Battery, Alternator, or something else?

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CYaBro

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Topic # 194960 31-Mar-2016 22:26

We have a 2001 Subaru Legacy GTB wagon that doesn't get used a lot anymore, since I got a new work car and wife got a cheaper car to run around in.

Due to it not being used often, plus having an alarm, the battery would go flat often so needed a jump start.

Eventually it got the point where the battery light came on and didn't got out so I thought the battery was needing to be replaced.

A few weeks ago I tried jump starting it again, and it would start and run OK but not long after disconnecting the other car the Subaru would stall. Tried jump start again and same thing, could not keep the Subaru running.

I assumed the battery was finally gone so got another one and installed it.

Car starts fine now, and runs great, but the battery light still stays on.

Just tested tonight with a multimetre, and with the car running there is about 14.2V across the battery, even with some electrics going and lights on high beam so I suspect that the alternator is OK too.

So what else could be causing the battery light to stay on?

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Yogi02

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Reply # 1523597 31-Mar-2016 22:34

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Could be as a result of a low voltage code (or other) in the computer. Might need to get it plugged in and check for codes and have them cleared?

CYaBro

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Reply # 1523598 31-Mar-2016 22:40

Yogi02:

Could be as a result of a low voltage code (or other) in the computer. Might need to get it plugged in and check for codes and have them cleared?

Thanks, that is something I was just thinking about.

Wonder if I can clear the computer myself without having to pay someone to do it? :)

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Aredwood

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Reply # 1524830 2-Apr-2016 20:56

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I doubt that the gtb would have active alternator control. So it would most likely be a failed voltage regulator in the alternator. Or the wire from the warning light to the alternator has shorted to ground. Should be easy enough to figure out from the wiring diagram if it has active alternator control or not.

blakamin

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Reply # 1524853 2-Apr-2016 20:59

+1 ^^^

CYaBro

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Reply # 1524894 2-Apr-2016 22:36

And what is active alternator control?

I know nothing about cars. 😱

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blakamin

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Reply # 1524906 2-Apr-2016 23:01

Computer checks for under or over voltage

richms

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Reply # 1524918 2-Apr-2016 23:09

A friend had a VW something-small and it would light that battery light if it thought the battery was stuffed, not like old cars which had it come on when it was just not able to charge.

Anyway, it was a dodgey connection on the battery that made it come on. Still started fine but everything else was iffy sometimes.

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Rickles

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Reply # 1525378 3-Apr-2016 20:02

Chris .... my Jeep does that after a complete flat battery, and they tell me it is an anti-theft 'feature'.

Cure was to disconnect one of the battery terminals for about 2 minutes, then reconnect and use starter cables.

geoffwnz

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Reply # 1525816 4-Apr-2016 15:26

CYaBro:

Yogi02:

Could be as a result of a low voltage code (or other) in the computer. Might need to get it plugged in and check for codes and have them cleared?

Thanks, that is something I was just thinking about.

Wonder if I can clear the computer myself without having to pay someone to do it? :)

If it's a fault code it would normally show up as a Check Engine light. These can be cleared by connecting a pair of connectors under the dash then doing a prescribed interpretive dance around the car while holding a particular scented candle.

Apparently disconnecting the battery for an hour or more should clear fault codes but given you've got a battery light, not the Check Engine light this might be something different. Also if the battery has gone flat that'll give the same effect as disconnecting it.

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Fred99

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Reply # 1526214 5-Apr-2016 10:25

The last car I had here with alternator light issue (SWMBO's late '90s NB MX5) even for a car that age, the warning light circuit was not simply showing if the battery was charging. It would go out when the car first started but light up after about 30 seconds running, presumably when the ECU had done self diagnostics. According to the FSM, there was a list of conditions which would trigger it. In that case the alternator was stuffed. After replacing it with a used one - I wasted some time attempting to pull the old one apart - at least confirming that it was failed diodes which in theory might have been able to be replaced but in practice it was never going to happen unless you had access to a full machine shop and lots of spare time, it was never made to be a serviceable item.

That car is 18 years old - we've had it for about 8. It's got a Panasonic branded battery in it which is possibly original as it wasn't new when we got the car. Even if not, 8+ years is good going if 18 seems unbelievable. I wonder if the lifespan is related to heat - the battery is in the boot, not the engine bay.

ubergeeknz

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Reply # 1526218 5-Apr-2016 10:36

Fred99:

I wonder if the lifespan is related to heat - the battery is in the boot, not the engine bay.

Definitely heat affects the lifecycle of the lead-acid batteries.

Sidestep

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Reply # 1526223 5-Apr-2016 10:47

CYaBro:

Yogi02:

Could be as a result of a low voltage code (or other) in the computer. Might need to get it plugged in and check for codes and have them cleared?

Thanks, that is something I was just thinking about.

Wonder if I can clear the computer myself without having to pay someone to do it? :)

Yes, in that year Subies there are a couple of wires, terminating in unconnected plastic plugs directly under the steering column.

Connect the correct 2 together, (from memory the 2 blacks) turn on ign dash lights will flash any stored codes via long or short flashes.

Connect the next 2 together, clears codes. Google will tell you the specifics for you car.

Bit of a roadside emergency method, probably better to borrow a code reader.

Aredwood

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Reply # 1526623 6-Apr-2016 00:19

CYaBro: And what is active alternator control?

I know nothing about cars. 😱

Where the ecu can vary the alternator output voltage. It is done to get slightly better fuel economy. As varying the output voltage varies how much engine power gets used by the alternator.

Note also that it is possible for an alternator to fail in that it will still charge the battery. But will quickly drain the battery when the engine is off.

Batman

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Reply # 1526668 6-Apr-2016 07:23

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The AA can diagnose this on the spot if you are a member.