I've been seeing how my dash lights (converted all to LED) have been diming to the point of beeing almost off with the blower on high and AC on and headlights on.. However with the car at like over 2K and above the dash lights and all come on again.

 

So I took some readings.. btw this is a Subaru reman alternator

 

Battery at rest: 12.6v

 

Running: 14.4v no load

 

With load (i.e. headlights & AC) 13.2v

 

After about 5 min running with the headlights and AC on voltage fell to 12.7 V running

 

 

Any clues?

alternator.

Should I've taken the reading at 3k?

It means that for some reason the battery isn't being charged with enough current to keep up with the load. There is a few reasons that can happen.

The easiest thing to point at is the alternator, and while that may be the cause, wiring is another big culprit.

Start by taking the positive connection off the alternator and cleaning it really well. Then take the ground off (it's on the top starter bolt), and clean that well. Clean the battery terminals and battery well. Any bad connection in the ground or charging wire will cause a drop and not charge the battery enough.

 

If it's still low, try adding a wire from the alternator case directly to the negative on the battery, see if that solves the problem. Especially if everything has been apart and together a bunch, you might have a bad grounding path from the alternator to the battery.

 

Oh, and also, before any of this there's one more check you need to do.

Check the voltage at the positive post on the alternator. See if you're experiencing the same drop there, if you are, it may very well be the alt.

Now that I think about it the positive wire has a fare amount of corrosion on the wire.. I was going to switch the wire out but what gauge wire should I use?

Now that I think about it the positive wire has a fare amount of corrosion on the wire.. I was going to switch the wire out but what gauge wire should I use?

 

The same size or bigger than what's there now.

From the sounds of things you still may need to replace the alternator. It seems to be failing above certain RPMs.

From the sounds of things you still may need to replace the alternator. It seems to be failing above certain RPMs.

 

Like I said I tested at idle with load but i know this never happened before.

I actually see this on all my Subaru's, two of which have recent genuine reman alts. If sitting at a long traffic light for example it will go from 13.4V or so at first and slooowwlyyy drop as low as 11 volts if it's sitting for like four minutes or more. That's with say a/c on hi, lights on, brake lamps lit. I think the once the alternators are hot, at idle they just can't put out enough current to run all that stuff.

Just to throw in one more item: check the belt tension. If it slips, it will slip more under load. Eliminate that and if you still have the problem, I'm with the replace the alternator camp.

Just to throw in one more item: check the belt tension. If it slips, it will slip more under load. Eliminate that and if you still have the problem, I'm with the replace the alternator camp.

 

Yeah but i have no squeal

In thinking about it, sitting stopped with a/c on hi, both rad fans running, blower motor on hi, brake lights on, fuel pump, and everything else who knows that could be 40 amp draw or more. The alternators on these things are rated at what 70 amps maybe, but that's going to be cool and at some rpm above idle. I think these alts just can't put out 40+ amps hot at idle. So then the battery starts discharging slowly, and you see the commensurate voltage drop, until the voltage drops enough that the total current draw drops to what the alternator is actually putting out.

I was thinking about this (sorry been busy this past week)

 

I was driving at night and I was leaving a parking lot and I sat at the stop sign for a few and my HVAC blower was on 2 with no AC on I noticed at idle I see my dash flickering more then normal.. I've converted the dash over to LED a good 5 months ago so It's caught my eye.

 

Anyways I'm still using the battery that came with the car I'd have to guess it's maybe 2 or 3 years old...

 

Anyways I was wondering maybe the battery is draining faster then normal because of it's age? Things I've noticed if I have the headlights on the dash turns on, maybe like 5 months ago my dash gauges would be all still lit up (if this makes any sense) but now it fades pretty quick to off because the LED's are 12 V so once they start to fall under 11.5 they fade.

 

I think it would be wise to do the battery first then at the same time do the cables then recheck again?

When the engine is running, the battery does no more than smooth the current coming out of the alternator, and absorb sudden draws. Like the radiator fans kicking in, etc. Other than that, the alternator should supply all the power for everything.

I'd expect a flicker maybe as the radiator fans kick in. They use a lot of current. But if you have low voltage at idle, you need to start looking at wiring and the alternator.

That's true, except if the alternator is not able to put out as much current as everything is drawing, as could be the case with a hot alternator at idle with lots of accessories running. Then the battery starts to slowly discharge and the voltage starts to drop. As the voltage drops, the current draw decreases. Eventually the current draw would either drop to what the alternator is putting out or if the voltage gets low enough I imagine she will stall out.

 

Now you did mention a 'flickering' on your dash lights. These alts are three phase, and if one or possibly two of the phases is faulty, you may notice a flickering, especially at idle because the alt is then operating at its lowest speed. Someone had mentioned before to put a voltmeter across the battery terminals but set to measure AC rather than DC. If you see much appreciable AC voltage there then one of the phases in the alt might be not outputting. That will also reduce its maximum average output, I would imagine by 1/3, so that would contribute even further to the voltage slowly dropping at idle with lots of accessories on.

Measuring AC ripple at the battery isn't because a phase failed in the alternator, it's because the diodes are going bad and not doing a good job rectifying the AC to DC.

 

If a given load is drawing the same wattage, as voltage decreases it will actually draw more amps, making the issue worse.

I forgot about checking the AC to see if I had a reading.. I remember before I totaled my old legacy the dashlights really flickered I really think that alt was on it's way out it got shorted a few times because of the wire was frayed on the output.

 

Anyways as soon as I get back I will check, I do no with no loads on it will provide about 14 V

 

I have no clue how old the reaman Subaru Alternator is.. I know the Subaru reman sticker is on it still so maybe not older then 5 years?

Measuring AC ripple at the battery isn't because a phase failed in the alternator, it's because the diodes are going bad and not doing a good job rectifying the AC to DC.

 

If a given load is drawing the same wattage, as voltage decreases it will actually draw more amps, making the issue worse.

 

Yep heat build up and all.

 

Too bad they have nothing but 85A replacements

If it is a constant kva load like an ac induction motor yes it will draw more current as the voltage decreases. But things like headlights, the defroster, and dc motors, the current draw drops as the voltage decreases.

Ah that makes sense now

Right resistive loads like incandescent headlights will see the current decrease as the voltage decreases (and hence the power decreases as well).

 

Even when the alternator says say 85A that's going to be with a cool alt and at something above idle like 3000 rpm or whatever they rate them at. At idle when it is hot the maximum output is going to be much less. I think when they say the rating whatever it is such as 3000 rpm they are referring to alternator rpm not engine rpm.

 

I have had times in the winter when stuck for long periods in traffic with the heat on full, rear defroster on, headlights on, foot on brake where the voltage got down to like 10.7 volts. That gets a little scary because you can hear the blower start to slow down and things like that. And you can't really turn any of that stuff when it's snowing hard. So I've learned to double foot it a little in those conditions, or sometimes pull the parking brake so I can let my foot off the brake to put the brake lights out, or go to neutral. Usually any of those seem to let it not have the voltage dropping then.

I was wondering does the alternator out of a '07 ForesterXT fit the '97 Outback? I think the Forester has a 3 plug connector?