Weird problem with my 91 taurus.

I started my car one day and found that the battery light was on. I called up my dad, since he was a mechanic for a few years back in the 80's. We looked at it, and took the alternator to a shop to have it bench tested. IT came out fine. We then tested the battery, it was fine too. Thinking it could be something else with the battery we hooked up a different one. The light still come on. I was looking all over in the Chilton Manual, and couldn't find anything else about it.

Do any of you guys know what could be causing the light to stay on? Oh yea, when I accelerate, it goes off, but when I let up, the light comes back on.

I believe the 1991 still has a seperate external voltage regulator. The voltage regulator is most likely your problem. I think it may be located somewhere behind your battery. I'm Not sure of the location, just look around and you'll see it if you now what they look like, I'm sure your dad does.

Welcome to the forum!

Keep in mind that a weak battery is not likely to cause a charging system indicator on the instrument cluster. An issue with the alternator or, as mechhound suggested, voltage regulator is most likely.

Make sure the belt is in good condition and not slipping as well.

Per the wiring diagrams available from www.autozone.com (http://www.autozone.com) in the repair guide section, it appears that your 1991 probably has the internal voltage regulator. However, per the "shopping" section, it would appear that it does use an external voltage regulator.

Do you have access to a multimeter and understand how to use one? You could monitor the voltage at the battery posts when the car is idling and the indicator is on, then switch over to the battery cables and see if they are different. Turn the headlights on and see how much the charge voltage drops.

The quick alternator/starter/battery load bench tests are not fool-proof.

-Rod

We tested it, and without the car running, the battery reads at 12 volts. With the car running, the air all the way up and the headlights on, it puts out 14 volts. What does the votage regulator do, and why would it cause a problem?

I just looked at my 1990 Taurus and it does in fact have the external voltage regulator. It is located rigt beside the battery on the fender. I believe your 1991 would be the same. They are easy to change and fairly inexpensive. The regulator tells the alternator how much charge to put out so the battery stays at the correct level of charge and not overcharged. Your light problem is somewhat typical of a bad regulator.

Hi, I have a 90 and mine needed a replacement alt. and regulator when the light came on. That still did not fix the problem. I found a blown resistor (on the back of the instrument cluster) and a shorted wire on the charge indicator light circuit. Check your wires resistance to ground and make sure you don't have a short... I hope you don't have as much trouble as I did...

good luck

Hi, After reading all the posts, Lots of good info here, Anyone ever think of engine to body GROUND circuit? Give it a shot, Clay..