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Check engine light and battery/alternator Q's


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7 replies to this topic

#1 hop

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Posted 02 November 2004 - 12:51 PM

I got the codes read and replaced the rear O2 sensor like they said. Now the light won't go off. Well, it did, when the mech. reset everything, but it came back on within 20 miles. I went back to the mech. and asked them what was up, and they didn't know. reset again, back on again after 30 or so miles. went back, and they told me that w/o changing both front and rear o2's they won't know for sure. maybe I got a bad replacement Bosch, but how to tell unless they take the car and do their tests for $48989 an hour? I don't want to go replacing everything but at the same time I'm a bit pissed off at this.

In other news, same thing with my battery and/or alternator. Now the car won't start when I leave the radio on for 10 minutes. I could change both, or I could change the battery (but a bum alt. will kill a new battery, right?) or I could change the alt. Sounds like that might be the way to go. Or I could just turn off the radio....

#2 Glenner55

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Posted 02 November 2004 - 12:59 PM

I had some wierd stuff going on with my 98 Gt and after replacing a number of parts traced it back to bad ground wire from the main fuse box. You may want to look at that. The issue being that the ECU is getting inconsistent voltage particularly at idle and it messes up the reading giving false check engine light signals. Apparantly Subaru has some issues with their electrics.

Glenn


I got the codes read and replaced the rear O2 sensor like they said. Now the light won't go off. Well, it did, when the mech. reset everything, but it came back on within 20 miles. I went back to the mech. and asked them what was up, and they didn't know. reset again, back on again after 30 or so miles. went back, and they told me that w/o changing both front and rear o2's they won't know for sure. maybe I got a bad replacement Bosch, but how to tell unless they take the car and do their tests for $48989 an hour? I don't want to go replacing everything but at the same time I'm a bit pissed off at this.

In other news, same thing with my battery and/or alternator. Now the car won't start when I leave the radio on for 10 minutes. I could change both, or I could change the battery (but a bum alt. will kill a new battery, right?) or I could change the alt. Sounds like that might be the way to go. Or I could just turn off the radio....



#3 urabus retserof

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Posted 02 November 2004 - 06:48 PM

If your alt. or battery is bad you will get all sorts of weird stuff going on. If you don't have a meter to check the output, just disconnect the battery with the car running to check it. If it stalls it's the alt.that's bad, if not get the battery tested.

It's impossible to troubleshoot without a good charging system.

#4 Cougar

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Posted 02 November 2004 - 09:04 PM

I would see if you can find a parts store that will read the codes again for you. If the O2 code is still there and the shop did the work on replacing the O2 sensor then they should put in another sensor for you for just cost of the part. If they can't tell a bad sensor from a good one then it seems they are not a place to go back to for service.

With your charging problems it would be convenient also if a parts store could do a check on the charging system. You may have a weak alternator or bad connection to the battery. If the battery is over 5 years old you are due for a new one anyways. It is a good idea to change the battery and alternator together. Then you know things should be fine for some time.

#5 kh0432

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Posted 13 November 2004 - 10:29 PM

DO NOT DISCONNECT THE BATTERY TO CHECK ALTERNATOR OUTPUT!!!! This worked in the 60's but don't do it on any computer equipped car. An alternator can be defective and still provide enough current to run the ignition system. Just because the engine does'nt quit does'nt mean the alternator is good. When you disconnect the alternator while the engine is running you run the risk of a momentary voltage spike damaging the computer. Very likely if the alternator has a bad diode. If the alternator is completely dead, the disconnected battery test will tell you that. But it's not 100% accurate and you can do some real damage.

#6 shimonmor

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Posted 14 November 2004 - 12:11 AM

If you don't have a meter to check the output, just disconnect the battery with the car running to check it.


Yeah, I have to agree that disconnecting the battery while the engine is running is a bad idea. The voltage regulator has a battery sense lead which tells the alternator what to put out. If you remove the battery, the battery sense lead will read 0 volts and the voltage regulator will tell the alternator to keep cranking up the voltage till the battery sense lead reads appropriately (which it never will since there is no battery) which leads to something blowing...most likely the diode pack. As far as I know...never remove a battery from a running alternator...as far as the computer stuff...don't know much about them.

#7 urabus retserof

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Posted 14 November 2004 - 04:43 AM

DO NOT DISCONNECT THE BATTERY TO CHECK ALTERNATOR OUTPUT!!!!


This procedure is no more dangerous than giving/getting a boost with jumper cables. Most makers recommend you don't do this either, but what would you call a guy that wouldn't help you in your time of need?

the alternator to keep cranking up the voltage till the battery sense lead reads appropriately (which it never will since there is no battery) which leads to something blowing


The output of the alternator is ultimately controlled by the rpm of the engine. That is why there is a high idle device to compensate for high drain items like the AC. The regulator does not control the alternator, but is designed to ensure that the preset voltage, and no more, is fed to the rest of the system. This crazy alternator runaway idea, is just that.

I did recommend the use of a meter, but in a pinch, this method does work. 99% of the time when an alt goes, there is no output and the car will stall.

#8 shimonmor

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Posted 14 November 2004 - 11:09 AM

Well, I'm no alternator expert but I have been taught to not disconnect a battery from a running alternator. I've never done it but I have removed about 6 alternators from engines on boats when people shut off their battery switch while the engine was running (thus disconnecting the battery). And each of those alternators was toasted and sent to an alternator shop to be repaired. Now, these engines were all diesels and not automobiles so maybe there is a difference but I think the theory of operation would be the same. So, I will have to disagree with your explanation until I hear more concrete proof. I'm always willing to learn new things and these forums have taught me a lot...but I'm not yet convinced by your explanation.

Here is a quote from a page I just looked up on Google (http://www.bcae1.com/charging.htm):

Warning!

Some people tell you that you can check your alternator by disconnecting it from the battery to see if the alternator can produce enough current to keep the engine running. BAD IDEA! Disconnecting the battery will subject the voltage regulator (and computer and audio equipment...) to significant voltage spikes which may cause an otherwise good alternator to fail. Even if there were no damaging spikes, this test would not indicate whether or not the alternator was good because the engine will easily run with a weak or failing alternator.

Simple Test:
If you want to see if your alternator is producing current, turn on your headlights when you're parked and the engine idling with the headlights shining on a wall (at night). Notice how bright they are. Then turn the engine off. The lights should get dimmer when you turn the engine off. If the lights get brighter when you kill the engine, the alternator was not charging sufficiently. When doing this test, the lights should be the only load (turn the stereo, a/c and other accessories off). With a heavy load, an otherwise good alternator may not be able to produce sufficient amounts of current at idle.


And here is another (http://autorepair.ab...y/aa101604g.htm):

6. Test the alternator by disconnecting the battery with the engine running.

A battery acts as a voltage stabilizer or filter to the pulsating DC produced by the alternator. Disconnecting a battery while the engine is running can destroy the sensitive electronic components connected to the electrical system such as the emission computer, audio system, cell phone, alarm system, etc., or the charging system because the peak voltage can rise to 40 volts or more. In the 1970s, removing a battery terminal was an accepted practice to test charging systems of that era. That is not the case today.





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