[Micron]

If the fuse is blown, there would normally be battery voltage across it. If the fuse is good, there should be no voltage across it.

I think you have stated that backwards?

 

[Dukdukduece]

According to the 2011 wiring diagrams, if the 120 A main fuse in the engine compartment fuse box is blown, there are still circuits that are powered, including the interior lighting, e.g., the map lights and the ceiling light (when manually switched on), and the horns.

That, of course, presumes the battery is good, and the wiring from the battery to the fuse box is good.

If the interior lighting doesn't work, fuse #10 (10 Amps) in the engine area fuse box should be checked. With the meter, also check to see if there's 12 V at both sides of the fuse. And, certainly, check the battery voltage at the two battery posts, then at the cable clamps.

The FSM doesn't seem to show any fuses (or fusible links) at the battery, as in the photo above, but it wouldn't surprise me if there is.

When the battery cables are connected in reverse, current from the battery has a direct path through the alternator. If (and this is important) the ignition switch was at OFF, most circuits are disconnected, and the remaining ones all have fuses with relatively lower values, so other than blowing fuses, those circuits should be intact. However, the current through the alternator goes through the main 120 A fuse, so it, and the wiring to it, could be damaged. In this regard, the 120 A main fuse, might not have blown, but 100+ Amps might have been going through the wires, and they could be fried. The latter includes the positive wire from the battery positive post to the fuse box and from there to the alternator output post, and, the ground wire from the battery negative post to the body and/or engine.

Thank you for your insight. Believe it or not... I do understand. I also feel that I have gotten in over my head with diagnosing and fixing. I can change the ECU but I am not convinced that is the problem. At this time I lean toward a fusible link or the anti-theft. Are there numerous fusible links beyond the engine fuse box? Would the lights, horn, brake lights work if the main fuse or the fusible link before the battery were blown? If the ECU was fried? Dang this is frustrating. (maybe because I'm not a mechanic but a dumbass for getting myself into this).

 

[Dukdukduece]

That's the same as the earlier generation. @walker circled the main fuse, which is held in by two bolts. Check it as suggested. If the fuse is blown, there would normally be battery voltage across it. If the fuse is good, there should be no voltage across it.

One of the two bolts goes to the battery positive. Measure the voltage between the battery negative post and each of the bolts at the fuse. There should be voltage on both sides if the fuse is good, and only one side if the fuse is blown.

Did you check to see if the map lights work, and if fuse #10 is good? If the map lights and ceiling lights don't work when manually turned on, then even if the main fuse were blown, there's still another issue.

Thanks so much for the input. I've got tonight and then it's tow truck time....uuughhhh!

 

[Dukdukduece]

Sorry I can't add more. I was a mek-anik for many years but never worked on Subarus (mostly trucks of all sizes) so while I wonder if you've pooched the electronics if all fuses and fusible links are OK, it would be irresponsible for me to coach you any further with one possible exception. If you think you've tripped the anti theft, I would start researching that if it were me. Hopefully someone who knows the Outback electrical system will chime in.

Thank You... one more stab at it tonight and then tow truck....

 

[Dukdukduece]

I think you have stated that backwards?

I assumed the same thing... I hope I assumed correctly.

 

[dukey33]

I think you have stated that backwards?

I see what OM is getting at.
If the fuse is blown, measuring across the fuse posts/terminals should have a potential (voltage), especially if there is a short to ground downstream of the fuse.
If the fuse is good, there will be equal voltage on both sides of the fuse so the meter will show zero potential.

I think it would be more reliable to measure each side of the fuse relative to ground and expect to have battery voltage on both sides.

 

[Micron]

I think it would be more reliable to measure each side of the fuse relative to ground and expect to have battery voltage on both sides.

Exactly.

 

[plain OM]

I think it would be more reliable to measure each side of the fuse relative to ground and expect to have battery voltage on both sides.

That's what I have in the second paragraph. (I have now crossed out the problematic sentences.) I messed it up doing an edit shortly after posting.

If the fuse is blown, measuring across the fuse posts/terminals should have a potential (voltage), especially if there is a short to ground downstream of the fuse.

However, it doesn't require a short to ground on the downstream side. Example:

The radio's memory is connected downstream of the fuse -- it draws only 1 ma continuously to save the stored stations, etc. In a 12 V power system, that current load corresponds to a 12,000 Ohm resistance (12/0.001). Many digital multimeter voltage ranges have an input resistance of 10 megOhms. When the meter is connected across the fuse, it's resistance is then in series (from the battery positive to ground) with the radio's 12,000 Ohms. With the battery positive on the upstream side of the fuse, the voltage across the meter leads will be 10,000,000/10,012,000 of the battery voltage; i.e., the meter will read close to battery voltage. Compare that to the voltage being zero if the fuse is good, and that leads to the conclusion the fuse is bad.

 

[Micron]

Why so complicated?
battery ground to fuse feed side: 12V
battery ground to fuse load side: 0V
conclusion = fuse bad

 

[johnre]

Why so complicated?
battery ground to fuse feed side: 12V
battery ground to fuse load side: 0V
conclusion = fuse bad

The confusion is whether the voltage measurement is done:

• just as you state above (i.e. two comparison measurements taken at both ends of the fuse against the same ground reference), or
• a single masurement taken differentially across the fuse, floating the voltmeter

Either way is valid as long as you maintain a disciplined approach in your measurements and understand the results you are getting. It will matter though how you document the approach to others.

FWIW I thought it was the second approach that was being described.

 

[John_K]

Please let us know how this turns out. Reversing the input voltage on a DC circuit may destroy components downstream of the voltage source without blowing fuses. It is interesting that the horn and simple light circuits may still be working because they are not polarity sensitive. On the other hand, the Subaru's sophisticated control modules may have been fried due to this event. I wish you good luck with this.

 

[apb]

Please let us know how this turns out. Reversing the input voltage on a DC circuit may destroy components downstream of the voltage source without blowing fuses. It is interesting that the horn and simple light circuits may still be working because they are not polarity sensitive. On the other hand, the Subaru's sophisticated control modules may have been fried due to this event. I wish you good luck with this.

Shoudn't decent design prevent reverse polarity from destroying things simply by interposing diodes to prevent current flow in the wrong direction? Even better would be a rectifier bridge circuit to make battery polarity irrelevant. Of course, that does cause a small voltage drop, which could be a problem if the battery is almost dead.

 

[Micron]

Where do you draw the line? What technology would we impose to prevent forgetting to torque wheel nuts?

Connecting a battery is very basic stuff and anyone who knows what they're doing AND is paying attention would never do this. I've never connected a battery backwards and I've done it thousands of times. Even if I were to put the positive clamp over the negative post, I'd have no problem because a) I'd see right away that it doesn't fit and b) the ground wouldn't be connected. Ground is off FIRST and on LAST and there are reasons for that. Note that I didn't say "negative" because that is not inclusive enough. The point is, you have to know more about what you're doing besides how to turn a wrench.

 

[apb]

Where do you draw the line? What technology would we impose to prevent forgetting to torque wheel nuts?

Connecting a battery is very basic stuff and anyone who knows what they're doing AND is paying attention would never do this. I've never connected a battery backwards and I've done it thousands of times. Even if I were to put the positive clamp over the negative post, I'd have no problem because a) I'd see right away that it doesn't fit and b) the ground wouldn't be connected. Ground is off FIRST and on LAST and there are reasons for that. Note that I didn't say "negative" because that is not inclusive enough. The point is, you have to know more about what you're doing besides how to turn a wrench.

I haven't replaced thousands of batteries, but in the many that I've done, I've never done it backwards, nor almost done it backwards. However, not everyone is as brilliant as you or I. In fact, because there is so much wrong data about group number interchangeability, it's very easy to end up with a battery that has the terminals on the wrong sides for your car, thus making it easy to put the battery in backwards, and hence install reverse polarity.

The incremental cost of reverse polarity protection is very small. What about tire pressure monitoring? I have never driven around with underinflated tires, despite hundreds of thousands of miles of driving. Anyone can easily tell if their tires are underinflated. Why bother with that expensive technology?

 

[SilverOnyx]

It is hard to know where to draw the line on nanny technology, but nobody is going to suffer from reverse polarity while driving at 70 mph on the freeway, but they could have a blow-out.

The damage from reverse-polarity will cost money but not lives.

Another fine point is that reverse polarity is the result of an inappropriate action, but low tire pressure is usually the result of neglect/inaction.

 

[z28dug]

The incremental cost of reverse polarity protection is very small. What about tire pressure monitoring? I have never driven around with underinflated tires, despite hundreds of thousands of miles of driving. Anyone can easily tell if their tires are underinflated. Why bother with that expensive technology?

Because .gov said so.

I agree with some of the previous posts. It's pretty difficult to install a battery with reverse polarity.....but obviously not impossible.

 

[johnre]

In fact, because there is so much wrong data about group number interchangeability, it's very easy to end up with a battery that has the terminals on the wrong sides for your car, thus making it easy to put the battery in backwards, and hence install reverse polarity.

The post position on a battery has nothing to do with the group number - rather, it's all about the letter "R" that is afterward (or absence thereof), meaning, of course, reversed position. Pretty simple and foolproof designation, as long as you're paying attention.

 

[Micron]

However, not everyone is as brilliant as you or I.

I'm was never brilliant - just schooled and trained.

 

[apb]

The post position on a battery has nothing to do with the group number - rather, it's all about the letter "R" that is afterward (or absence thereof), meaning, of course, reversed position. Pretty simple and foolproof designation, as long as you're paying attention.

Unfortunately, I've seen auto parts places that give the wrong version for various vehicles.

 

[Rymar]

Actually reverse polarity protection, at the kind of currents seen in car batteries (hundreds of amps) is expensive, and difficult to do well.

A full wave bridge by itself will not work: current needs to flow both into and out of the battery, and full wave bridges allow current flow in only one direction. Additionally, the voltage drops would kill the whole calibration of the charging voltage, so the alternator output would need to be turned up.