matt167 Posted November 8, 2015 Share Posted November 8, 2015 2004 wrx, had a dead batt the other day, now has a cel. Have not gotten to scan code yet.. It had a code last week for a knock sensor, first it had ever triggered a code in my care. I reset it and figured if it came back, I would replace sensor. But this time I think it might be alt related? I have not got to test diodes yet but its drivable still, and charging so I think if anything, the rectifier is gone Quote Link to comment Share on other sites More sharing options...
987687 Posted November 8, 2015 Share Posted November 8, 2015 If it's charging fine and you didn't fix the knock sensor, it's probably the knock sensor... Just go to autozone or VIP or something, they'll usually scan the code for free. Quote Link to comment Share on other sites More sharing options...
matt167 Posted November 8, 2015 Author Share Posted November 8, 2015 (edited) I have a scanner. Its not my car tho, so I haven't been able to get to it... It could stay charging if the diodes were bad. It would put a lot of ac voltage in the system tho. Edited November 8, 2015 by matt167 Quote Link to comment Share on other sites More sharing options...
Bushwick Posted November 8, 2015 Share Posted November 8, 2015 There are ways to test for the AC issue. You could also drive to an Auto Zone, unbolt the alternator, walk in with it, and have them test it (call first to make sure they have the tester) OR go to an actual generator shop that specializes in alternator upgrades/repairs as they'll be able to test bench it and repair, and in many cases might have a loaner they can hand you in the meantime (need to ask) if they'll need to order something. If alt is bad, it should put a charge light on the dash unless they did away with them by 04'? Remove the serpentine belt and start the car or just unplug the alt at it's connector instead. Any lights on the dash now appearing? That'll be your warning light for if/when it fails. If knock sensor is bad, replace it. If the engine has a turbo, it's more important to have that working correctly as you don't want erratic timing being abruptly added and removed under harder acceleration. My knock sensor that failed a couple months ago had a break in the pigtail wire of the actual sensor itself, and under engine movement it sometimes canceled the CEL only to reappear later. They are cheap and easy to replace, so quit dragging feet. You'll also get better mileage and performance will return. Quote Link to comment Share on other sites More sharing options...
mikaleda Posted November 8, 2015 Share Posted November 8, 2015 Its possible, but its not likely. Its easy enough to have the alternator and battery tested buy most any auto parts store or shop For free since they are hoping they will be able to sell you one or the other. If your battery is old and sounding like its on its last legs you should probably replace it anyway. Quote Link to comment Share on other sites More sharing options...
matt167 Posted November 8, 2015 Author Share Posted November 8, 2015 (edited) I can do an A/C voltage test on the alternator if I can get to the car. It's not mine tho, it's my GF's and I won't see her or the car till at least tomorrow night.. I just want to make sure shes not stranded before I can check the car. she does have a real volt gauge in the center clock replacer gauge pod, and knows to alert me if it drops below 12v or goes above 15v I didn't change the knock sensor when the code came up because the turbo sensors are in the $80 range, not the usual $20. So I wanted the code to come back once before I swapped the sensor. It will limit the advance I think with the sensor code, so it's safe to drive at least as long as she does not hammer on it but she does not drive like that. Batterys on the replace list due to the no start Edited November 8, 2015 by matt167 Quote Link to comment Share on other sites More sharing options...
987687 Posted November 8, 2015 Share Posted November 8, 2015 If you're gonna test for bad diodes with a volt meter it has to be a true RMS meter, that is, the meter measures the frequency and calculates the RMS voltage. If you use a cheapo volt meter it assumes any AC voltage is 60hz because that's all most people ever use it for, since the ripple coming out of an alternator is more than 60hz you'll get inaccurate readings, usually way higher than what's actually there. 1 Quote Link to comment Share on other sites More sharing options...
matt167 Posted November 8, 2015 Author Share Posted November 8, 2015 I do use a cheapo meter, and have found it accurate when I have diagnosed bad rectifiers, that's how I was shown and always done it that way.. As I recall, .5V A/C is the max limit. I tested the alternator on my 2001 Forester which I'm pretty sure is the same alt as the WRX, and got .02V A/C and 14.5v charge voltage, so I can use those as baselines knowing that it should be at least close to that. sometimes with high A/C it will also output a low D/C voltage while charging too... Over the summer, I had a 1995 Chevy pick up of a friend of mines. batt registered 12.6v with engine off. When engine was started, voltage went down to 11.6 but alternator was charging. When I checked the A/C voltage, it was putting out 15v of A/C.. I put in a $40 junkyard alternator in it, and problem fixed.. Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted November 8, 2015 Share Posted November 8, 2015 There are no codes that pertain directly to the alternator as far as I know, (at least not on that model year) but a charging problem resulting in low voltage can set other codes. 1 Quote Link to comment Share on other sites More sharing options...
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