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Discussion Starter · #1 ·
Fellas I need some help. Have an 02 7.3 about 3 days ago noticed my voltage light or the “battery light” as most people call it pop up on my gauge cluster when I started my truck to warm it up. Stayed on until I got in it and took off. As soon as I started driving it it went off. Did the same thing the next day so I tested alternated with it running and seen the alternator was only sending out about 12.8v so I replaced it. Put the new on and it’s still only sending out about 13v. Batteries are right around 12.4v with truck off and 12.8ish with truck running. Just to make sure I didn’t get a bad alternator pulled off another one off my other truck that is good put it on and same thing. Tested to make sure voltage was going from alternator to batteries. (One multimeter lead on ground of battery other on the post of the alternator) tested the plug for the alternator and have 12.8v with the key in the on position. All seems well between battery and alternator and the plug that plugs into alternator. Had a guy tell me to check I believe the number 29 fuse under the dash which is a 10 amp and it is good as well… so I am stumped. What am I missing here. Where am I losing voltage? Or why is a brand new alternator only producing about 13v? I thought maybe I have a bad ground somewhere? I’m assuming if I do it has to be a ground that goes from block to frame but I can’t seem to find one to check? Or am I not seeing a fuse somewhere that needs checked? Any help whatsoever would be highly appreciated. Thanks in advance
 

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12.4 resting voltage is a bit low being about 80% capacity. I would start with putting the truck batteries on a charger for several hours while looking at all connections. Your resting voltage should be right at 12.7 maybe 12.6 for full charge.

You have two primary grounds on the front bottom side of the engine that go directly to the batteries on the perspective side. You should also have a ground on the passenger side of the engine block going to the frame this cable is about 12 gauge and maybe a foot long. I do not remember which side but you also should have a 12 gauge or so going from one of the negatives on the battery to a fender if not both sides.

Remove all cables from the batteries and clean them properly with soap and water and a good brush. When you put the connections back on use a good conductive grease made for electrical connections or put them back on dry. DO NOT USE DIELECTRIC GREASE it is an insulator....

While engine off you should have battery voltage at the lug on the alternator if not your fused link is likely bad.

I believe the 2002 still has the start relay on the passenger fender if so you should have a cable from the passenger battery to a molded plug to it. This plug has battery voltage, from there it jumps over by another wire to the glow plug from another cable as well under the same lug if memory is correct is the two fused links from the alternator. This lug must be battery voltage. Engine off here as well.

Pull the molded plug off the alternator and check for battery voltage there as well, if memory serves the center pin should be hot. Again engine off. If you do not have battery voltage at the plug your alternator will never come on line.

Do not just wiggle the lugs check the cable for good condition..... Any corrosion will increase resistance in the line and lower the passed voltage

Bit of a long read but this should get you going.

Edit.....
Also remember Voltage on a battery only tells part of the story. If you want the rest of the story you need a good load tester not the cheep hand held that even most of the part stores have. Those hand held testers work for car batteries not trucks.
 
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Anytime weird electrical issues come up in '01's and above, you want to check a connection that's feeding the fusebox and making the transition between B+ and the Fusible Links feeding it (compare in-cab Voltage to what's coming out of the batteries, but your '02 does not have that starter relay on the inner well, I'd use the battery post and the in-cab "Outlet" (cig ltr))

 

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Discussion Starter · #4 ·
Anytime weird electrical issues come up in '01's and above, you want to check a connection that's feeding the fusebox and making the transition between B+ and the Fusible Links feeding it (compare in-cab Voltage to what's coming out of the batteries, but your '02 does not have that starter relay on the inner well, I'd use the battery post and the in-cab "Outlet" (cig ltr))

Man I can’t thank you enough! that was the problem! Put a new connection block (or whatever it is actually called) and cleaned the lugs and instantly had 14.4 at the alternator and within a couple mins had 14.2 at the batteries. Mine was so corroded I was only getting about a 4v reading on the connection. And it basically crumbled apart when I tried to take the nut off to clean lol What a terrible location/design. Anyways thanks again sir
 

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Discussion Starter · #5 ·
12.4 resting voltage is a bit low being about 80% capacity. I would start with putting the truck batteries on a charger for several hours while looking at all connections. Your resting voltage should be right at 12.7 maybe 12.6 for full charge.

You have two primary grounds on the front bottom side of the engine that go directly to the batteries on the perspective side. You should also have a ground on the passenger side of the engine block going to the frame this cable is about 12 gauge and maybe a foot long. I do not remember which side but you also should have a 12 gauge or so going from one of the negatives on the battery to a fender if not both sides.

Remove all cables from the batteries and clean them properly with soap and water and a good brush. When you put the connections back on use a good conductive grease made for electrical connections or put them back on dry. DO NOT USE DIELECTRIC GREASE it is an insulator....

While engine off you should have battery voltage at the lug on the alternator if not your fused link is likely bad.

I believe the 2002 still has the start relay on the passenger fender if so you should have a cable from the passenger battery to a molded plug to it. This plug has battery voltage, from there it jumps over by another wire to the glow plug from another cable as well under the same lug if memory is correct is the two fused links from the alternator. This lug must be battery voltage. Engine off here as well.

Pull the molded plug off the alternator and check for battery voltage there as well, if memory serves the center pin should be hot. Again engine off. If you do not have battery voltage at the plug your alternator will never come on line.

Do not just wiggle the lugs check the cable for good condition..... Any corrosion will increase resistance in the line and lower the passed voltage

Bit of a long read but this should get you going.

Edit.....
Also remember Voltage on a battery only tells part of the story. If you want the rest of the story you need a good load tester not the cheep hand held that even most of the part stores have. Those hand held testers work for car batteries not trucks.
Thank you for the reply and taking the time to post. Very good info. Problem is resolved now!
 
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