Alternator issues
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Cross-Country
Joined: Sep 2024
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Alternator issues
So I have a 1987 f250 with the carbureted 351 w and for a while it’s been having battery issues and a week ago I was driving it and my lights went very dim, radio cut out and the voltmeter dropped pretty much to zero and the truck stalled. So naturally I assumed a bad alternator so I bought a new one and installed it. (This alternator is for a 1990 but the 1987 alternators were all different from the old one that was on it and this one matches it perfectly) so I mounted it in and wired a new connection in. Sure enough I start it and I disconnect the battery to see how it’s working and it immediately stalls. When the truck is off the battery reads 12.4 v and when the truck is running it is at about 11.9 v the the voltage is the same on each side of the fuseable link that is on the wires from the alt to the battery so that is not the problem. I want to test the alternator to see if it’s even working. Where do I test it? It’s not the easy ones where there are posts or open connections to test on. There is just the two plastic connections so where do I test this thing to make sure it’s working? Any help is much appreciated
Hotshot
Joined: Jul 2020
Posts: 10,534
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From: Near Salt Lake City
Well, it sounds like your new alternator is not charging
Could be a fuse link got burnt up from the old alternator
Your 87 alternator should have a post you can ground to full field it
That post is a torx screw on the back of the internal regulator (says ground to test) on the plastic with an arrow pointing to the screw
Doubt yours will start charging if the fuse link is burnt but that's how to test it
You need a power distribution diagram out of a 1987 F series EVTM
That will show the fuse links and the alternator connections
Good luck
Could be a fuse link got burnt up from the old alternator
Your 87 alternator should have a post you can ground to full field it
That post is a torx screw on the back of the internal regulator (says ground to test) on the plastic with an arrow pointing to the screw
Doubt yours will start charging if the fuse link is burnt but that's how to test it
You need a power distribution diagram out of a 1987 F series EVTM
That will show the fuse links and the alternator connections
Good luck
Hotshot
Joined: Jun 2015
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…So naturally I assumed a bad alternator so I bought a new one and installed it. (This alternator is for a 1990 but the 1987 alternators were all different from the old one that was on it and this one matches it perfectly) so I mounted it in and wired a new connection in. Sure enough I start it and I disconnect the battery to see how it’s working and it immediately stalls. When the truck is off the battery reads 12.4 v and when the truck is running it is at about 11.9 v the the voltage is the same on each side of the fuseable link that is on the wires from the alt to the battery so that is not the problem.
Just because the battery has 12v doesn’t mean it is good. What happens when the battery is load tested? It sure looks like the new/reman alternator isn’t charging. Now to figure out if the new part is junk. At one point did the charging system work?
Cargo Master
Joined: Mar 2007
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Too many assumptions for me. I would check the basics. Does the alternator have power to it? Are the basic cables any good [ground and power] from the alternator to the battery.
There are way too many cheap rebuilds out there. I have never had an alternator bench tested before I bought it and I have never had an issue. But then again, I would never buy any house brand alternator especially if it has a lifetime warranty. A lifetime warranty just guarantees you will be replacing alternators/starters every 6 months. Usually, on the side of the road. Read the fine print, it says that
God forbid I purchased an alternator on the internet. What do you do with the core? When there is an issue [and yes there will be one at the price point they are selling it at], do you really want to deal with someone on the phone/email? Buy locally from someone that you can build a relationship with. Someone that can test the alternator if there is an issue.
There are way too many cheap rebuilds out there. I have never had an alternator bench tested before I bought it and I have never had an issue. But then again, I would never buy any house brand alternator especially if it has a lifetime warranty. A lifetime warranty just guarantees you will be replacing alternators/starters every 6 months. Usually, on the side of the road. Read the fine print, it says that
God forbid I purchased an alternator on the internet. What do you do with the core? When there is an issue [and yes there will be one at the price point they are selling it at], do you really want to deal with someone on the phone/email? Buy locally from someone that you can build a relationship with. Someone that can test the alternator if there is an issue.
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Cross-Country
Joined: Sep 2024
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Thanks for replying, the battery should be fine I’ve used it in other trucks and have 0 issues with it. I’ll do some testing when I get home on the altanator. One question I did have was on the connection that goes into the alternator there is 3 wires, 2 big one small. When I wired the new pig tail in I got the wires mixed up. Obviously I put the little one together but kind of had to guess with the two big ones as they are identical. If I wired the big ones in wrong would it cause issues like this?
Lead Driver
Joined: Sep 2002
Posts: 9,553
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From: North Texas
Not to me, they both run power back to the fusible links to the 'noid. Which is why I would eye that cable for continuity.
Are you able to return it for a refund? A 3G swap can be done cheap and the 105 or 130A is a far better unit.
Are you able to return it for a refund? A 3G swap can be done cheap and the 105 or 130A is a far better unit.
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Fleet Owner
Joined: Oct 2005
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From: Kentucky
2G alternator basic wiring diagram:
Ensure the Light Green/Red wire has power when the key is in the Run position. That is the exciter voltage to turn on the alternator. Without it, the alternator functions as an expensive idler pulley.
Ensure the Light Green/Red wire has power when the key is in the Run position. That is the exciter voltage to turn on the alternator. Without it, the alternator functions as an expensive idler pulley.
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Don't do this. Bad things happen. Diodes can fry. Regulator can fry.
Fleet Owner
Joined: Oct 2005
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From: Kentucky
I suspect an open fuse or open in the circuit. Under load that 3 volts will most likely be gone. You need the full 12 volts to turn on the alternator. This is where a Ford EVTM manual comes in handy.
That exciter voltage goes through the instrument cluster on it's way to the alternator. Does the Battery lamp illuminate when you turn the key from Off to Run? It should.
That exciter voltage goes through the instrument cluster on it's way to the alternator. Does the Battery lamp illuminate when you turn the key from Off to Run? It should.
Fleet Owner
Joined: Oct 2005
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From: Kentucky
If the bulb illuminates when the key is cycled from Off to Run, that means there is supply voltage and a path to ground from the sensing circuit inside the distributor.
Are you sure that Light Green/Red wire is only measuring ~3 VDC?
Has the Battery lamp been swapped to LED?
Are you sure that Light Green/Red wire is only measuring ~3 VDC?
Has the Battery lamp been swapped to LED?
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Cross-Country
Joined: Sep 2024
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Yes I’m 100% it’s reading 3.28 v constantly. It has not been changed to led that I know of, on this truck the green and red wire comes up from the alternator and goes to a connection by the battery. On this connection the one side has the green and red wire and the other side has 4 wires 3 of the wires read around 11.5 v and the other has nothing but I think it’s a ground. So shouldn’t the green wire be getting the 12 volts or close to? The connection looks good to me, no corrosion.