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OK - I'm at my wit's end! Please excuse this lengthy explanation in advance - here's the story:

Just bought our first Subaru ever - 1997 Legacy Outback 2.5L with 185K miles - from a used car lot. The dealer had a local shop go over it to make sure there were no "big" problems. Everything seemed to work fine, just some cosmetic flaws from a baseball-size hailstorm we had here at the end of June. (The perfect car for my teenage son to drive.)

Within a week, he started having to jump-start it almost every time he stopped somewhere. After some messing around at home, we jumped it and took it to a parts store for a free charging system check. They said the battery was fine (just a bit low), but there was no charging voltage out of the alternator.

So we bought a "new" remanufactured alternator from our local Subaru dealer and installed it. I don't think we checked the alternator output / charging voltage right away - we stupidly assumed that the problem was fixed. Ha! Within a week, again, my son was having to jump-start the car every time he stopped anywhere. (Luckily, we live where everyone's a Good Samaritan and it's easy to get help.)

We thought maybe the battery was just too deeply discharged for the alternator to recharge it during his quick trips around town. So we put the battery on a trickle charger overnight, put it back on the car the next morning, and sent him off to school. After just a few trips around town, the battery was dying again. We spent a couple of weeks screwing around with it again - checking connections, ground continuity, etc. Finally took it to have a battery check again and got the same story - battery was fine, alternator was not charging. This time we decided to make sure it was a bad alternator - we took it out of the car and had the local NAPA store bench-test it for us. Result = no DC charging voltage = bad alternator.

So we took the "new" alternator back to the Subie dealer and did a warranty swap for another "new" alternator. We stopped on the way home and had it bench-tested at NAPA just to make sure it was good - according to their test, it was. We installed it last night, turned the car on, checked the charging voltage, and NO CHARGING VOLTAGE AGAIN!!!

I have checked and double-checked every ground connection in the engine compartment - all fine. I have tested the continuity on the battery to alternator cable - it's fine. I checked for a short between the two contacts on the alternator harness/connector - not even a highly-resistive short. I have checked the voltages on the alternator connector with the key switched on (engine not running) and get about 12V across the board - same as battery - normal, I think? But when the engine is running, the alternator output to the battery reads the same voltage as the battery (slightly lower than it tested right before I started the engine) - between 12.2 and 12.8V, depending on how many times I've started it since the battery was charged - and it's dropping.

The battery / charge indicator light (and all other idiot lights) comes on when the key is switched on and immediately goes off when the car starts, as if there's nothing wrong with the charging circuit. The CEL has come on a couple of times over the past month, but just for a short time and then goes back off. (It has also been reset repeatedly from disconnecting the battery for charging.)

HELP!
 

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I'm just starting to have an issue with mine, fluctuating lights at anything over idle [think I have a failed diode].

You seem to have checked everything I would have suggested. You did check the continuity between the alternator case and the battery neg terminal after installation right ?

If it's similar to the 2nd gen I have, it should be internally regulated so I can't think of anything in the car that would "short it out". Usually the reason for failure on a reman or newly installed new alternator is a bad rebuild, failed part, damage during shipping etc. It is possible for a unit to bench test ok then fail shortly after.

If it get to it tomorrow and figure out some of the test I read, I might have a better Idea of what issue you might be having. If you take it back to the store and it bench tests ok, you obviously have a wiring issue. Some of the older cars I've had, had a thing where , if the dash light didn't work correctly, the alternator would not give an output current. I know you said it lights up but, if all else checks out, maybe a direction to go.

First thing would be to have it tested again to make sure it didn't get another "bad" unit. If it's good, check back in and I'll let you know if I come up with anything.

Good luck
 

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Maybe pull the code(s) of CEL in autoparts, usually free ... the trouble code(s) might give you a clue of why.
 

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Discussion Starter #4
Thanks for the suggestions!

Yes, I checked the continuity between the alternator case and ground/battery neg.

I'm heading out now and will try to stop at the auto parts store on my way home to have the alternator tested again and see if they can read the trouble codes (if any) for me

I'll let y'all know what I find out.
 

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Make sure that the "always on" 12 V is at pin 2 of the connector, as shown in the attached diagram. I believe the diagram is for the 97, but if not, it would be similar. There has to be a source of current for the rotor in order for the alternator to work, and it comes from the battery in this way. It is this current that the internal regulator controls to maintain the alternator output in the proper range (around 14 V at 2000 rpm). On some this source is fused, for example on my 07 it's a 7.5 A fuse, but this particular diagram doesn't show it.
 

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Yes, the "always on" at pin 2 is good = positive battery terminal voltage (no resistance).
 

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Just got back from having the alternator bench-tested. I drove the Subie to the store first and they used their portable tester on the battery / charging system. The result was "good" on the battery, and the alternator tested "weak" = putting out only 12.5V (the battery voltage) and less than 30A and having a weak regulator. Then I took the alternator off of the car and had them bench-test it. The result was a "pass" on all tests. So, according to the bench test, the alternator isn't weak, it's good. (They got ~14.5V out of it.)

The only thing I'm seeing that's a little suspicious is a .4V voltage drop on the switched pin (#1) on the connector to the alternator versus the battery voltage. Which means that when the car is on, there's a .4V differential between those two pins on the alternator. Could that be causing it not to charge? What is that pin supposed to be at when the engine is on?
 

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And no trouble codes on the OBD system.

Side note on the OBD code reader: O'Reilly's said they can't read the diagnostic codes for you anymore - liability issues. But, they will loan you a code reader (after you put down a $242 deposit) so you can do it yourself. If you get it back to them within 48 hours, they refund your deposit. The NAPA store just said no, they can't read your codes, but they could sell you a code reader.
 

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I believe it should be at the full alternator output. That's because the "ALT 1" goes to one side of the "charge" warning light. The other side goes to fuse #15 (in cabin fuse box) and then to the IGN pin on the ignition switch. The IGN, with the switch at ON, is connected to the small "2" at the upper right of the diagram in my earlier post, which goes to the common point of the battery and alternator output (MB-10.)

With the ignition switch at ON but the engine not started, the light goes on because there's 12 V from the IGN pin to the light, and the other side is to the regulator in the alternator, where it is essentially at ground. When the engine is started, pin 1 goes up to alternator output voltage, as does the IGN, so the light goes out.

As the alternator works fine on the bench, I suspect that there is a fault either in the supply at pin 2 (but you've verified that there is no resistance all the way to the battery positive terminal), or in the charge light line which is holding it down -- perhaps a short. Remove the connector from the alternator and the pos. battery terminal, and check for shorts in the pin 1 line to ground, and check it for continuity back to the positive cable, with the ignition switch ON. (There will be some resistance due to the bulb, fuses and switch, but altogether I would guess not more than 10 -20 Ohms.)
 

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Discussion Starter #10
"Remove the connector from the alternator and the pos. battery terminal"

Do you mean disconnect the alternator from the battery? disconnect the 2-pin connector from the alternator? or both?
 

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Sorry. Not clear.

Disconnect the two-pin connector at the alternator to provide access to the pin #1 on the harness. But, because this goes back through the ignition switch to the battery, and would be raised to 12 V if the ignition switch is at ON, it's necessary to disconnect the battery. This way, continuity from Pin 1 through the ignition switch to the positive battery cable (not the battery terminal) can be confirmed, and leakage/shorts to ground between the pin 1 of the harness to ground can be identified (which wouldn't work if there is 12 V there).

I might also disconnect the large cable at the alternator, just to eliminate the effect of the alternator on measurements on the pin 1 harness wiring.

Hope this is better.
 

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OK - disconnected positive battery cable, cable from output post of alternator, and alternator harness - results from the continuity / short check:

1. continuity from pin 1 on the alternator harness to the positive battery cable is good = only 12 ohms;

2. it looks like there's a short to ground somewhere, because there's only 24 ohms between pin 1 and ground / neg. battery terminal.

AARGG!!
 

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Discussion Starter #13
Thanks for your help! Looks like I have a serious hunt ahead of me...
 

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it looks like there's a short to ground somewhere, because there's only 24 ohms between pin 1 and ground / neg. battery terminal.
Not necessarily. There are other connections to the IGN contact that are too numerous to identify. So 24 Ohms isn't a short to ground.

While the 12 Ohms in the bulb line looks good, it might be misleading.

In some systems, the circuit from the IGN terminal through the warning light also supplies the starting field current (via pin 1) that the alternator needs to begin generating as soon as it starts turning. (When the engine is off, but the ignition is on, current flows from the IGN terminal through the bulb and through the field coil of the alternator to ground.)

Once the generator gets to full output (as determined by the regulator) the current for the field coil is supplied internally from the bulb circuit to the alternator output, so the alternator becomes self-sustaining. At this point the light goes out. But if the starting supply through the bulb fails, the alternator will never start generating as it starts turning.
And if this is the way the Subaru alternator works, the loss of the starting current could explain the lack of alternator output.

(When the alternator is bench tested, the starting supply is provided from the bench and that could be why the alternator tests good on the bench, but not in the car.)

How might this happen? Not likely, but one possibility is if the bulb is bad. It might work initially, i.e., when the ignition switch is just at ON; but when the alternator starts to draw more current for the field coil as the engine begins to turn, the light goes out. This could be because it, or something in that line, is going open circuit. That cuts the field supply and the alternator doesn't generate, while, unfortunately, at the same time the light is out.

Or, the 12 Ohms could be higher than it should be.

So, how might this be verified? It's a bit of a jury-rig, but I wonder if pin 1 could be connected, through a bulb similar to the type used for the warning light, to the battery while the engine is running. This would simulate the warning light circuit, providing a substitute source of starting current through the bulb. If the alternator output suddenly jumps up to 13 -14 V, then this shows the previous absence of the starting current. I believe this can't do any harm because it's only paralleling the existing path from the battery through the ignition switch, and warning bulb, to pin 1.

Edit: This website has a diagram of this type of circuit and explains how the warning light circuit is used: http://www.alternatorparts.com/understanding_alternators.htm
 

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Discussion Starter #15
I think I can handle that. I'll try it tomorrow and let you know the results.

Thanks again for all the helpful suggestions - electrical problems are so frustrating!
 

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Is this starting to sound like a bad connection in the keyswitch in the run position ?
 

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ETC

Yes. That is a possibility, along with an intermittent warning bulb or fuse #15, or a bad physical connection in the wiring.

But it certainly seems as if the initial starting current for the field coil is either absent, or is lost as soon as the engine turns, and the alternator never reaches the required output to transfer the field supply and become self-sustaining.

The parallel bulb approach would, I think, prove this out. If the output suddenly jumps up to normal, and remains there when the shunt bulb is removed, it verifies the absence of the starting current. Then at least the troubleshooting can be focussed. I think that can be taken in stages, from the alternator connector harness at pin 1 to the bulb, from the bulb to the ignition switch etc. And just changing the bulb and the fuse might be a good start.
 

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OK - sorry for the long delay. Family stuff (kids' soccer game, etc.) took precedence today.

I tried the shunt bulb wire-up: removed the 2-pin alternator harness and replaced it with a direct connection to battery positive terminal on pin #2 and a connection to battery positive terminal through 1.4W bulb on pin #1. It made no difference in the behavior of the alternator.

A couple of times last night and this morning when I had the car running, the voltage across the battery would go up to about 13.0V (0.4V above the battery voltage when the car was turned off). Seems like maybe the alternator is trying to put out a charging voltage at times but it's extremely weak.

I'm still stumped...

Is there anything that could drag down the output of the alternator that much without blowing a fuse?
 

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I'm still stumped...
It has us all stumped.

Brainstorming here . . . .

Looking back at post #7, when the alternator was tested in the car, the voltage and current capacity appeared to be low. Yet when it was taken out and bench tested, it was fine.

With the in-car test, I believe the alternator was connected to the car normally, and the test involved connecting a heavy load on the electrical system (across the battery?) and measuring the current and voltage with the engine running. Was the engine idling or was the rpm raised to 1500 - 2000 rpm (which is usually where it has to be for max output)?

Also, where was the current being measured? Was it at the load tester, or at the large terminal on the alternator? If it was at the load, then perhaps it wasn't including the current used to run the engine etc. (I'm not trying to be critical of what was done; I only want to eliminate any doubts as to what the measurements mean.)

If there is an unusually heavy load on the alternator, one that's enough to drag the voltage down to the mid-12 V range, it would have to be close to the amperage rating of the alternator itself. It should be possible to measure the current at the large terminal. At this stage I'm trying to determine if the alternator is, in fact, being overloaded when installed in the car. (The alternative is that something else is clamping it's output down.) To be overloaded, the current drag at the main terminal would have to be close to the alternator's rated output, whereas it would be far less normally, with a fully charged battery.

More brainstorming . . .

From post #1
I have checked and double-checked every ground connection in the engine compartment - all fine. I have tested the continuity on the battery to alternator cable - it's fine. I checked for a short between the two contacts on the alternator harness/connector - not even a highly-resistive short. I have checked the voltages on the alternator connector with the key switched on (engine not running) and get about 12V across the board - same as battery - normal, I think?
(underline added)

With alternator connected normally, and the key switched ON (engine not started), the battery warning light is on, right? If pin 1 on the alternator goes to the light (as seems to be the case from the wiring diagram and the diagram in the linked web page), and from there to the ignition switch and from there to the battery, then the voltage at pin 1 with the light illuminated should be noticeably less than the battery voltage. If pin 1 is at the battery voltage, then how can the bulb be lit as there would be battery voltage on both sides? There has to be a voltage drop across the lit bulb. Is the battery warning light bright, comparable to the other warning lights?

And another off-the-wall . . .

What about the parking brake/brake fluid low warning light? In the above scenario, is there any difference in the voltage at pin 1 with the parking brake applied or released? And does the parking brake light noticeably change intensity with the parking brake lever up or down? (I'm back to suspecting absence of field coil start current. The battery warning light and brake warning light are interconnected in the combination meter . . . .)
 

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Looking back at post #7, when the alternator was tested in the car, the voltage and current capacity appeared to be low. Yet when it was taken out and bench tested, it was fine.

With the in-car test, I believe the alternator was connected to the car normally, and the test involved connecting a heavy load on the electrical system (across the battery?) and measuring the current and voltage with the engine running. Was the engine idling or was the rpm raised to 1500 - 2000 rpm (which is usually where it has to be for max output)?

Also, where was the current being measured? Was it at the load tester, or at the large terminal on the alternator? If it was at the load, then perhaps it wasn't including the current used to run the engine etc. (I'm not trying to be critical of what was done; I only want to eliminate any doubts as to what the measurements mean.)
The in-car test, performed by an employee at the auto parts store with a portable battery tester, was done entirely with the tester connected across the battery, and the alternator connected in the car normally. It was done with the engine idling only.


If there is an unusually heavy load on the alternator, one that's enough to drag the voltage down to the mid-12 V range, it would have to be close to the amperage rating of the alternator itself. It should be possible to measure the current at the large terminal. At this stage I'm trying to determine if the alternator is, in fact, being overloaded when installed in the car. (The alternative is that something else is clamping it's output down.) To be overloaded, the current drag at the main terminal would have to be close to the alternator's rated output, whereas it would be far less normally, with a fully charged battery.
My thoughts, also. The alternator is rated at 85 amps, so the current draw would have to be close to that, which would fry any fuse or fusible link in the car that it was pulled through. (Unless it's being drawn through multiple circuits, of course.)

With alternator connected normally, and the key switched ON (engine not started), the battery warning light is on, right? If pin 1 on the alternator goes to the light (as seems to be the case from the wiring diagram and the diagram in the linked web page), and from there to the ignition switch and from there to the battery, then the voltage at pin 1 with the light illuminated should be noticeably less than the battery voltage. If pin 1 is at the battery voltage, then how can the bulb be lit as there would be battery voltage on both sides? There has to be a voltage drop across the lit bulb. Is the battery warning light bright, comparable to the other warning lights?
Yes, with the key ON, engine not running, and alternator connected normally, the battery warning light is on. With the engine (and therefore, the alternator) not running, the connection on the alternator at pin #1 is essentially a ground, which lights the bulb. However, when the alternator harness is disconnected, that opens the circuit, and the battery warning light does not come on with the key ON, engine not running. At that point, the voltage at pin #1 on the harness (not on the alternator) reads about 0.4V below battery voltage. I assume the battery drop is from the light bulb.

The attached file is a chart with the measured voltages on the alternator harness, the alternator output post, and the battery, for all connected/disconnected and key on/off conditions.

Is the battery warning light bright, comparable to the other warning lights?
No, if anything, it looks a little dimmer that the other warning lights, but only slightly, if at all.

What about the parking brake/brake fluid low warning light? In the above scenario, is there any difference in the voltage at pin 1 with the parking brake applied or released? And does the parking brake light noticeably change intensity with the parking brake lever up or down? (I'm back to suspecting absence of field coil start current. The battery warning light and brake warning light are interconnected in the combination meter . . . .)
The voltage at pin #1 on the alternator harness (harness disconnected, key on, engine not running) does change ever so slightly, but very consistently, when the parking brake is applied (and the brake warning light illuminates). The change is only 0.03V. However, the battery voltage itself does not change at all.

The brake light does noticeably change intensity when the parking brake is applied - it gets brighter. Then it dims back to the same intensity as the battery light when the parking brake lever is lowered.
 

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