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Alternator not charging: Dash light was to blame. Seriously.

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Been struggling for 3 days to get a 98 Legacy Outback to charge. Apparent dead alternator, but not true after all. The battery light in the dash is wired in series with the L terminal of the alternator. The dash sends current through the light to the alternator to power it up. If the light is either burned out or not installed, the alternator will appear to be dead. Found that out the hard way. Took the dash into a Carquest to find replacement illumination bulbs. In the process of plucking bulbs out, examining them, and putting them back, the battery light got put somewhere else that isn't even used on this car. That means the ignition wiring now fails to send current to the alternator and it just sits there. 3 new alternators later, it charges great. An ammeter and voltmeter in the dash would be great. Or, just look at the battery light as you start the car to see that it lights temporarily in run but not start position. If it doesn't, the light's burned out and will fail to alert you if the alternator does actually fail.

 

There are 2 terminals on the side of the alternator. One (the big fat white one) senses battery voltage, and the other (smaller black/white stripe) comes from the dash. Both have to be up at battery voltage to trigger the alternator to charge the battery through the big lug on top of the alternator. I could have diagnosed the problem, had I understood what the two terminals do, but all I had was the standard, useless manuals you get at the parts stores. Nothing in them that was very useful to me.

 

Hope this info helps someone out there.

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This was fairly common in older cars. In newer cars there is supposed to be a resistor in parallel with the bulb so the alternator still charges even if the bulb burns out.

I would have to suspect that the resistor in your dash cluster is burned out or has a poor solder joint, or is damaged in some other way.

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Your statement about the battery warning light needing to be working is absolutely correct. I had to learn this the hard way also many years ago when I was working on a Ford truck. There are a lot of cars that use this design so the warning lights go into the test mode when you turn the ignition on and so you can verify they do work. The "L" or lamp lead as it is called passes current through the battery warning lamp and on to the exciter of the alternator to build up the field of the alternator. If the lamp lead is open for some reason you have no field built up in the alternator and that means, "yous got nut'un", for alternator output. When the alternator starts working as it should then voltage is back fed on that lead and that makes the warning light turn off since there is no ground potential on that lead anymore, at least until something happens to the alternator. Then the light will turn on and you get the warning to check the alternator.

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I might rig up a simple sensing circuit that has a single red LED in the dash that comes on and blinks only if the system voltage falls below some voltage, like 13V. That is above the 12.6V of a battery and less than the charging voltage. That would give ample warning to the driver that they need to do something.

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If it remains problematic, you can wire in a 12v key "ON" source and bypass the factory BS and connect to the field "ON" on the alt. That way, every time you start the car it's charging w/o worries.

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If it remains problematic, you can wire in a 12v key "ON" source and bypass the factory BS and connect to the field "ON" on the alt. That way, every time you start the car it's charging w/o worries.

But by doing that you would defeat the warning light circuit and wouldn't know when you do have a charging problem. It would also effect the other warning lights testing process.

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If it's really a problematic issue, put a 12v+ voltmeter gauge in (they even make ones that plug into the lighter outlet so it'd be super easy to monitor; if the light stays on in the dash, pull the bulb) and use that as it'll be more accurate and let you know in advance if the alt or battery are going south as opposed to a dummy light. i.e. if a battery usually holds 12.04 volts after sitting for 1-2 days before starting, and all of a sudden it's only holding 11.01 volts before starting, you'll know something is up, whereas a dummy light wouldn't even come on for that if the alt is working and the engine cranks enough to start. If the charging volts are typically 14-16v with no accessories on, and starts dropping to 12v while running and drops lower with accessories on but stays just above the dummy light threshold of OK/not OK, a gauge will tell you in advance rather than down the road and the "not OK" get's tripped. Alternators typically last quite awhile as it is, so unless you are running more accessories than what the alt can handle, it really doesn't need 24/7 monitoring. 

Edited by Bushwick

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Fixing the warning light circuit for the alternator is a simple job to do. Installing a voltmeter to monitor the charging system is a very good thing to do. Equus makes a nice one that just plugs into the cigarette lighter socket for a very reasonable price.

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^ If you have an actual 12v gauge meter laying around and have an old lighter socket cable (power inverters have them, as do old, unused cell phones, etc.) you can easily wire it to the gauge for occasional in-car testing too.

 

Also, is this issue common with older Legacy models? Or is it only problematic with certain, specific year/engine combos?

Edited by Bushwick

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