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Old 11-24-2004, 01:19 PM   #1 (permalink)
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Question signs of bad alternator?

Guys,

I think my alternator is going bad, but I need a second opinion. Here are the simptoms

1. When the car is off battery voltage is 12.5V
2. At idle (700 RPM, no load) voltage is ~14V
3. At idle with full load (heater on, lights on, defogger on) voltage drops to ~12.5V

From my experience with my other car, this indicates a bad alternator, since it has to always keep the voltage above 13V no matter what. If it does not, the battery is drained at idle. Am I right?

Can it be anything else? The voltage is fine when I cruise. Can it be belt slipping? I don't want to adjust my idle to fix that. Can anyone run his car at idle with full load (item 3) and check the battery voltage?

Thanks
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Old 11-24-2004, 02:04 PM   #2 (permalink)
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Default Re: signs of bad alternator?

Quote:
Originally Posted by Blue94VR4
Guys,

I think my alternator is going bad, but I need a second opinion. Here are the simptoms

1. When the car is off battery voltage is 12.5V
2. At idle (700 RPM, no load) voltage is ~14V
3. At idle with full load (heater on, lights on, defogger on) voltage drops to ~12.5V

From my experience with my other car, this indicates a bad alternator, since it has to always keep the voltage above 13V no matter what. If it does not, the battery is drained at idle. Am I right?

Can it be anything else? The voltage is fine when I cruise. Can it be belt slipping? I don't want to adjust my idle to fix that. Can anyone run his car at idle with full load (item 3) and check the battery voltage?

Thanks
Those aren't symptoms of a failing alternator. You're battery won't start draining until you are under 12V and idling even at full load doesn't even last that long. New these cars had either 90A alt's or 110A alt's. This is just sufficient enough so you shouldn't really expect over 13V at idle under full load. Now when you're getting 12.5V with only a few accessories and you're above 2000 rpm's you may have an issue.

Last edited by falconey; 11-24-2004 at 02:12 PM.
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Old 11-24-2004, 02:42 PM   #3 (permalink)
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Default Re: signs of bad alternator?

It is normal; with all those loads the alternator can not produce enough voltage and current to overcome the demand at idle. Roughly 700RPM is about 2100 rotor RPM. Stock Alternators produce it's full output at about 6000 rotor RPM and that is about 2000RPM in your car.
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Old 11-24-2004, 03:23 PM   #4 (permalink)
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Default Re: signs of bad alternator?

so a stock alternator at 2000rpm(engine) should produce 14.3v? or 13.6? or something else...

i have squealling belts and low readings from my turbo timer volt gauge. i adjusted for the belts, but they are starting to come back. and i still have what i consider "bad" volatage. and a tester revealed 12v at idle 11.x with the load switch on
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Old 11-24-2004, 03:56 PM   #5 (permalink)
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Default Re: signs of bad alternator?

Thanks. I'll keep my eye on it though. I remember reading in Haynes and couple of places online that it should always stay above 13V, but maybe this does not apply to our alternators.
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Old 11-24-2004, 04:14 PM   #6 (permalink)
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Default Re: signs of bad alternator?

whenever i have my accessories (radio,lights,heat,wipers ie..) my alternator will be about 12-12.5 volts but when i turn the heat off it jumps back up
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Old 11-25-2004, 07:36 AM   #7 (permalink)
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Default Re: signs of bad alternator?

@ flux & blue

If you haven't upgraded your charging system wiring do so with 0, 2 or 4 guage wire. These are the wires from alt to bat +, bat - to ground, starter. Also a ground kit will help with electic flow.

@flux

You definitely don't want to be going under 12v although that's not a fair test when you put full load at idle. Your symptoms are not a sure sign that you're alternator is bad. Like you said it could be your belt problem. If the belts aren't revving as fast as normal that's less energy produced. Also if you haven't done my above suggestion try that it will help.
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Old 11-26-2004, 07:06 PM   #8 (permalink)
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Default Re: signs of bad alternator?

FALCONEY

i just finished putting together a ground kit, i need to make one more from the alt ground tho. i think i will do a 4 gauge power wire too... thanx for the idea.

this morning for the first time that i remember my voltage was at 13.8 so i'm really starting to think i have a belt problem. they seemed pretty loose when they were squealing so i went as tight as possible before they started to squeal again... every now and then they get alittle whiny tho... i also found (my logger) that my tach is off about 150 short of what it reads.

so now i have to mess with the BISS screw. shes just acting wierd lately. i'm getting quicker spoolup since i did the wire kit/ belt adjustment but my gas/mph went down from avg of 350 on the tank to 280.... i gotta do some more logs and check my tps curve. i'm kinda baffled. i want to figure out whats goin on before i install my MBC and do the vac reduc.

damn thos throttle body/ elbows go back on tough! xcuse the rambling i'm just tryin to show the bigger picture
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