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Unplugged instrument panel will not let alternator charge battery

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1996 Legacy L.

 

After putting my car back together following a vexing electrical problem, I was happily driving the car around for 2 days with no issue.  I still had a mess of wiring hanging out under the dash since I wanted to make sure the electrical problems were resolved before reassembling the lower dash panels.

 

I finally secured the lower dash panels and wiring, did a quick restart test and then went to bed.

 

This morning I drove the car for 20 minutes with all lights on, radio, phone charger when I noticed the Check Engine light came on.  The car bucked a couple of times, then popped out an ABS light.  I recognized this combination - I have seen before on failed alternators.   I turned off all unnecessary electrics and tried to stay off the brakes (brake lights) to get the car as close to my work as possible.  I noticed the fuel gauge was bottomed out and was puzzled why I could not see a battery light in the midst of this.

 

I drove another 10 minutes in this state of tension knowing I was exclusively on battery power in Bay Area morning commute traffic.

 

I managed to get the car into my work parking lot and was trying to back into my space when the car finally completely died.   The additional 55w reverse lights sucked out the last of the battery charge.   I ended up pushing the car into my spot for the last 10 feet.

 

Throughout the work day, I replayed what happened the night before and concluded that I had not plugged in the left side of the instrument panel's connector.  The battery warning light is on that circuit.   Without that light on the charge circuit, the alternator will not charge the battery properly.

Fortunately some wise folks on this forum have already discussed this at length....http://www.ultimatesubaru.org/forum/topic/143889-alternator-not-charging-dash-light-was-to-blame-seriously/  ! :unsure:

 

When I got off work, I quickly opened up the instrument panel and plugged the connector back in *dur*, and got a jump from a co-worker.  The car started right up and drove home with no issues other than a Check Engine light that was an artifact of the power failure when the P0500 Vehicle Speed Sensor.  This was reset with my OBD reader and we are back in business.

 

Public Service Announcement - make sure the battery light works and your connectors are installed properly! :horse:

 

 

 

 

 

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Current to the exciter passes through the warning light which excites the field windings inside the alternator. If there is no current to the exciter for some reason then you get no output from the alternator.

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I'm confused - I've driven multiple 80's and 90's subarus without the instrument cluster installed at all. including a 99 legacy sedan back and forth to work which has the same instrument cluster as yours. And an XT6 and I think my 2002 outback.

 

It sounds like it's an intermittent issue that just happened to resolve itself after the jump just like it did to allow you to drive it the first two days.

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The car can run without the cluster installed but the alternator won't be charging the battery, unless the wiring has been modified by someone.

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Wow, you're certain?

I've driven one back and forth to work in an automatic.

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If the battery is good you could drive a couple trips on just the battery.

How many days did you drive to work without the cluster?

 

The way the alternator is wired on the 95-99 Legacy cars the battery light circuit provides the current for the field exciter. Without that, the alternator will not charge.

Theoretically, if you rev the engine high enough the field can self-excite, but I dont take you for the race car type.

 

Might be a fun experiment for you. Pull the cluster out (or just unplug it) start the engine and check the charging voltage.

 

Btw, got that sprocket yesterday Gary. Thanks again

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Great, glad you got it. I'll throw away the tracking number I forgot to send you! ha, right no racing or revving involved but now I want to read the electromagnetism behind self exciting, thanks for hosing my morning.

 

I don't recall how many times i drove it, certainly not an extended time since its weird and unsafe not knowing speed and engine temps.

 

Good to know, I could see this ending bad: "fly to Florida & drive back legacy with no cluster".

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Many cars use the battery warning light circuit to provide power to the exciter inside the alternator. I had to learn this the hard way many years ago while working on a Ford pickup where the alternator wouldn't charge the battery (the warning light was making a bad connection in the bulb socket). The circuit also is used to test the warning lights and makes a connection to ground to turn on those lights when the alternator isn't running. When the engine fires up the alternator field builds up then voltage is back fed on the exciter lead and the current can't flow though the lights, so they turn off and the TEST mode is complete. If the alternator has a problem then the battery warning light will turn on.

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Theoretically, if you rev the engine high enough the field can self-excite, but I don't take you for the race car type.

 

 

I think it depends on the alternator and if it's been modified. On my old 96' Mark VIII, I had the alt rewound to a 290 amp beast due to having a heavy draining amplifier system + all the electronics that were in the car needing more. Anyhow, the shop failed to mention that the alt would no longer kick on at idle speeds, which is useless in a daily driver. I had to rev to around 800 RPM before it'd even charge, despite still having the factory exciter on/off connected. 

 

Started thinking about it, and if it needed a certain RPM to even get the "OK" to want to charge, got the idea to call an undersized pulley shop I found through Summit Racing (undersized meaning they make pulleys to slow everything down) and explained the situation. They needed crank and alt pulley OD, what idle was, and what RPM the alt needed before even wanting to charge. Cost nearly $80, but they custom ground a new pulley that allowed the alt to spin roughly 250 rpm faster at idle. Worked like a charm.

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$80 for a custom alt sounds like a superb price. Did it solve the issue?

800 rpm to charge hahaha that's great!

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$80 for a custom alt sounds like a superb price. Did it solve the issue?

800 rpm to charge hahaha that's great!

That's like the 88 mph thing from Back to the Future!

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