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Discussion Starter · #1 ·
I'm unable to drive faster than 60, without the battery light coming on.

Battery voltage: 12.4 / 12.5 - Passenger / Driver side.

Not yet tested the output of the alternator.

Seems odd the alternator would/could function properly unless I race the engine, or run +/- 1900 RPM (about 60 mph), triggering the battery light to come on, as well as the voltage gauge to drop very low.

I suspect the belt tensioner is calling it quits.

Am I on the right track?
 

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What does the volt meter show when the ALT light is on - and when it's off? Have you looked at the belt with the engine running, or pushed against it with the engine off? If it's slipping on the alternator, there will be an obvious buildup of black powder on the face of the alternator around the pulley; particularly on the side the belt rolls off.

But it's also worth checking the battery terminals, alternator case, alternator bracket, alternator output stud, and the wiring that runs behind the battery for damage. Click these & read the captions:

. . . .

IDK where the 7.3L block ground is, but you should find & clean it:



You should confirm the voltmeter with a DMM:

 

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Discussion Starter · #3 ·
When the light comes on, the voltage gauge drops significantly. When I let off the fuel pedal, light goes out and voltage gauge come back up pretty close to my trucks normal position.

I have watched the belt while the engine was running. I couldn't see any shift in the belt tracking. Belt edges appear to be in good shape and no cracks show. Also, the face of the alternator housing is clear of any black powdery residue.

Appreciate the reference material.

I'll for sure, clean the block ground and pull the alternator to clean up the mounting bosses. Will get some numbers on the output of the alternator too.

Thanks Steve!
 

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While you have the alternator off check or spin all of your pulleys. I usually loosen my belt once a year just to spin and check for play in my pulley's just trying to catch a bad pulley before it leaves me on the side of the road. I have had to replace the pigtail going to my Alt. because it was making intermitten contact and it was making my battery light come on. themue67
 

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When mine was doing similar things it was bad "slip rings". I could easily see it when I pulled the regulator off the alternator. New alternator and no more problem. Cheers!
 

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Discussion Starter · #6 ·
While you have the alternator off check or spin all of your pulleys. I usually loosen my belt once a year just to spin and check for play in my pulley's just trying to catch a bad pulley before it leaves me on the side of the road. I have had to replace the pigtail going to my Alt. because it was making intermitten contact and it was making my battery light come on. themue67
Fully intend to do that. I may opt to replace bearings instead of pulleys, if I can source the right ones.

When mine was doing similar things it was bad "slip rings". I could easily see it when I pulled the regulator off the alternator. New alternator and no more problem. Cheers!
Thank you, Patrick! Googling "3G alternator slip ring" helped me understand with a picture. Lol

Will pull the VR before cleaning the bosses.
http://i200.photobucket.com/albums/aa221/1968_falcon/IMGP1468.jpg
 

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I have had a number of alternators that would work quite fine under a certain RPM but go over that RPM and suddenly there is no charge, which sounds just like what yours is doing. A new or a good rebuilt one should fix you right up.
 

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I couldn't see any shift in the belt tracking.
It won't move fore-aft, so you wouldn't see the track shift. It's just going to slow down imperceptibly. But if it's the result of a bad tensioner, you'd see the belt going slack, or the tensioner NOT moving, or a puff of dust/smoke/rubber powder when the belt slips.
Also, the face of the alternator housing is clear of any black powdery residue.
Then look on & around the harmonic balancer. If it's slipping, you should WASH the belt & pulleys. Start with the engine off, and use a long-handled brush to get as much of the belt grooves (on the belt AND on the pulleys) as you can, using SimpleGreen/PurplePower and LOTS of water. Then start it, but skip the brush this time. :winking:

It's dangerous, but I sometimes use a wire brush on the pulleys with the engine idling if there's heavy rust or thick varnish. I don't hold the brush firmly (so my hand doesn't go with it if it gets into the belt), and I don't put my head in line with the belt (so a flying brush doesn't bounce of my skull). But I still don't recommend it. It's much safer, and only takes a little more time to drop the belt and deglaze the pulleys with the engine stopped.

 

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The slip ring and brushes will go bad about every 100,000 miles. Getting a rebuilt alternator is a real crapshoot, and when mine neededrebuild I tried to buy a new Ford one and even they were rebuilt. Consider rebuilding your own. I got the rebuild kit for $30.00 give or take, and it worked great.
https://alternatorstarterrebuildkits.com/product/3g/
 

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Discussion Starter · #10 ·
So, the battery light (alternator) came on, and stayed on Wednesday afternoon. Pretty well told me what to do next... lol

Called a rebuilder but can't wait on him. Looked at used, then have mine rebuilt... Decided it best to do O'reilly's limited lifetime route. $140 and 15 minutes in the parking lot, She's happy to go another round. ;)

>:) Among gingerly bathing my idler pulleys, belt, etc... Discovered in more reading that it may help running a ground wire from the passenger side battery. That's the battery that shows less capabilities in a load test. May as well run a switch for the glow plugs too being that I'm near the gulf coast; they aren't really called for, save the two months or so of what we call winter.

It won't move fore-aft, so you wouldn't see the track shift. It's just going to slow down imperceptibly. But if it's the result of a bad tensioner, you'd see the belt going slack, or the tensioner NOT moving, or a puff of dust/smoke/rubber powder when the belt slips.Then look on & around the harmonic balancer. If it's slipping, you should WASH the belt & pulleys. Start with the engine off, and use a long-handled brush to get as much of the belt grooves (on the belt AND on the pulleys) as you can, using SimpleGreen/PurplePower and LOTS of water. Then start it, but skip the brush this time.
The belt drive system all looks good. Tensioner in good shape too. They are in need of a good cleaning. May as well put a new belt on, putting the old one in the tool box.



It's dangerous, but I sometimes use a wire brush on the pulleys with the engine idling if there's heavy rust or thick varnish. I don't hold the brush firmly (so my hand doesn't go with it if it gets into the belt), and I don't put my head in line with the belt (so a flying brush doesn't bounce of my skull). But I still don't recommend it. It's much safer, and only takes a little more time to drop the belt and deglaze the pulleys with the engine stopped.
You'll understand, Steve, I laughed hard at this! I could see this being done and ending very bad. Lol

I'm in no rush so, will take my time. My truck, she's suffering from that rusty colored gunk all over the pulleys and idlers. Plus, the idlers are caramel colored now. Can't be beneficial so, off it'll come!



The slip ring and brushes will go bad about every 100,000 miles. Getting a rebuilt alternator is a real crapshoot, and when mine neededrebuild I tried to buy a new Ford one and even they were rebuilt. Consider rebuilding your own. I got the rebuild kit for $30.00 give or take, and it worked great.
https://alternatorstarterrebuildkits.com/product/3g/
Mine had a couple of burnt diodes too. I did look into doing this and it would have cost me the same to have my buddies shop rebuild it for me. :/

Appreciate you folks tolerating my speculations. Glad I asked before throwing parts at it, on my own hunches. ;)
 

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Discussion Starter · #11 ·
I have had a number of alternators that would work quite fine under a certain RPM but go over that RPM and suddenly there is no charge, which sounds just like what yours is doing. A new or a good rebuilt one should fix you right up.
Good to see you're still around, helping people out when they need it. You're exactly right. A rebuilt alt. fixed me right up.

Thought I lucked out with a junkyard find. 2011, 6.2L gasser, 150AMP Alt. Missing the one wire plug in but otherwise, a direct bolt in swap. Good people at Mississippi Truck Dismantlers.
 

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...belt..., putting the old one in the tool box.
I use a bungee cord to keep mine on the underside of the hood. If the one on the engine ever shreds, I don't have to take a step to grab the backup belt, and then get back on the road. You didn't mention if your toolbox is at home, or on the truck.
 

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Yeah, it'd suck to open the hood & find the old belt resting on top of the engine, huh? ;)

 

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Neither is near the fan. They'd fall on the back of the engine.
 
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