Alt. voltage yo-yoing! WTF.....
Thread Starter
Drifting
Joined: Jul 2004
Posts: 1,321
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From: Dallas Texas
Alt. voltage yo-yoing! WTF.....
I'm about sick of the alternator swinging from 12.5 to 13.9 while driving/stopping. Fuel pressure drops, car feels a lil sluggish! As anyone come up with a fix? I've taken voltage reading from the back of the alt., had it tested, checked grounds, checked everything. And I know a bunch of other guys in my area with the exact same problem. Is there a fix or are we stuck?
Shahram
Shahram
Thread Starter
Drifting
Joined: Jul 2004
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From: Dallas Texas
Safety Car
Joined: Aug 2000
Posts: 4,933
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From: San Rafael CA
Check the connections at the starter. The POSITIVE voltage coming off the alternator goes to the starter motor, then to the battery. Check that the connections are clean and tight at the starter.
CAUTION ... that cable is HOT at all times ... do not let a wrench contact the POSITIVE cable/post and metal parts of the car such as the engine.
CAUTION ... that cable is HOT at all times ... do not let a wrench contact the POSITIVE cable/post and metal parts of the car such as the engine.
Thread Starter
Drifting
Joined: Jul 2004
Posts: 1,321
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From: Dallas Texas
Tech Contributor
Joined: May 2008
Posts: 3,100
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From: Somers, CT and Clermont, FL
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16
The battery could have an intermittant shorted cell.
Try a different battery to see if the voltage becomes stable.
If that doesn't fix it,I would take another hard look at the alternator.
Try a different battery to see if the voltage becomes stable.
If that doesn't fix it,I would take another hard look at the alternator.
Team Owner
Joined: Jul 2006
Posts: 22,167
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From: Kannapolis NC
BlackZ06 gave you the first thing to try. If the nut isn't loose on the starter, the starter post may be burned internally. Disconnect the battery, use a wrench to turn the nut on the positive wire on the starter, if it won't tighten (the post turns), you'll need a starter. I've seen it twice now on two other people's cars. Both cars had headers, so maybe the heat fries the bakelite the post is insulated with, but that's just a guess. Check that before you start buying batteries and stuff.
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Burning Brakes
Joined: Feb 2005
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From: Fayetteville AR
I would check the Alternator. One of our club members said his voltage readout would jump constantly even if driven at a steady speed. Then the car was acting up (sputtering). The fix was a new alt. The dealership told him $600. He told him no thanks and purchased a rebuilt one at AutoZone for like $200.00 
Tech Contributor
Joined: Apr 2007
Posts: 10,924
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From: Forney Texas
do you have an underdrive pulley? When you drive, the voltage should go up and when you come to a stop, the voltage should drop because the alternator is turning slower. If you have an underdrive pulley, the voltage will drop at a higher rpm than it would without it. Is this your complaint? The radiator fans probably draw the most current and when they're on, you should expect a voltage drop as well. The more current load you place on the alternator, the lower your voltage will be. This will be amplified at idle.
Tech Contributor
Joined: May 2008
Posts: 3,100
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From: Somers, CT and Clermont, FL
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16
do you have an underdrive pulley? When you drive, the voltage should go up and when you come to a stop, the voltage should drop because the alternator is turning slower. If you have an underdrive pulley, the voltage will drop at a higher rpm than it would without it. Is this your complaint? The radiator fans probably draw the most current and when they're on, you should expect a voltage drop as well. The more current load you place on the alternator, the lower your voltage will be. This will be amplified at idle.
Very good point!
Thread Starter
Drifting
Joined: Jul 2004
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From: Dallas Texas
No undy's for me. I just don't like the voltage drop when the fans come on or after some hot laps! I can't understand why these cars will not put out 13.7 or higher when there is a load or heat starts to build up like my other cars do. And we can't put a higher amp rating becuse the stupid "SYSTEM CHARGE FAULT" will keep coming on!
Shahram
Team Owner
Joined: Jul 2006
Posts: 22,167
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From: Kannapolis NC
Did you check the starter post as has been suggested?
My car sits on 14.2 volts at idle with or without fans, at highway speed, it does not vary more than a few tenths of a volt. You haven't mentioned whether you checked the post on the starter and I have told you of two cases where it caused what you are seeing. Eventually, the starter will not turn over if you don't fix it. If you haven't checked it, at least take a look or stop asking for advice.
My car sits on 14.2 volts at idle with or without fans, at highway speed, it does not vary more than a few tenths of a volt. You haven't mentioned whether you checked the post on the starter and I have told you of two cases where it caused what you are seeing. Eventually, the starter will not turn over if you don't fix it. If you haven't checked it, at least take a look or stop asking for advice.
Safety Car
Joined: Aug 2000
Posts: 4,933
Likes: 30
From: San Rafael CA
Did you check the starter post as has been suggested?
My car sits on 14.2 volts at idle with or without fans, at highway speed, it does not vary more than a few tenths of a volt. You haven't mentioned whether you checked the post on the starter and I have told you of two cases where it caused what you are seeing. Eventually, the starter will not turn over if you don't fix it. If you haven't checked it, at least take a look or stop asking for advice.
My car sits on 14.2 volts at idle with or without fans, at highway speed, it does not vary more than a few tenths of a volt. You haven't mentioned whether you checked the post on the starter and I have told you of two cases where it caused what you are seeing. Eventually, the starter will not turn over if you don't fix it. If you haven't checked it, at least take a look or stop asking for advice.
Thread Starter
Drifting
Joined: Jul 2004
Posts: 1,321
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From: Dallas Texas
Racer
Joined: Apr 2006
Posts: 256
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From: Huntsville AL
You can do a search for it (not just on Vettes either) to see what it is. But basically you add larger wires from the Alt to the battery, battery to ground and engine block to ground. I used 4ga wire for mine, some people use 2 or 0ga (but I think those generally have larger stereos or just like overkill..lol).
I dont know that this will solve your problem though. The stuff the others guys have said should be checked first (if you havent already). My car had a couple grounds work loose once and the nut attaching the power wire to the starter got loose once too. Any of these things can cause problems for you. BTW.. you did say you had checked that Alt and verified that it is actually varying right? I currently have a High Voltage issue with my car where the IPC reads 3-4 volts higher than the Alt is putting out. After much testing I am about 95% sure its the IPC itself that is messed up. I have one on order to fix it so we will see if thats it.
I dont know that this will solve your problem though. The stuff the others guys have said should be checked first (if you havent already). My car had a couple grounds work loose once and the nut attaching the power wire to the starter got loose once too. Any of these things can cause problems for you. BTW.. you did say you had checked that Alt and verified that it is actually varying right? I currently have a High Voltage issue with my car where the IPC reads 3-4 volts higher than the Alt is putting out. After much testing I am about 95% sure its the IPC itself that is messed up. I have one on order to fix it so we will see if thats it.
Instructor
Joined: Feb 2007
Posts: 100
Likes: 5
I empathize because the Vettes have so much more detailed information than most cars do, and monitoring your DIC is addictive. Still I don't think there is an issue.
The guy who said that he could see his radio lights flicker had an issue, but if the only way you can tell is through the guage; probably not an issue.
Safety Car
Joined: Aug 2000
Posts: 4,933
Likes: 30
From: San Rafael CA
* The voltage regulator on the alternator is flakey, or your PCM is flakey and is commanding the alternator on/off at the wrong times.
* Your battery has a bad cell
Since you say the car has no problem starting ... I'll go for the first one .... the voltage regulator is bad.
Tech Contributor
Joined: May 2008
Posts: 3,100
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From: Somers, CT and Clermont, FL
St. Jude Donor '09-'10-'11-'12-'13-'14-'15-'16
No undy's for me. I just don't like the voltage drop when the fans come on or after some hot laps! I can't understand why these cars will not put out 13.7 or higher when there is a load or heat starts to build up like my other cars do. And we can't put a higher amp rating becuse the stupid "SYSTEM CHARGE FAULT" will keep coming on!
Shahram
Shahram
The alternator's internal voltage regulator has built in temperature compensation,which LIMITS the output voltage when temperatures are hot.
This voltage limit is needed to minimize overcharging the battery,which can result in electrolyte loss.
A 12 volt battery has six 2.2 volt cells in series,which add up to 13.2
volts when it's FULLY charged.
I think you're asking a HOT alternator to output a voltage higher than
it's designed to do.
