Exciting my alternator (1 Viewer)

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Joined
Sep 4, 2009
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Location
Madison Alabama
The alternator is good but it won't charge my battery. I determined the alternator was not getting input to excite the stator. My ignition switch is good and I have continuity from the switch to the altenator plug. Puzzling. I was thinking that I could tap into a "keyed hot" wire in the engine compartment, use a 15A inline fuse and plug it into the alternator. It looks like a 18ga wire at the altenator. Any thoughts?

vehicle info: 79 desmogged FJ40. The altenator is a two wire with the regulator on top. The white/blue goes to the battery et al and the black/yellow comes from the switch.
 
How do you know that the alternator isn't getting excitation power? Is your engine fuse good?

Why go to the trouble of running another wire when you already have one?
 
I beleive what Pin_Head is saying is to pull the alternator connector and put the key in the ignition at the run position (do not start). Then take a voltmeter and check the removed connector for 12v and on which socket. Report back what you discover.
 
BTDT, I checked the connector into the altenator in off, acc, and run and got nothing, I checked the switch to see that it connected the "batt" to the "ACC", "IG" and "ST" as it is supposed to, and I checked the continuity from the switch to the altenator. So I am stumped. It does not seem possible that I could have continuity and power through the switch and it still not get 12v at the altenator. I'll keep looking, I have today off.
 
BTDT, I checked the connector into the altenator in off, acc, and run and got nothing, I checked the switch to see that it connected the "batt" to the "ACC", "IG" and "ST" as it is supposed to, and I checked the continuity from the switch to the altenator. So I am stumped. It does not seem possible that I could have continuity and power through the switch and it still not get 12v at the altenator. I'll keep looking, I have today off.

So you are saying that when the ignition switch is in the IG position there is 12v at that ignition terminal and from that terminal there is continuity to the alternator connector. Where are you putting your meter on the ignition switch side when measuring continuity? I'm assuming that you are not using an insulation penetrator to contact the wire and that the alternator connector is in free space. You already know that you cannot have 12v at one end of a wire that test with good continuity without having 12v at the other end of the wire. You need to go through the system again from the ignition switch to the alternator. Make sure that the ignition switch and/or the ignition harness do not display intermittent contact.
 
It does not seem possible that I could have continuity and power through the switch and it still not get 12v at the altenator. I'll keep looking, I have today off.

It depends on what you mean by continuity, which is why I asked you for the details about how you checked.

You could have a bad connection which would show up as maybe 10 ohms resistance or more in continuity in the wire and this would be enough resistance to get a big voltage drop at the alternator. You need to check the voltage at the alternator plug with it connected to see this, because the open circuit voltage will read battery voltage even with a bad connection. Alternatively, you could have a short in the alternator excitor wire which shows up as continuity, but no power makes it to the alternator.

If you check continuity without disconnecting both ends of the wire, you can get fooled by other paths on the same circuit.

Did you check the engine fuse?
 
The engine fuses are good. I took out the ignition switch and checked continuity in the wire off of the switch to the connector off of the alternator. I had 1.5 ohms. The short idea sounds plausible. So, to the original question, if I substitute another wire to the altenator shouldn't it work as planned.
 
You need to isolate the wire and disconnect it to check for shorts or you will get fooled by all the other circuits that are connected to the switch.

1.5 ohms isn't great, but it is hard to accurately measure resistances that low with a cheap meter.

Go ahead and temporarily run a wire from the battery to the alternator to see if the problem goes away.
 
OK, here's the update. Pror to this, I had to run a wire to my idle solenoid from a keyed 15A fuse.

So today, I taped into that wire and ran it to the alternator input. The alternator charges the battery at 14.5 volts.

I still don't get a reaction from ammeter but that could be so many things I won't worry about it. The fusible link is new, it's wored per the book and I'm bet I just have a bad meter. I will probably put in a voltmeter instead.
 
Note that the alternator and the idle solenoid run off of the same engine fuse. You are having trouble with both, so this doesn't sound coincidental. Maybe the fuse isn't as good as you think it is.
 
The solenoid runs off of the emissions control computer and after it stopped sendiing the signal for the sleonoid, I bypassed it and then took the whole computer out. Step by step I am replacing the wiring harness.
 

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