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Old 05-30-12, 03:59 PM   #1
Bocatrip
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Default Alternator output

I have an ancient Sears engine analyzer which I hooked up to my LS430 to check the alternator voltage output. While the car was warming up it showed 13.6 volts and once hot went down to 13.2 with the a/c on at idle. I was not able to rev it up to see if it went up. I have been having my battery checked since it loses a charge after sitting for 30 days. I have never in 2 years had an issue with my car not starting. I checked my Infiniti G35 when cold with the same analyzer and it showed 14 volts. Does anyone know what the normal output should be for our cars and is it possible that a bad battery can have an affect on the readings? If I never had a dead battery in all this time should I still be thinking that my alternator is weak? Any feedback would be appreciated. Thanks all.
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Old 05-30-12, 07:54 PM   #2
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I think its a problem if the alternator dips below 12 v at idle with all accessories on. 13.6 at idle with nothing much on is likely normal but I am only a weekend hobby diy car guy so I could be stone cold dead wrong. If it were me I'd take it to my local Walmart - they can do a free battery and electrical check - takes 5 minutes and you get a print out on the results. I always buy my batteries at Walmart and have never had a problem in 20 years. Do not under any circumstances take your car to NTB.
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Old 05-30-12, 09:08 PM   #3
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I think its a problem if the alternator dips below 12 v at idle with all accessories on. 13.6 at idle with nothing much on is likely normal but I am only a weekend hobby diy car guy so I could be stone cold dead wrong. If it were me I'd take it to my local Walmart - they can do a free battery and electrical check - takes 5 minutes and you get a print out on the results. I always buy my batteries at Walmart and have never had a problem in 20 years. Do not under any circumstances take your car to NTB.
Thanks for the info Jabberwock. I wish someone else on the forum was able to take readings so I could make a comparison. No 2 cars are the same with alternator output and it would be nice to know. I'm planning on trying to get the battery replaced anyway because it is 5 years from the manufacturer's date which is pushing it here in Florida. A fresh battery always helps.
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Old 05-30-12, 09:47 PM   #4
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A battery voltage measurement won't tell you much about the alternator. You need to measure battery voltage at three times:

1) With car off, just battery voltage no load, no charge
2) With engine running, but everything switched off that can be switched off (should be north of 13.3VDC
3) With engine running ad everything that uses power turned on. high beams, HVAC, blower fan max, bun warmers, audio, wipers, rear defrost, ....

If battery is Above 13VDC on test 3 your alternator is fine. If battery is below 12VDC on test 1 the battery is suspect.

My bet is battery is done, they don't last forever. Most auto parts stores do a battery charging system health test for free.
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Old 05-31-12, 06:43 AM   #5
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A battery voltage measurement won't tell you much about the alternator. You need to measure battery voltage at three times:

1) With car off, just battery voltage no load, no charge
2) With engine running, but everything switched off that can be switched off (should be north of 13.3VDC
3) With engine running ad everything that uses power turned on. high beams, HVAC, blower fan max, bun warmers, audio, wipers, rear defrost, ....

If battery is Above 13VDC on test 3 your alternator is fine. If battery is below 12VDC on test 1 the battery is suspect.

My bet is battery is done, they don't last forever. Most auto parts stores do a battery charging system health test for free.

Performed the following test this morning:

Please note that all tests were done with an old Sears Analyzer from the battery terminals only. I have nothing connected to the the alternator.

1) With car off overnight and not running the battery read 12 Volts at the terminals
2) Starting the car cold with everything off, the car read 13.8 Volts but the voltage does drop over time.
3) With everything on (lights, heated seats, a/c, radio) the alternator read 13.1 volts for approximately 10-15 minutes.
4) With shutting everything back off, the alternator read 13.2 volts
5) Shutting the car off, the battery read 12.4 volts but I could see it dropping over time (if that is normal).

My only concern is, why does the alternator start out cold putting out 13.8 volts and after warming up stay at 13.2 volts? Is the regulator controled by the car's ECU (engine control unit?) I read somewhere that if the alternator puts outs at least .5 volts over the battery reading at rest it will charge the battery. Regardless, I will attempt to get the battery changed today.
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Old 05-31-12, 07:10 AM   #6
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Your alternator seems fine to me. If the battery is 5 years old you are probably doing a good thing by changing it before it leaves you stranded. Its such a cheap and easy thing to do and in fact many places will install it free of charge if you buy it there. I'm one of those people that "if it aint broke don't fix it" but batteries usually don't give you a warning like in the old days which for me was the 70's. Back then a car would be slow to crank for a while and you had time to replace the battery. The last 2 batteries I replaced (2 different cars 02 LS430 and an 07 Acura TLS) were all the same way. They seemed perfectly fine, a few hours later went to use the car and nothing......

I bought my car used and I was looking through all the maint. receipts I was given a few days after I bought it and it had a battery installed at a Lexus dealer 7 years ago. Although it seemed perfectly fine I said to my wife that I was going to change the battery in the next few weeks just to be safe. I came home one day and parked the car in the garage as always and 3 hours later went out to run one of the kids somewhere and it was nearly dead. Things started beeping, the steering wheel wouldn't move into position, the mirrors stopped half way and I couldn't even get a click out of the starter. I yanked out the battery, brought it to the parts store for a test and they said it tested okay. I bought a battery anyway, popped it in and its been great ever since.
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Old 05-31-12, 09:16 AM   #7
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Originally Posted by Bocatrip View Post
Performed the following test this morning:

Please note that all tests were done with an old Sears Analyzer from the battery terminals only. I have nothing connected to the the alternator.

1) With car off overnight and not running the battery read 12 Volts at the terminals
2) Starting the car cold with everything off, the car read 13.8 Volts but the voltage does drop over time.
3) With everything on (lights, heated seats, a/c, radio) the alternator read 13.1 volts for approximately 10-15 minutes.
4) With shutting everything back off, the alternator read 13.2 volts
5) Shutting the car off, the battery read 12.4 volts but I could see it dropping over time (if that is normal).

My only concern is, why does the alternator start out cold putting out 13.8 volts and after warming up stay at 13.2 volts? Is the regulator controled by the car's ECU (engine control unit?) I read somewhere that if the alternator puts outs at least .5 volts over the battery reading at rest it will charge the battery. Regardless, I will attempt to get the battery changed today.
It seems your alternator is fine. It was able to maintain battery voltage at idle with max load. It is putting out the power.

There is an outside chance it is a phantom electrical load (key off current). This can happen due to the stator plate diodes in the alternator going bad and leaking current when the car is off and sitting. Its a rare issue though so I would try the battery first. 5 years is getting to end of life for a battery and your hot climate is harder on them.
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Old 05-31-12, 12:50 PM   #8
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Originally Posted by curiousB View Post
It seems your alternator is fine. It was able to maintain battery voltage at idle with max load. It is putting out the power.

There is an outside chance it is a phantom electrical load (key off current). This can happen due to the stator plate diodes in the alternator going bad and leaking current when the car is off and sitting. Its a rare issue though so I would try the battery first. 5 years is getting to end of life for a battery and your hot climate is harder on them.
Thanks everyone for their great imput.....

I just got an AC Delco Professional 7 year battery installed (identical to the battery that was replaced). It's a free replacement for the first 50 months and considering the original battery was still cranking after 5 years I went for it again. My reliable local mechanic had my alternator showing 13.3 volts with a/c on which is similar to my 13.1-13.2 volt output with everything on. Although my mechanic told me he would have preferred to see 13.5 volts from the alternator under load, I have a feeling that if the original battery and this alternator output took me for the last 2 years without an issue, hopefully I should be good for a few more years.
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Last edited by Bocatrip; 05-31-12 at 12:56 PM..
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Old 05-31-12, 12:50 PM
 
 
 
 
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