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Yesterday evening a I was leaving work my volt meter dropped from 14 to about 13.2. When I stop at like a red light the car idles reral rough....like its sorta wanting to die. Im not sure but I think the alternator is dying.

one quick way to test is to start the car; and while it is running disconnect your negative terminal off the battery. if the car stays running for a little then shuts off then the alternator is still working properly or decent. if your car shuts off almost instantly than your alternator might be starting to crap out on you. not completely 100% effective, but it can help to give you a general idea and is quick and easy to do.

I thought that diddnt work on modern cars.

^ it shouldn't... the alternator is suppose to provide enought power to keep the car running and charge the battery. so if you disconect the battery, the car should stay on. i think a better way would be to start the car and then turn every electrical accessory on and then see if the alternator can supply power to everything... or you could just take it to your local auto parts store. most places will test your alternator and your battery for free

well one way to tell is your batt light will come on and all those symptoms will happen. Once your batt light comes on turn off your car for about a minute or two and turn it back on and let it idle, if the light is off when you turn on the car on and about 30 sec later it comes back on then you can be sure its your alt. ( Trust me Ive been through about 3 in a month bc my positive lead kept getting disconnected)

I drove home last nite w/ lights & stereo on...i urned the ac on for about 15 sec....I diddnt get a light

well then I dont think its the alternator then, bc it would completely drain your electrical system, ex. Power windows would roll up very slowly, your interior and exterior lights would be dim and etc.

from what i understand, disconnecting the battery while the car is running can mess up solid state components. it was a good test on older cars, but with new technology its a risk i wouldnt take.

it sounds to me like the alternator is taking a shit. if charging goes down AND the car runs rough at idle, those are telltale signs. Also, turn on a lot of accessories like headlights an radio and see if the car seems to run even worse. thats another way to check it. whats the battery's static voltage (car off)?

IIRC like 12.2-12.5......Ill check at my next smoke break

Ok...on/of volts are about like I said...12.2-12.4

running w/ all accrssories off 13.2-13.6

radio on/lights on....same

a/c on....runs same for about the first minuite then slowly starts to drop...5 min and im @ 11.8-12



If I goose the gas it will jump to approx 13.8 then work its way back down.

MoparNeonMan wrote:Yesterday evening a I was leaving work my volt meter dropped from 14 to about 13.2. When I stop at like a red light the car idles reral rough....like its sorta wanting to die. Im not sure but I think the alternator is dying.

do you have any mods like a cai.. you might need to relocate your air temp sensore. I really dont think its your alt thats taking a dump, it just sounds more like the tipical rough idle from a neon.. mine does it all the time. i noticed it reduce when i installed new spark plugs..

Check you AC voltage not just DC. Should be less than 200mV.

half_twisted wrote:do you have any mods like a cai.. you might need to relocate your air temp sensore. I really dont think its your alt thats taking a dump, it just sounds more like the tipical rough idle from a neon.. mine does it all the time. i noticed it reduce when i installed new spark plugs..

the air temp sensor hardly affects the way the car runs. even without it in the air stream the car should run fine. sensing a difference in temperature is only going to slightly change the way the computer controls the fuel/air mixture and timing. Even with an ambient reading of 100+ degrees, it will compensate. The car might not get the best mileage or make quite the same power, but it will run fine.

batteries and alternators use AC only. DC voltage doesnt apply. the PCM changes AC to DC for certain signals, but thats irrelevant in this case.

The FSM states that battery voltage should be somewhere in the 14v range (13.8-14.7v or something like that). if its the alternator, this number will be quite low when the engine is running (anything below 13.5v would be considered low IMO).

how many miles are on your alt? battery?

stock and stock...approx 85K and three years next month

i'd say more then likely it is your battery, go to a parts store with it and they will test it for free and tell if you if the battery is still good.. this way, if its the battery you will know.. if its not, then you will know its the alternator.. idle charge should be 13.7-13.8V and should be 14v+ at anything over 1800rpm. And unhookin the batterys negitive terminal won't tell you anything, so dont do it.

I don't think you're alternator is the problem. Sounds like something else is causing a low/rough idle situation and the voltage is being affected that way. The 13.5 volts you get isn't the alternator. If it was the alternator not working you would be dropping to 12v or lower from a static 12.2/12.4. You're still getting at least a volt over base voltage at idle, which to me doesn't indicate a problem, and if you rev it I'm sure it's well over 14v.

Keep in mind that alternator RPM affects it's output. Anybody who's got an UDP knows this.

fixitmattman wrote:I don't think you're alternator is the problem. Sounds like something else is causing a low/rough idle situation and the voltage is being affected that way. The 13.5 volts you get isn't the alternator. If it was the alternator not working you would be dropping to 12v or lower from a static 12.2/12.4. You're still getting at least a volt over base voltage at idle, which to me doesn't indicate a problem, and if you rev it I'm sure it's well over 14v.

Keep in mind that alternator RPM affects it's output. Anybody who's got an UDP knows this.

I know all about the UDP issue...especially with the stereo I run...the stereo has been OOC for about 3 months. When I rev the car i might spike 14 for a half second...but usually its no higher than 13.8

Ok...took it to a Firestone @ lunch.
Not the battery.


Not the alternator.



The guage.


Autometer cobalt guage is not even 6 months old and its junk. We had the life charging feed from the tested and I compared it to the guage reading.

idle 13.8-14.2

guage reads 13.2-ish


full load dropped to 13.4

guage reads 12-ish

so the guage is a full volt off. Im gonna heck my connections tonite (if theres enough light) and Ill call Jegs tomorrow about getting it replaced.

Still doesnt account for the extremely rough idle tho....Ill pull the plugs saturday and see if that helps.

OB wrote:

batteries and alternators use AC only. DC voltage doesnt apply. the PCM changes AC to DC for certain signals, but thats irrelevant in this case.

HUH????

The car and all batteries run on DC!
You check AC to make sure your diodes in the rectifier are functioning properly not allowing the AC voltage that is actually produced by an alternator get out and mess up the electronics and battery. The computer will monitor and control voltage not rectify the AC to DC or act as an inverter to change DC to AC.

You can have a car that has proper DC voltage all day long but acts funny and has charging problems if one or more of the Diodes has failed letting excessive voltage into he electrical system. Friend just found his problem just that way on his CARAVAN.

I got things a bit mixed up, my apologies. AC is produced by the alternator, then rectified to a DC signal for use in the battery. Not sure what I was thinkin when I wrote that. the above post is indeed correct. my bad