Good Battery + Good Alternator = Low Voltage CEL?
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  1. #1
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    Good Battery + Good Alternator = Low Voltage CEL?

    OK. Maybe somebody can help me. First my battery started to go dead. It would recharge and last a couple of days. I go and have it tested at the parts store and they say the alternator is bad. I take it home and while the car is still running I disconnect the negative lead to the battery. The car still runs. I turn on the heater, defogger, etc. and the car struggles, but with a little gas it still runs without the battery. I take the car to another store now because I am thinking the alternator must be good and they give me the same verdict: bad alternator. So I take off the alternator and the guy at the repair shop says it is fine. I decided to replace the battery but the CEL is still on.

    Good Battery + Good Alternator = Low Voltage?

    Any Advice? What else could be causing this?

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  3. #2
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    How long since the battery was replaced? It might take a few trips before the CEL turns off.

    Why did you replace the battery? Did anyone tell you it was bad?

  4. #3
    the deal finder!
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    if you have a vag, clear the code, and see if the CEL reappears.

  5. #4
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    Voltage regulator!!!!!

    If VWs still have a voltage regulator, that is what is shot. There's actually two things that could be wrong - the charging circuit could be bad or the voltage regulator could be bad. The charging circuit includes all of the wiring from the battery to the alternator (both positive and ground) and the exciter wiring. The voltage regulator is a little part that gives you the 14.4V that you are supposed to get and keeps it from frying anything.

  6. #5
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    Not sure about My Passat but, in the past i have had car run with a bad alternator, it may not be charging to full capacity and thats y your battery dies quickly. Also in the past (other cars)...didnt need the battery to drive the car just to start it (had a battery stolen out of my car...had it jumped and drove home WITHOUT a battery). A voltage regulator will also cause this type of problem...had the exact problem in my Mini 1275GT when i lived in Spain ...it ATE voltage regulators...would replace for $5 from the junkyard every 2 weeks

  7. #6
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    Quote Originally Posted by atomicalex
    Voltage regulator!!!!!

    If VWs still have a voltage regulator, that is what is shot. There's actually two things that could be wrong - the charging circuit could be bad or the voltage regulator could be bad. The charging circuit includes all of the wiring from the battery to the alternator (both positive and ground) and the exciter wiring. The voltage regulator is a little part that gives you the 14.4V that you are supposed to get and keeps it from frying anything.
    Isn't the regulator built into (attached to) the alternator?

    When a shop tests the alternator, is the regulator tested at the same time, or do they look directly at the alternator's output?

    1.8tRouble: do you have a multimeter? Get one and look at the alternator's output voltage when the engine is idling warm with no added loads (defroster, vent fan, headlights, etc.) The voltage should be something like 14.4V. You can probe it at the battery terminals. Then turn on all sorts of other crap (fan [no AC], high&low beams, rear defroster, etc.) and check the voltage again. It should go down by a volt or two, but should not be below 12V. Then turn off the car, turn something electrical on for 5-10 seconds (headlights, fan, etc.) then probe the battery voltage. It should be 12.5 or so.

  8. #7
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    If you are testing the alternator output you should be checking for AMPS for charging, you can check at the battery to see if the voltage there is going up; without current (amps) you can all the volts you want but nothing will charge.

  9. #8
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    Quote Originally Posted by ursuspolaris
    If you are testing the alternator output you should be checking for AMPS for charging, you can check at the battery to see if the voltage there is going up; without current (amps) you can all the volts you want but nothing will charge.
    Yeah, but checking the voltage is much easier.

    If the voltage is off (as in not correct), then checking the current isn't really necessary. If the voltages are all good, then you can go and check the current (making sure that the multimeter can handle big currents).

  10. #9
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    Quote Originally Posted by ralfp
    Yeah, but checking the voltage is much easier.

    If the voltage is off (as in not correct), then checking the current isn't really necessary. If the voltages are all good, then you can go and check the current (making sure that the multimeter can handle big currents).
    you can have 100V and 100mA and it'll take FOREVER to charge...reading Voltage on the way to the battery isn't enough (if you think you have an alternator/charging problem)

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    The second time I had the store test everything I got a few numbers. At idle the voltage was ~13.4V, with heavy load it was about 13V. This is normal per a VW representative. Then I look at the amps... At idle it was about 20A, with heavy load it was -20A! This made me believe that the alternator was not fully working and that at heavy load the car was actually pulling some current from the battery causing it to go dead. This is not consistent with the disconnected battery test, though, where the alternator provided enough power for the car to operate under load.

    After this I took the alternator to the shop where they tested it and found it to be good (90A output.)

  12. #11
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    Quote Originally Posted by ursuspolaris
    you can have 100V and 100mA and it'll take FOREVER to charge...reading Voltage on the way to the battery isn't enough (if you think you have an alternator/charging problem)
    But if it reads 100V you know something's more than wrong. My point was that it's faster and easier (and also safer) to check the voltage. It's a good thing to check first; it's not the only thing to check.

    I'm an electrical engineer, so I must be correct. [If only that were true]

  13. #12
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    LOL..I work with Engineers all the time and....well ....just well I was an aviation electrician in the Navy have 18+ years of experience and actually worked at a company that tested industrial batteries (GNB Industrial Power part of Exide) so I know a little about electricity and batteries it's all good though we're both on the same page just different sides of it...lol

  14. #13
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    Quote Originally Posted by 1.8tRouble
    OK. Maybe somebody can help me. First my battery started to go dead. It would recharge and last a couple of days. I go and have it tested at the parts store and they say the alternator is bad. I take it home and while the car is still running I disconnect the negative lead to the battery. The car still runs. I turn on the heater, defogger, etc. and the car struggles, but with a little gas it still runs without the battery. I take the car to another store now because I am thinking the alternator must be good and they give me the same verdict: bad alternator. So I take off the alternator and the guy at the repair shop says it is fine. I decided to replace the battery but the CEL is still on.

    Good Battery + Good Alternator = Low Voltage?

    Any Advice? What else could be causing this?
    "The second time I had the store test everything I got a few numbers. At idle the voltage was ~13.4V, with heavy load it was about 13V. This is normal per a VW representative. Then I look at the amps... At idle it was about 20A, with heavy load it was -20A! This made me believe that the alternator was not fully working and that at heavy load the car was actually pulling some current from the battery causing it to go dead. This is not consistent with the disconnected battery test, though, where the alternator provided enough power for the car to operate under load.

    After this I took the alternator to the shop where they tested it and found it to be good (90A output.)"
    *
    *
    *
    *

    I just wanted to retouch this subject as I have not found the problem yet. I had the codes checked and what it read was "System Voltage Low Voltage" Also, the idle would fluctuate at startup before I had the codes cleared, afterwards it was OK. A trip or two later the CEL came back and the idle was again fluctuating at cold start-up.

    P.S. When I took the alternator to the repair shop the guy said that sometimes the Bosch alternators would work sporadically. I wonder if this is the case...

    Anybody with some insight?

  15. #14
    Lisa Simpson
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    Only that this sounds like my Passat.

  16. #15
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    you may have an intermittent short in your battery...maybe check the impedance of it....with an intermittent short in the plates, you could have sulphation and shortened battery endurance. It is possible that it would test out ok during a short term test like at Autozone or the like.

  17. #16
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    Quote Originally Posted by ursuspolaris
    you may have an intermittent short in your battery...maybe check the impedance of it....with an intermittent short in the plates, you could have sulphation and shortened battery endurance. It is possible that it would test out ok during a short term test like at Autozone or the like.

    It is a brand new battery. I replaced it after the alternator tested OK.

  18. #17
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    Check the wires too. I once had a failed battery cable (not on the Passat) where there was corrosion beneath the insulation. Impossible to see yet it increased resistance in the conductor.

  19. #18
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    Today I checked the voltage again while running the car. Surprise, it was battery voltage. This is getting weird. Maybe there was an intermittent problem with the alternator and now it just gave out. Or maybe it is in the wiring. The battery cables are brand new so if it is wiring it must be from the alternator. Any suggestions on how or where to check this wiring? I guess the large lead goes to the battery but where does the small one go?

    If everything checks out I will take it off and have it bench tested again.

    P.S. When I removed the alternator before I went for the aggressive approach. Since it looks like I may have to remove it again I am very glad I did. To do this I loosened the lower bolt and pulled it out as far as I could, cut it off, pushed what was left back into the hole, and pulled off the alternator. Then I sawed off the protruding portion of the nut on the alternator and drilled out the the threads so the new bolt would slide in. When I re-installed it I just put the bolt in from the back and used a nut/washer/loctite combo to secure it. 15min. install or removal now. VW Rules.

  20. #19
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    Update: Hooray, the problem is fixed.

    The ground to the chassis was loose. What a headache for such a small fix. Anyway, I can remove or install my alternator in about 20min now in case it ever does go out.

  21. #20
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    haha.....what an ordeal.

  22. #21
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    Quote Originally Posted by 1.8tRouble
    Update: Hooray, the problem is fixed.

    The ground to the chassis was loose. What a headache for such a small fix. Anyway, I can remove or install my alternator in about 20min now in case it ever does go out.
    Which ground? (Battery? Alternator? ...so the next guy doesn't have to search everywhere in the engine bay)

  23. #22
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    Yes, what ground?!! I'm having a similar problem with my 2000 1.8T.

  24. #23
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    Quote Originally Posted by badmoodguy
    Yes, what ground?!! I'm having a similar problem with my 2000 1.8T.
    I would consider the ground to be battery earth point to chassis. Never knew the alternator had a ground (negative) point attached. The only way a majority of alternators get the earth point is from the body mounting bolts.

  25. #24
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    Quote Originally Posted by 1.8tRouble
    OK. Maybe somebody can help me. First my battery started to go dead. It would recharge and last a couple of days. I go and have it tested at the parts store and they say the alternator is bad. I take it home and while the car is still running I disconnect the negative lead to the battery. The car still runs.

    Any Advice? What else could be causing this?
    This I find hard to believe, disconnecting the battery whilst the engine is running WILL blow the alternator regulator! Do not try this at home kids

  26. #25
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    Steuart

    I agree!! NEVER disconnect the battery while the engine is running
    The voltage regulator needs to see battery voltage for a referance..



  27. #26
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    I had an '81 Jetta 1.8 where the alternator ground was through a braided cable hidden under the alternator. The heavy spade connector under the alternator got a crack and I had fluctuating voltage for years.

    I didn't find the problem until I used some Gunk engine cleaner and it dissolved the rubber alternator mounting washers. This resulted in the alternator occaisionally getting a good ground and blowing headlights.

    I also had a slim ground cable between the body and engine. It was an extension of the negative battery negative braid but VW had sleeved it for the loop that went between teh engine and body. This loop fills up with water and road salt and corrodes the braid to a mere shadow of it's former self. The replacement cable had no sleeve and so no long duration corrossion problems.

    Engineer John

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    There are a couple of ways to use a voltmeter to find bad connections.

    Connect the meter across any suspect connection while there is a load on it. Anything above about 0.1 V indicates a connection that is deteriorating. If the voltage goes up to 3V there will probably be heat generated under load depending on the amount of current flowing.

    Connect from the alternator positive terminal to the battery positive terminal when the car is running with headlights on to check the positive charging side.

    Connect from the alternator case to the negative battery terminal to check the ground charging side.
    If you connect to the terminal instead of the connector you
    add the connection resistances at each end and the cable resistance. This helps to eliminat a string of items at once.

    Also check the voltage from the engine block to the alternator case.

    These checks should narrow down the search quickly.

    Engineer John

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