Batteries or Alternator
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Elder User
Joined: Dec 2002
Posts: 797
Likes: 1
From: UT
Batteries or Alternator
I was driving home last night, noticed the headlights looked a bit dim. Voltage on the dash also looked quite a bit low too. I think I have a low constant draw from the stereo, as the batteries won't quite last 2 full weeks (drive the car during the week) without being recharged. So I grabbed the meter while it was still running, headlights off, 10.1v on both batteries. I'm thinking Alternator....
Lead Driver
Joined: Dec 2006
Posts: 7,092
Likes: 145
From: San Diego, CA
Once the truck is running, disconnect both batteries and check the voltage on the cables again. A dead battery with failed cells will be quite a draw on the system. microdevil45 is right, you should see 14.4VDC output from your alternator at full rated output (perfect cables and higher RPM). At idle, you should definitely see in the 13VDC range so that you would be charging your batteries (around 12VDC fully charged).
Determining the problem is the hardest part. There are so many possibilities that it makes it difficult to figure out what is the problem and what is the result of the problem.
I hope this helps and best of luck finding the culprit!
Determining the problem is the hardest part. There are so many possibilities that it makes it difficult to figure out what is the problem and what is the result of the problem.
I hope this helps and best of luck finding the culprit!
Senior User
Joined: Jan 2007
Posts: 355
Likes: 0
From: Wichita, KS
I agree with brian42, test your alt output first. If your not comfortable doing this, drive it to closet autozone, etc and have them test it. If you have a small stereo draw, it shouldn't kill these batteries. Sounds like bad batteries to me though.
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Elder User
Joined: Dec 2002
Posts: 797
Likes: 1
From: UT
Thanks guys. I will charge the batteries up tonight, start it, unplug them and measure the voltage. I'm leaning toward the Alt. because its the original stock one with 78K on her. If its the batteries, bye bye Optima (2 sets in 4 years).
Lead Driver
Joined: Feb 2000
Posts: 5,374
Likes: 46
From: Palm Bay, Florida
Have a qualified auto electric shop test your charging system, and batteries, NOT AUTOZOO.
Optimas used to be a GREAT battery when they were made here in the USA, but since they moved they are crap-I was about to buy a set when I heard that.
MY local auto electric shop sold me 2 750cca Motorcraft batteries 2 years ago, so far no problems, lots of opinions on battery brands and CCAs......MY guy says NOT to get the 850cca Motorcrafts because the extra material inside is too much for the same size case as the 750cca and the battery cannot do its job properly.
And unless you have a heck of a demand on your alternator 78k is nothing, mine has 115k and is perfect as new and others have LOTS more miles than I do on an original/stocker.
ALSO: IF it is determined now or later that your alternator is bad have IT rebuilt by a local, known reputable auto electric shop, DO NOT buy a reman from any auto parts store chain-they are generally crap.
Optimas used to be a GREAT battery when they were made here in the USA, but since they moved they are crap-I was about to buy a set when I heard that.
MY local auto electric shop sold me 2 750cca Motorcraft batteries 2 years ago, so far no problems, lots of opinions on battery brands and CCAs......MY guy says NOT to get the 850cca Motorcrafts because the extra material inside is too much for the same size case as the 750cca and the battery cannot do its job properly.
And unless you have a heck of a demand on your alternator 78k is nothing, mine has 115k and is perfect as new and others have LOTS more miles than I do on an original/stocker.
ALSO: IF it is determined now or later that your alternator is bad have IT rebuilt by a local, known reputable auto electric shop, DO NOT buy a reman from any auto parts store chain-they are generally crap.
Lead Driver
Joined: Dec 2006
Posts: 7,092
Likes: 145
From: San Diego, CA
By the way, both of my alternators are the originals (110A rated output) and they are still going strong at 217K.
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Elder User
Joined: Dec 2006
Posts: 718
Likes: 2
From: Chattanooga, TN
I use a VOM meter and check the voltage between each cell - you can then see which cell is causing the overall low voltage. This is done by probing each cell with the lead stuck down in the acid and read the voltage. At the end cells just probe the cell with one lead and the post with the other lead and read the voltage.
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Lead Driver
Joined: Dec 2006
Posts: 7,092
Likes: 145
From: San Diego, CA
I use a VOM meter and check the voltage between each cell - you can then see which cell is causing the overall low voltage. This is done by probing each cell with the lead stuck down in the acid and read the voltage. At the end cells just probe the cell with one lead and the post with the other lead and read the voltage.
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Elder User
Joined: Dec 2002
Posts: 797
Likes: 1
From: UT
I've also heard they(Optima's) aren't what they used to be. My first set went south and would no longer hold a charge. I sent them back under warranty, and they sent me two new ones. Freight alone cost as much as one new battery. If they turn out to be bad again I will go back to lead acid's. My original Moto's lasted way longer. My thought was the Red Tops would last longer on some of the light gravel roads I drive to get to the trail head....IDK.
My stereo system isn't that big of draw the way I use it. I do have two amps, 1-12" and 2-10" subs. But I rarely turn it up...
These HO look interesting:
https://www.dcpowerinc.com/fit/2000~...6-190-oem.html
My stereo system isn't that big of draw the way I use it. I do have two amps, 1-12" and 2-10" subs. But I rarely turn it up...
These HO look interesting:
https://www.dcpowerinc.com/fit/2000~...6-190-oem.html
Thread Starter
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Elder User
Joined: Dec 2002
Posts: 797
Likes: 1
From: UT
So last night I charged both batteries fully. Tonight I started the truck and unplugged the positive on both batteries to measure voltage off the alternator. It immediately killed the motor. Each battery measured 11.4v
I'm thinking the batteries might not be 100%, but the alternator isn't charging them either...
I'm thinking the batteries might not be 100%, but the alternator isn't charging them either...
Fleet Mechanic
Joined: Apr 2011
Posts: 1,441
Likes: 6
From: Marseilles IL
If your batteries are charged and read 11.4 they are dead. A fully charged battery should be at least 12.5. Optima batteries if I remember don't have enough cranking amps for our trucks anyway. Optima batteries have to be used every day, if not they won't last! I have had many Optimas and if they sit they die fast. If there siting they need to have a battery tender on them at all times. Not worth the price or trouble. Get a good set of Interstate or Motorcraft. I don't know if our trucks are different but usually if you pull the battery cables off the battery and the vehicle dies the alternator is bad. Truck running with charged batteries and good alternator the voltage should be 14 to 14.2v.
Thread Starter
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Elder User
Joined: Dec 2002
Posts: 797
Likes: 1
From: UT
If your batteries are charged and read 11.4 they are dead. A fully charged battery should be at least 12.5. Optima batteries if I remember don't have enough cranking amps for our trucks anyway. Optima batteries have to be used every day, if not they won't last! I have had many Optimas and if they sit they die fast. If there siting they need to have a battery tender on them at all times. Not worth the price or trouble. Get a good set of Interstate or Motorcraft. I don't know if our trucks are different but usually if you pull the battery cables off the battery and the vehicle dies the alternator is bad. Truck running with charged batteries and good alternator the voltage should be 14 to 14.2v.
The odd part is my brother has a '99 with Optima Red Tops just like mine. His can sit a month without being started and it starts no problem.
Looks like the Optima RT is 800CCA. Motorcraft Max is 750CCA.
