98 Legacy. Driving in stop and go, the battery and break light came on. But only when rpm was below 2000. Step on the gas and the lights turn off while driving or idling. Speculating for a bad alternator, I thought that when I am over 2000, the battery still charges. I kept the rpm high as often as possible. Other that the lights everything looked normal, no dimming of lights or anything. After more than an hour, all of a sudden, I was well over 2000, the car jerked like crazy, had almost no power, but was still drivable. I went for the next possible spot on the side of the road and it died. I flatbedded it home. The battery had 12.2 V, tried to start it but could only crank very slowly. Brought the battry back to life with a charger, started the car no problem to drive it in the garage here at home.

 

My questions: Why is the break light coming on with the battery light? Could it be just the diodes? Could these symptons be enough to be absolutly sure it’s the alternator? I checked the battery connections, they are fine.

 

Thanks.

AC current from the alternator's rectifier is defeating the diodes in your guage cluster.

 

At this point from the extended AC output your battery is also shot.

 

Replace both.

 

GD

I have a good used Alt if you want it. $30+shipping

Alternator belt tight?

In modern cars, when altenators die, they backfeed so the brake light comes on along with the battery light.

 

Either buy the used ALT here, or get one cheaply from one of the on line suppliers. If you buy one from a store, If you pay more then 68.00 for it, they are ripping you off and selling you the wrong alt

 

https://www.subarugenuineparts.com/oe_parts_cat.html

 

(plus 50.00 core charge and shipping)

 

Its not uncommon for them to sell an imprezza alt for a legacy. It doesnt work.

 

 

nipper

[...]My questions: Why is the break light coming on with the battery light? [...]

One side of each of the dash warning lights is connected, through the ignition switch, to the "hot" side of the battery. When the key is first turned on, before the alternator field is energized, the brake warning light gets a ground connection (via a diode) through the alternator's field winding. This allows the brake warning light to be tested at the same time as the other dash lights. Once the alternator field is self-excited, the brake light (and battery charge warning light) go out because both ends of each of those two lamps will have about the same voltage on them. Since the brake warning light is connected in that manner, if the alternator field voltage dies, both the charge and brake warning lights will come on.

 

Aternator output is dependent on the field being energized. If that stops happening for whatever reason, even if the rest of the alternator is okay (including coils, rectifier diodes, etc.), there won't be any output, and of course the battery voltage will keep dropping even if engine rpm is kept high.

 

As long as the battery is not too old and accepts a decent charge, resolving the alternator problem should be the end of the difficulty.

Well you may want to check to see if this car has had the alt replaced under the recall. 98 Legacys had a recall on the alternator...this is exactly what mine did and was replaced for free under the recall.

Thank you all for the answers. The belt is good and tight as should be.

 

Thanks OB99 for the explanation of the circuit. Wouldn’t it be easy to completely separate the circuits? Too expensive for Subaru? Another question would be why my lights went off with higher rpms. Was just one winding broken? I know, it’s just time for a new one, but I am just wondering wether it’s possible to make a conclusion from the symptoms. When I have time I ll take that thing apart and test the components.

 

Yes, the alternator was on recall. I was twice at Penisula Subaru in Oakville Ontario with an appointment, first they said they are behind schedule, then the mechanic was sick, and then I just forgot about it and that’s to blame me. I heard other bad things about them and in the meantime their licence was taken from them.

 

I just called Subaru of Canada, they were not very helpful, sticking to the paragraph. They want the car at the dealer, basta. That would cost me another 90 bucks or so to flatbed. And maybe the serial no. on the alt is such that they would refuse the recall repair. But SoC, when giving them that serial, is not able to confirm if recall is indicated. Only the dealer can do that, and only if the alt is in the car.

 

Imdew: So I am interested. Is it from a legacy? How old? How much would shipping be?

 

Thanks again

[...]Thanks OB99 for the explanation of the circuit.

You're welcome.

 

 

Wouldn’t it be easy to completely separate the circuits? Too expensive for Subaru?[...]

I'm not privy to Subaru's design decisions, but I can surmise based on the wiring diagram. The other dash warning lights already either normally have one end grounded at initial key-on (for example, oil pressure is non-existent, so the sender contact is closed), or are attached to sophisticated circuits that include the test function (example, ABS). The brake warning light normally only comes on with either low fluid or parking brake on; if the fluid isn't low, and the parking brake isn't set, the lamp wouldn't otherwise light at start-up. It seems Subaru was looking for a way to test the lamp that didn't require additional circuitry beyond the diode.

You're welcome.

 

 

I'm not privy to Subaru's design decisions, but I can surmise based on the wiring diagram. The other dash warning lights already either normally have one end grounded at initial key-on (for example, oil pressure is non-existent, so the sender contact is closed), or are attached to sophisticated circuits that include the test function (example, ABS). The brake warning light normally only comes on with either low fluid or parking brake on; if the fluid isn't low, and the parking brake isn't set, the lamp wouldn't otherwise light at start-up. It seems Subaru was looking for a way to test the lamp that didn't require additional circuitry beyond the diode.

 

And is standard practice in all automobiles. Self tests also are required by the goverment(s).

 

nipper

And is standard practice in all automobiles. Self tests also are required by the goverment(s).

I know that others (Toyota, for example) do also use the alternator field connection approach. However, it's my understanding that as long as the testing requirement is met, the actual method by which it's done is up to the manufacturer.

[...]Another question would be why my lights went off with higher rpms. Was just one winding broken? I know, it’s just time for a new one, but I am just wondering wether it’s possible to make a conclusion from the symptoms. When I have time I ll take that thing apart and test the components.[...]

It may not be easy to find the problem, even during a "post mortem". It's not uncommon for the generating portion (windings, brushes/slip-rings, rectifier diodes, etc.) to be okay, but for the voltage regulator (built into the alternator) to fail completely or be intermittent.