Erradic Voltage
Racer
Joined: May 2008
Posts: 316
Likes: 0
From: San Deigo California
My 2 cents.
Remember you are measuring BATTERY voltage. And this should not vary so much unless:
A) the battery is bad
B) there is high resistance in a connection from the battery.
So - check that the battery connections are tight, not corroded, etc. If they look good, get out your volt meter. Put it right on the battery - not on a connector, but get in there and make sure the voltmeter probes are actually touching the battery. Then see if the meter in the car reads / fluctuates the same as the voltmeter. If it does, I don't know the problem, but guess bad battery. If it doesn't, then keep moving the voltmeter probes back along the main power cables until you see where the voltage is varying. The point of change is the problem. If you live in San Diego call me 858-243-2736 and I'll help.
Remember you are measuring BATTERY voltage. And this should not vary so much unless:
A) the battery is bad
B) there is high resistance in a connection from the battery.
So - check that the battery connections are tight, not corroded, etc. If they look good, get out your volt meter. Put it right on the battery - not on a connector, but get in there and make sure the voltmeter probes are actually touching the battery. Then see if the meter in the car reads / fluctuates the same as the voltmeter. If it does, I don't know the problem, but guess bad battery. If it doesn't, then keep moving the voltmeter probes back along the main power cables until you see where the voltage is varying. The point of change is the problem. If you live in San Diego call me 858-243-2736 and I'll help.
Team Owner
Joined: Mar 2008
Posts: 21,502
Likes: 461
From: Southern Utah
My 2 cents.
Remember you are measuring BATTERY voltage. And this should not vary so much unless:
A) the battery is bad
B) there is high resistance in a connection from the battery.
So - check that the battery connections are tight, not corroded, etc. If they look good, get out your volt meter. Put it right on the battery - not on a connector, but get in there and make sure the voltmeter probes are actually touching the battery. Then see if the meter in the car reads / fluctuates the same as the voltmeter. If it does, I don't know the problem, but guess bad battery. If it doesn't, then keep moving the voltmeter probes back along the main power cables until you see where the voltage is varying. The point of change is the problem. If you live in San Diego call me 858-243-2736 and I'll help.
Remember you are measuring BATTERY voltage. And this should not vary so much unless:
A) the battery is bad
B) there is high resistance in a connection from the battery.
So - check that the battery connections are tight, not corroded, etc. If they look good, get out your volt meter. Put it right on the battery - not on a connector, but get in there and make sure the voltmeter probes are actually touching the battery. Then see if the meter in the car reads / fluctuates the same as the voltmeter. If it does, I don't know the problem, but guess bad battery. If it doesn't, then keep moving the voltmeter probes back along the main power cables until you see where the voltage is varying. The point of change is the problem. If you live in San Diego call me 858-243-2736 and I'll help.
Burning Brakes
Joined: Oct 2006
Posts: 1,011
Likes: 6
My guesses are:
1) Battery is bad, going to get worse. You could just do nothing and the battery will totally fail sometime soon.
2) Bad cable connections. The voltage in our Cadillac bounced around (from about 11.9 to 15.3 volts) the entire 1,500 mile trip to south Florida. I pulled the rear seat to get to the battery and tightened the ground and all other connections back there - the variations in voltage stopped.
1) Battery is bad, going to get worse. You could just do nothing and the battery will totally fail sometime soon.
2) Bad cable connections. The voltage in our Cadillac bounced around (from about 11.9 to 15.3 volts) the entire 1,500 mile trip to south Florida. I pulled the rear seat to get to the battery and tightened the ground and all other connections back there - the variations in voltage stopped.
Thread Starter
Advanced
Joined: Jun 2007
Posts: 56
Likes: 0
From: Pharr TX
Thanks again for all your feedback guys. Truly in you all's debt. 
I haven't gotten any service charging system messages. My baby has 26,159 miles on her.
I ended up pulling the alternator yesterday and tried getting it bench tested but the local auto zone, advance auto parts, and o'reilly didn't have the connectors to test it.
My dad went to drop it off today with a buddy of his who repairs alternators. He is going to check it out for me and repair if necessary.
I will definitely keep you all posted on the results.
I haven't gotten any service charging system messages. My baby has 26,159 miles on her.
I ended up pulling the alternator yesterday and tried getting it bench tested but the local auto zone, advance auto parts, and o'reilly didn't have the connectors to test it.
My dad went to drop it off today with a buddy of his who repairs alternators. He is going to check it out for me and repair if necessary.
I will definitely keep you all posted on the results.
Thread Starter
Advanced
Joined: Jun 2007
Posts: 56
Likes: 0
From: Pharr TX
Ok so the results are in. For those of you who guessed alternator...you are correct. Regulator and diodes need replacing. Price for repair $120.
I sincerely thank all of you for your help. This is an awesome site.
I sincerely thank all of you for your help. This is an awesome site.
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Race Director
Joined: Apr 2007
Posts: 16,025
Likes: 1,656
From: Toronto, Canada
C7 of the Year - Unmodified Finalist 2021
This was typical behaviour of GM alternators and I've owned a few GM vehicles so it wasn't difficult to assess.
Team Owner
Joined: Mar 2008
Posts: 21,502
Likes: 461
From: Southern Utah
Last edited by cclive; Jun 3, 2008 at 07:46 PM.
Melting Slicks
Joined: Apr 2002
Posts: 2,397
Likes: 55
From: Dove Mountain, AZ
If I were you I would still get a load check done on the battery. On my 04 I had taken it to the dealer for the volatage fluctuation and they said it was fine, on week later I went to start the car and it was dead. They put in on a truck and took it to the dealer. The battery had a bad cell and took out the alternator. They had to replace both.
The 05 was rock steady just above 14 volts and then started with the fluctuation. Finally the battery was dead one morning (18000 miles on the car) and I jumped it and took it to the dealer. They replaced the battery
(bad cell) and now it is rock steady again and it has been 3 months.
So it would not hurt to have a load check done.
The 05 was rock steady just above 14 volts and then started with the fluctuation. Finally the battery was dead one morning (18000 miles on the car) and I jumped it and took it to the dealer. They replaced the battery
(bad cell) and now it is rock steady again and it has been 3 months.
So it would not hurt to have a load check done.
Pro
Joined: Jul 2007
Posts: 554
Likes: 0
I've been having some funny voltage readings on my 05 for the past week. The voltage has been fluctuating anywhere from 11.8 - 14.3 within the same drive. I've noticed that in the afternoons (3pm - 5pm) the voltage seems to drop more and be unstable as compared to the mornings and evenings when it's cooler outside. I'm not sure if this even has anything to do with it or not...just an observation. I was hoping you all could help me out with any advice or expertise. I appreciate it.
Brian
Thread Starter
Advanced
Joined: Jun 2007
Posts: 56
Likes: 0
From: Pharr TX
Let me try again..."for those of you who diagnosed the problem as being the alternator..."
If I were you I would still get a load check done on the battery. On my 04 I had taken it to the dealer for the volatage fluctuation and they said it was fine, on week later I went to start the car and it was dead. They put in on a truck and took it to the dealer. The battery had a bad cell and took out the alternator. They had to replace both.
The 05 was rock steady just above 14 volts and then started with the fluctuation. Finally the battery was dead one morning (18000 miles on the car) and I jumped it and took it to the dealer. They replaced the battery
(bad cell) and now it is rock steady again and it has been 3 months.
So it would not hurt to have a load check done.
The 05 was rock steady just above 14 volts and then started with the fluctuation. Finally the battery was dead one morning (18000 miles on the car) and I jumped it and took it to the dealer. They replaced the battery
(bad cell) and now it is rock steady again and it has been 3 months.
So it would not hurt to have a load check done.
I sure will.
Team Owner
Joined: Mar 2008
Posts: 21,502
Likes: 461
From: Southern Utah
If the battery is the original one, then it could be up to four years old...it will definitely not pass a load test as a new one would. Any electrical system in a car should be able to maintain 13.8 to 14.5 volts under any reasonable load, including high current accessories like window defrosters or heated seats. If the voltage while running is fluctuating more than a few tenths of a volt, then there is a problem that should be fixed. Lower voltage will damage a battery if continued for a long time. All voltage regulators have a voltage that they will maintain and it can be a bit different for each one due to manufacturing tolerances, but once you know what that voltage is, it shouldn't fluctuate much or something is wrong. All this applies when the engine is running at speed, at idle sometimes the alt is not being spun fast enough to keep up, but at 80mph on the highway, more than a few tenths of a volt is not acceptable.