[shadowgames81]

The battery light on the dash is coming on but kind of blinking. Sometimes doesn't blink at all and runs good. The car wont start unless I jump it. Question is would the light come on cuz its not charging the battery cuz its no good or is it more than likely a bad alternator?

[SALIV8]

You can have a local autozone or whatever test your battery.
I would think its the alternator myself, or a bad battery connection

[87foxbodylx]

have them both tested at autozone.. its free

[nolife]

My guess is the alternator

If the battery or a terminal was the problem. You could jump or push start it and everything would work fine while it was running. The alternator light would not be on and the voltmeter would be in the normal range but jumpy. Jumpy meaning if you turn the flashers on, it would move a lot between cycles of the lights.

If the alternator was the problem. You can jump the car but eventually everything is going to die. You are basically living off of the battery and it gets weak fast. The alt light would normally be on as well because its output voltage is too low (a really low rpm, underdrive pullies, and lights on may cause it to come on as well).

In your case..
Id say an alternator with a low output because a winding or an internal voltage regulator problem, the voltage regulator has a bridge rectifier in it and it is common for this to go bad which results in half the output. This is kind of keeping you going, you have not died completely but the alternator light is on at times as well.

Short story, take the advice of the others and have AutoZone check it for free.

As a side note. I have bought one at some parts store in the past. Ford uses the same alternator for many cars but the alternator case can be put together in one of three ways 120 degrees apart from each other. They did not have a specific part number for the Mustang per say but the guy was able to take it apart and align it correctly so it matched my original one and the pigtails were in the right place.

[shadowgames81]

I'm not having any fluctuating in lights or dimming or no voltmeter bouncing. The battery light just kinda comes on then goes out. idles stays fine runs for who knows how long cuz ran it for a while and no dying so no clue. Don't wanna take it out cuz haven't got any recent plates or stickers on it.

[cobraetti]

question about the voltmeter bouncing, when I turn on my directional the meter goes up and down along with the blinker rythem. Also it reads low on the voltmeter when at idle(I do have u/d pullies but with stock alt. pulley) How can I be sure that the alt. is weak or faulty?

[Tony_90fox]

Mine will drop a bar or so on the volt meter when i turn on my heater or lights. but it stays charged and everything.

[nolife]

Originally Posted by shadowgames81 I'm not having any fluctuating in lights or dimming or no voltmeter bouncing. The battery light just kinda comes on then goes out. idles stays fine runs for who knows how long cuz ran it for a while and no dying so no clue. Don't wanna take it out cuz haven't got any recent plates or stickers on it.

The fluctuating in lights and voltmeter bouncing is if your battery is bad. When you have a new load on the system, the initial power is drawn from the larger much quicker reacting source which is the battery and the alternator eventually makes up the difference (very basic description). Without a battery or with a very weak battery, nothing is there to make up the initial surge or to cover the load and everything is drawn directly from the alternator. The alternator can not meet the demands as quickly and that is why you would see those fluctuations.

I may not understand the whole picture but I still think it is your alternator. I guess if the car has been sitting for a long time, it may be a really weak battery but even in that situation, the warning light should not come on. Maybe your not driving it is not letting some of these theories of what is wrong play out the whole way to help narrow narrow down the problem.

AutoZone tests the alternator inside the store on a bench tester, you do not need your car to be there, same with the battery.

[nolife]

Originally Posted by 89 ragtop question about the voltmeter bouncing, when I turn on my directional the meter goes up and down along with the blinker rythem. Also it reads low on the voltmeter when at idle(I do have u/d pullies but with stock alt. pulley) How can I be sure that the alt. is weak or faulty?

Well I guess it depends on if it has always done that? HAHAHA I guess you have to use more of a comparison over time but you may be at a point where your battery is not as good as it was in the past. The fluctuations are a result of the battery being at a lower voltage then the alternator is putting out. As the battery gets older (or a terminals gets more corroded), the batteries voltage and capacity goes down (again, basic non electrical description). The bigger the gap, the more the fluctuations. You can measure it with a voltmeter. The battery while car is not running should be about 12.5 volts. With the car running it will be about 13.5. The more stuff you turn on, the lower that voltage gets.

Back in the day, I had a lot of stereo equipment in my Mustang and I also had periods when the car sat for long periods of time and the battery drained. Both of these play havoc on the battery and I got very familiar with the voltmeter movements.
As the case with a large stereo (several high power amps). The battery provides the initial surge of power to the stereo as it thumps away. The alternator makes up for it as it can but the stereo can easily pull more power then the alternator can make up during extended loud volumes. Over time, these surges are depleting the batteries life almost like small charge and discharge cycles. I averaged about 1 battery every 12-18 months (many different brands, they all sucked equally). You could take that same theory to under drive pulleys as well. Same concept but the alternator is now putting out lower current at idle and more is coming from the battery. Not bad but not ideal. Ideally, the alternator provides ALL power and provides a small trickle charge to the battery at the same time. The battery is making up for small differences like the turn signals or the initial rush of cranking the heater to high but nothing else. Anything other then that ideal situation wears down the battery faster (they are cheap to replace anyway).

As for a car sitting.. A battery that gets depleted because you left something on or just from the computer system draining it over time is not good as well. This reduces the capacity of the battery and puts it in a state where it is harder to charge back up then normal. If your battery was old or questionable before that, it is definitely bad now. To recover from that takes a good driving around with the alternator putting out a decent amperage (not just sitting in the driveway at idle) or a decent battery charger (something more then 6 amps/hr).

The issue is most people have a battery that is some where in between all of these conditions.

One thing I forgot to mention in any of my replies is the serpentine belt or tension not being adjusted right, unlike cause though as you would notice slippage on the other devices as well.

[shawn91stang]

take your negative terminal off with the car running if it stays running then your battery is crap if it doesnt then something is wrong with your charging system. Also check your wiring at your alternator and the wiring that goes to your starter relay because there are wires coming from the alternator there that charge the batteries. And also check your ground you must have a very good ground. If you cannot find any of this go to autozone like 87 said and get your alternator and battery checked out

[shadowgames81]

ya I worked at advanced auto I know about all the testers I am just hoping to not have to take it in. I drove it for a month before it set / no problem but since I left it sitting and let the battery die and had it recharged about 4 times I finally got everything done and tried starting it up and it wouldn't start just jumped it and had the ligh on and all

[SALIV8]

Originally Posted by shawn91stang take your negative terminal off with the car running if it stays running then your battery is crap if it doesnt then something is wrong with your charging system. Also check your wiring at your alternator and the wiring that goes to your starter relay because there are wires coming from the alternator there that charge the batteries. And also check your ground you must have a very good ground. If you cannot find any of this go to autozone like 87 said and get your alternator and battery checked out

Ive been told never to disconnect the battery while the car is running- it could fry the computer

[mikebert]

I stopped reading halfway through, when you said you had U/D and stock alternator pulley, so I hope this helps:

Get a smaller alternator pulley!