where flexuating from 10. Xx to 13. Xx to 14. Xx. The other wire I couldn't get a reading.

Tiny
Greenchevymalibu
Jun 9, 2012.
I'm testing the pigtail that plugs in to the alternator?

Tiny
Greenchevymalibu
Jun 9, 2012.
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.

Caradiodoc
Jun 9, 2012.
Pigtail was plugged in. I'll test again and look for corrosion.

Tiny
Greenchevymalibu
Jun 9, 2012.
Ok I got the battery light off but now I have a check engine light on. P0123. It still also dies when ur slowing down/ low idle.

Tiny
Greenchevymalibu
Jun 15, 2012.
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.

Caradiodoc
Jun 16, 2012.
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