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2008 Outback LL Bean 3.0 R
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Discussion Starter #1
Car in question is a 2000 Outback with 250k+ miles on it. New battery purchased within last 12 months

Car needed a jump on its most-recent drive, and completely shut off at the end of the trip - coasted into the driveway. Car did not respond to a jump and no dash lights came on, nothing. Battery read about 11.3 volts on the multimeter. Charged the battery up to full strength (confirmed 12.5v reading with multimeter) and reconnected it - open door chime works, but still no dash lights or any response to key in ignition.

Found that the main fuse (80A with two 8mm screws holding it down) was blown, so we installed a new one. The new one blew as soon as we connected the battery. Any suggestions? We've tried cleaning the grounds we were able to locate but that made no difference.
 

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Check the alternator? Pull it and find a local auto parts store that can test it.



EDIT: re-read, twice. OK, may be more.......
 

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2008 Outback LL Bean 3.0 R
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Discussion Starter #5
You obviously have a short somewhere. Does this car have a lot of rust?
There's certainly rust present given its age and location (in VA about 2 hours south of DC, our roads got physical pieces of salt for a while but for the past several years they've used the spray instead); I'm very green when it comes to diagnosing electrical stuff, how would I go about finding a short if I can't connect the battery without blowing that main fuse?
 

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Just a wild guess, but are you sure you connected the battery up to the correct posts? There's another thread here where someone didn't do that and it blew the main fuse.
 

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Discussion Starter #7 (Edited)
Just a wild guess, but are you sure you connected the battery up to the correct posts? There's another thread here where someone didn't do that and it blew the main fuse.
99.999% sure. I plan on picking up another fuse or 2 so we can try again this weekend, but as I remember we were pretty careful about making sure the connections were correct when we re-attached.
 

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how would I go about finding a short if I can't connect the battery without blowing that main fuse?
A digital multimeter could help find the problem without having to blow fuses. The fuse blows at 100 Amps or more, and that high a current, in itself, even for a moment, could cause additional problems. Blowing fuses to find a short really isn't a good idea.

The meter can be used to measure the resistance from the fuse contacts in the fuse box, to ground, when everything is turned off and the battery is disconnected. As the fuse blows as soon as the battery is connected, the meter reading at the downstream, load side, fuse contact will probably be very low, i.e., indicating a short.

Once the short is verified by the meter, the next thing would be to disconnect the cable from the alternator output post and check the resistance again. A shorted alternator can conduct very high currents, and cause the fuse to blow. If removing the cable from the alternator eliminates the measured short, then the alternator is the problem. "Fault found, no fuses hurt in making this diagnosis."

The main fuse is the only fuse that has such a high rating. Pretty well all other circuits are fused at lower levels. Consequently, if the problem were in another circuit, the fuse for that circuit would blow first. That tends to narrow down the areas where a fault would blow the main fuse. The alternator itself is one. The other is in the wiring between the fuse and the connections to those other circuits.

Related wiring diagram attached.
 

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R.I.P. 2009 OB 2.5i 5MT
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There's certainly rust present given its age and location (in VA about 2 hours south of DC, our roads got physical pieces of salt for a while but for the past several years they've used the spray instead); I'm very green when it comes to diagnosing electrical stuff, how would I go about finding a short if I can't connect the battery without blowing that main fuse?
Do you have an ohm meter? Can you measure the resistance across the fuse contacts?
 

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Discussion Starter #10
Do you have an ohm meter? Can you measure the resistance across the fuse contacts?
I believe the entry-level multimeter I have has an ohm setting that allows resistance to be measured but I'll need to double check

A digital multimeter could help find the problem without having to blow fuses. The fuse blows at 100 Amps or more, and that high a current, in itself, even for a moment, could cause additional problems. Blowing fuses to find a short really isn't a good idea.

The meter can be used to measure the resistance from the fuse contacts in the fuse box, to ground, when everything is turned off and the battery is disconnected. As the fuse blows as soon as the battery is connected, the meter reading at the downstream, load side, fuse contact will probably be very low, i.e., indicating a short.

Once the short is verified by the meter, the next thing would be to disconnect the cable from the alternator output post and check the resistance again. A shorted alternator can conduct very high currents, and cause the fuse to blow. If removing the cable from the alternator eliminates the measured short, then the alternator is the problem. "Fault found, no fuses hurt in making this diagnosis."

The main fuse is the only fuse that has such a high rating. Pretty well all other circuits are fused at lower levels. Consequently, if the problem were in another circuit, the fuse for that circuit would blow first. That tends to narrow down the areas where a fault would blow the main fuse. The alternator itself is one. The other is in the wiring between the fuse and the connections to those other circuits.

Related wiring diagram attached.
Thanks @plain OM ! So if I understand this correctly, I should (with battery disconnected):

1) Measure resistance by correctly configuring my multimeter and placing the device's red pin on the metal the fuse connects to, with the black pin on a ground (any ground? Nearby spot on body frame? Somewhere else?)

2) Remove the red connector cable from the alternator and perform the same resistance measurement > a significant difference in results from step 1 indicates a bad alternator

Is that accurate?
 

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So if I understand this correctly, I should (with battery disconnected):
. . .
Is that accurate?
Yes.

Just so we're on the same page in regard to what the meter can do and what the settings should be, could you post the make and model, or a photo? I suggest this because there could be electrical differences between a digital meter and and analog type with a moving pointer.

In step one, the negative (black) test lead can go to the negative battery cable clamp. It's close to the fuse box area, and, of course, is removed from the battery.

The main fuse connects to two points in the fuse box. With the black lead on the negative battery cable, check the resistance at both points. Both, normally, would read a high resistance. But as there appears to be a short, in this case, one would likely show a low resistance. That's the one of interest.
 

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The main fuse connects to two points in the fuse box. With the black lead on the negative battery cable, check the resistance at both points. Both, normally, would read a high resistance. But as there appears to be a short, in this case, one would likely show a low resistance. That's the one of interest.
That depends whether a new fuse has already been installed. IF a new fuse is already installed, then measuring from either side of the fuse is equivalent.

With the blown fuse still installed, only one side of the fuse will show low resistance. That's the side you want to be using.
 

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Set the meter dial to measure resistance, about the 7-oclock position "2000" or "20k". It probably doesn't matter much which point in the green outline for ohms (Ω).

What does the meter say when the 2 probes are not touching anything? Maybe "OL" or "Open" or maybe just a huge number.

Touch the 2 meter probes together & see what it says. Probably just a very low value. You're looking for the difference between a large number vs. a small number. Maybe 1 ohm or less for a "short-circuit" compared to 1000 ohms or more after you fix the problem (or disconnect the problem).
 

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Discussion Starter #17
I got to take an initial run at this today:

- Black pin on (disconnected) battery negative cable, red pin on metal fuse contact closest to battery = 0.00 ohms

- disconnected red cable and connecter clip from alternator = started at 3ish ohms and climbed to 12ish.

Thoughts? No visible chewed threw/frayed wires that we were able to find.
 

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If I got this right, you unplugged the alternator and disconnected the cable from the alternator and checked from battery negative cable, still disconnected from the battery post, to the fuse and the resistance came up to about 12.

Did you isolate the alternator cable end so it was not touching anything metal on the car?

Ultimately you want the meter to indicate infinite. The same it would show you with the meter on and the leads were not touching.

With the meter showing 0.00 that means there is a connection to ground through that fuse. That's what is blowing the fuse.

The 12 means that the connection is still there, only the resistance is higher. If the alt cable was resting on metal, that may have been the reason for you getting 12.

If you had the cables disconnected at the battery and the alternator, neither cable touching metal or the battery, and the meter showed resistance, the 12, instead of infinite, there is still a short somewhere.
 

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Black pin on (disconnected) battery negative cable, red pin on metal fuse contact closest to battery = 0.00 ohms
Not sure which end of the fuse that is, relative to the wiring diagram. Nevertheless, there shouldn't be 0.00 Ohms between either side of the fuse and the negative battery cable.

However, was the meter set to the 200 Ohm range? (See annotated photo)
IMG_2164.jpg

When on the 200 Ohm range, there's only one digit after the decimal point; i.e., with a dead short across the meter sockets, it could theoretically read 00.0. Two digits after the decimal point, such as 0.00 is more likely the 20k Ohms range.

If the meter was set to the 20k Ohms range, the 3 - 12 readings would actually be 3000-12,000 Ohms. This would not be a concern.

To confirm, with the red cable off the alternator, measure the resistance from the large terminal on the alternator to the negative cable, using the red/positive meter test lead at the alternator. Use the 200 Ohm range. If the resistance is indeed very low, the alternator has an internal short. (As I see cardoc has already mentioned.)

I presume there was no fuse (or only a blown fuse) in place when the measurements were made.
 
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