Subaru Forester Owners Forum banner
1 - 13 of 13 Posts

·
Registered
2004 Forester XT
Joined
·
6 Posts
Discussion Starter · #1 ·
Hi

Would really appreciate some help with this one:
(NB We are in France not US!)
Problem started off with a broken alternator belt. This was replaced by a local mechanic who had recovered the vehicle.
On driving home, got a full set of warning lights and a dead battery. Mechanic came out and diagnosed a failed alternator.
New alternator fitted (not a Subaru one) and whilst charging ok, engine management light came on with an error code P0558.
Took the Forester to nearest main dealer (90 minute drive away) who said that the problem was the alternator and quoted 3 weeks for a replacement plus a cost of €800 for the part alone.
Returned home and sourced another alternator which the suppliers guaranteed was a compatible.
This was fitted - no change; P0558 error remains.
Sourced another alternator, this time a second hand Subaru (Mitsubishi) one with correct part number. Again no change.

Has anyone out there got any thoughts, bearing in mind that the value of the Forester is probably less than what the dealership would charge me to fix the problem.
(Apart from this, the car runs really well - 140,000 miles - and loathe to lose it!

Cheers
roxyboris
 

·
Create Value
2004 FXT 5MT
Joined
·
267 Posts
Hi roxyboris,

At first glance it looks like P0558 is related to the brake system, not the alternator/charging system.

The dealer you went to appears to be a waste of time and money. My wife replaced the alternator on our 2004 Forester XT with no prior mechanical experience and no help other than YouTube, if she can do it, you can too.

It sounds like there are two potential issues:

1. alternator
2. your brakes and the P0558 code

Regarding the Alternator:
Borrow a voltmeter from a friend. Assuming your battery is not damaged or dead, it should read around 12 to 13v when the car is off.
Turn the car on, accessories and lights off, measure the battery voltage again with the car running, it should be reading around 13v.
If it is reading 11v or lower while the car is on, then you likely have an alternator issue. If it is reading 12.5 or 13.5v, your alternator is probably fine.
There are numerous threads on these forums on how to diagnose a failing alternator, so I won't repeat those steps in detail here.

Regarding the P0558:
Are you experiencing any issues with your brakes?
Have your brakes recently been serviced?
Do your brakes feel hard to push down?
 

·
Registered
non-subaru
Joined
·
911 Posts
The alternator is likely fine. Your BATT light would be on if it was bad.

Like cre8value suggested, p0558 is related to your brake booster.
Edit1: Thinking about this some more, I wonder whether, when the alternator was replaced, the vacuum line to the brake booster wasn't knocked off or pinched. Maybe the connector to that brake boost sensor got unplugged.

Edit2: Never having seen a p0558 code, I did some searching and came across this article:
It suggests that a wrong alternator can actually cause that code. The two seem totally unrelated to me but, this being the internet, it has to be 100 percent correct, right? ;)
 

·
Registered
2004 Forester XT
Joined
·
6 Posts
Discussion Starter · #4 ·
Hi

Thanks for the comments and advice.

Having run out of patience with the dealer and local guy, did fit the 3rd alternator myself. Apart from dropping a couple of sockets into the engine pan - no problem.

Battery is 12.9v when off; 14.6v when running with no lights or accessories running.

Re the Just answer.com link - this what originally sent me down the 'is it the alternator?' route. The dealer and Subaru UK both suggested it was the wrong alternator which is why I bought a genuine part.

Apart from the light the car is running fine. Brakes are ok. I will check out the brake booster suggestion tomorrow.
 

·
Registered
non-subaru
Joined
·
911 Posts
Sounds like your alternator is just fine. With the alternator at the proper 14V, can you reset the ECU?

Edit: Being intrigued by your P0558 fault, I asked google about it some more. It came up with several threads on other forums on other model subarus that basically describe your problem - so it seems to be more than anecdotal.
No idea what a break booster code would have to do with a bad alternator, though. Very strange.

There's apparently a third wire that's used on certain years and models' alternators. When it is swapped with one that does not have that third connection, that P0558 code appears.
Since you now so have a correct alternator, perhaps check that three pin connector to the alternator and see whether it makes it back to where it needs to go. Perhaps one of those wires broke when the original alternator was replaced.

Since yours is indeed making 14V now, the following is probably moot but does describe a hack to make the car accept other 'two connector' alternators:
Maybe probe those three pins they talk about to see whether those signals make sense. I'd imagine that the sense voltage is in some proportion to the load, for instance.

Also, several threads say that P0558 means "P0558 Alternator Circuit Low" when it is reported by a Subaru scan tool. I didn't realize it was legit to just remap generic codes but Subaru may have done that.
It should mean "P0558 Brake Booster Pressure Sensor Circuit High" but I've never seen a pressure sensor in the brake booster system - and have had Subarus since 1984. (Then again, I never went looking for one because I've never seen a P0558 code.. )

If they reused that code for this purpose, maybe the ECU measures what the alternator does across one of those three wires. If one is broken, you see the code. That is just a guess but worth looking into.
 

·
Registered
2004 Forester XT
Joined
·
6 Posts
Discussion Starter · #6 ·
Have re-set the ECU but fault returns after 10 seconds or so. Have checked the blue signal wire to the ECU and doesn't appear to have a problem.
Guess will have the bite the bullet and go back to the dealer!
 

·
Registered
non-subaru
Joined
·
911 Posts
Have re-set the ECU but fault returns after 10 seconds or so. Have checked the blue signal wire to the ECU and doesn't appear to have a problem.
Guess will have the bite the bullet and go back to the dealer!
That does suck. Please do document the outcome here, tho.
That weird seemingly unrelated code no doubt sends people down a dark alley with a butter knife,
 

·
Registered
2004 Forester XT
Joined
·
6 Posts
Discussion Starter · #8 ·
Update as requested: finally got a Subaru dealer to check it out. They said it was the alternator - for sure. New one ordered by them - took a month to arrive from Japan. Spent day yesterday at dealership while they fitted it. No change. Battery charging ok, they confirm signal wire is ok. they now say it needs a new ECU at €1500 plus fitting. Ffs - I will have spent more than the car is worth!
I can get a recovered ECU but this is a whole new world for me - can someone tell me if there is an issue with the security system if I just swap them over? I see on the re-selling sites that the ECUs are sold with a key, barrel, immobilizer as well. I apologise if this a really dumb question!
 

·
Registered
non-subaru
Joined
·
911 Posts
Update as requested: finally got a Subaru dealer to check it out. They said it was the alternator - for sure. New one ordered by them - took a month to arrive from Japan. Spent day yesterday at dealership while they fitted it. No change. Battery charging ok, they confirm signal wire is ok. they now say it needs a new ECU at €1500 plus fitting. Ffs - I will have spent more than the car is worth!
I can get a recovered ECU but this is a whole new world for me - can someone tell me if there is an issue with the security system if I just swap them over? I see on the re-selling sites that the ECUs are sold with a key, barrel, immobilizer as well. I apologise if this a really dumb question!
Remember that article I mentioned before?
In it, is says "Subaru Foresters do apparently have an ECU software upgrade available from the Subaru dealer, but for all the other models there is no software upgrade fix so the following work around procedure can be performed. "

I hate to think the dealer is not being honest with you but (1)An alternator, for one, does not take a whole day to install. Since they bill in hour increments, that's all they should charge you.
(2) A new ECU may very well fix it because it has ^that^ firmware upgrade as being part of it. They should know that. It is apparently a fix Subaru has identified for Foresters of your vintage.

If I were you, you have little to lose. I'd do the modification as described in the article. See whether that gets rid of the problem.

I don't know whether replacing an ECU requires programming and setup by a dealer. If I had to guess, I'd say yes - they need to do something to make it work. You could get into a situation where, after you replace the ECU, you'd still have that problem because the replacement ECU does not have that firmware update. So you'd be back to square 1.
Id' bite the bullet. See if that modification fixes it. It seems like a cheap and easy thing to try. You can always undo it, should it not work.
 

·
Registered
2004 Forester XT
Joined
·
6 Posts
Discussion Starter · #10 ·
Thanks for your comments.

To be fair, they also had to do some work on the front suspension. They reckoned that they spent 3 hours trying to work out why the new alternator hadn't solved the problem and didn't charge me for the extra time. (Probably because I was so ****ed off.)

I think my next step is to speak to Subaru UK and see what they have to say. Will post back on here for anyone who is interested.
 

·
Registered
2003 Forester 2.5XS manual
Joined
·
7 Posts
So here's me a couple of hours ago, poking around under the bonnet, looking for a "Brake Booster Pressure Sensor" after downloading P0558, convinced the OBD reader was stuffed, or the the ECM was stuffed , or I was just going crazy! So the alternator I fitted last weekend WAS wrong - and now I just have to put the noisy one back on tomorrow, clear the code, and Voila!
You guys have saved my sanity. Cheers.
 

·
Registered
non-subaru
Joined
·
911 Posts
@AM Suzuki Good. Glad this proved useful.

It is a head scratcher as to why Subaru reused the P0558 brake booster pressure sensor code. Probably because Subarus don't have "Brake Booster Pressure Sensors" and someone must have thought that new codes cost money. :rolleyes:
Seems hosed to me because it sends people down rabbit holes for no reason.
 
1 - 13 of 13 Posts
Top