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Thread: Alternator voltage drop at batteries

  1. #1
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    Angry Alternator voltage drop at batteries

    earlier this year i installed a newly re-built alternator with a potentiometer set at 14 volts. At the alternator it was showing 14 volts plus, but at the batteries could barely muster 12 volts !

    needless to say, operating my car on a daily basis drains the batteries and usually need a charge on a weekly basis...

    any ideas where the power loss could be or rationale?

    i think this has been a chronic problem with my 8 and its just come to light this year... i used to think i had a parasitic drain but i now think this was the problem all along...


    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  2. #2
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    Possible dead batteries. Batteries can be fully charged, but when you load them up, they won't hold that charge.
    Start by getting them both load tested (best if they are removed from the vehicle and tested individually).
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  3. #3
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    ....also 14V is a little low, 14.6V is the correct no-load voltage.
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  4. #4
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    Quote Originally Posted by TxGR8White View Post
    Possible dead batteries. Batteries can be fully charged, but when you load them up, they won't hold that charge.
    Start by getting them both load tested (best if they are removed from the vehicle and tested individually).
    i had both batteries tested and they CAN and WILL hold a charge... clearly thats not my problem...

    - - - Updated - - -

    Quote Originally Posted by Timm View Post
    ....also 14V is a little low, 14.6V is the correct no-load voltage.
    thanks but thats not the point... only getting 12 volts to the batteries...

    - - - Updated - - -

    looking to change up my GM... sometimes those modules control too much!! Wuffer told me he had a club guy with similar problems and after changing up the GM it went away...

    However im not as convinced...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  5. #5
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    you sure it is charging the batteries ? (blue cable to alternator not connected, tells it to charge) Measure voltage at batteries while running (should be around 14v).
    BMW 850 CSI march 1994 (EU spec), SOLD. https://www.classiccars-forsale.com/...94-bmw-850csi/
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  6. #6
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    I honestly do't think batteries are getting charged, like clubE31 suggested double check alternator install, I bet that is your problem.

  7. #7
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    Quote Originally Posted by dragon850 View Post
    I honestly do't think batteries are getting charged, like clubE31 suggested double check alternator install, I bet that is your problem.
    i had the alternator installed professionally by a BMW shop, trust them implicitly, so does Wuffer... besides this is my 2nd Alternator in 3 years... i will try the cleaning scenario...

    - - - Updated - - -

    Quote Originally Posted by clubE31 View Post
    you sure it is charging the batteries ? (blue cable to alternator not connected, tells it to charge) Measure voltage at batteries while running (should be around 14v).
    no its not charging the batteries... this is my problem... not enough voltage to the Batteries! Alternator is push north of 14v... barely 12v at the batteries...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  8. #8
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    Thank you for the valuable information! I will endeavor to clean all connection points...

    see private message...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  9. #9
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    Quote Originally Posted by cdo5oo1a View Post
    oxidized connection points will cause too much voltage drop in the total current loop. Must check and clean (deoxidize) all the connections in the current loop including the ground strap from the engine to the car and it's two connection points, the negative cables from the batteries where they bolt to the car and the positive cable connection points on the left and right side of the engine bay.

    Attachment 611970

    this type of meter at the lighter socket will show you what is happening when the engine is running and when the engine is off.

    You must also clean and then tighten all female battery terminals, and clean all four battery posts. (wire brush tool)
    left and right side of engine bay??????????
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  10. #10
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    battery connections can be tricky I had an issue with my truck where it seemed as tho the battery was going. It has side mount terminals and it turned out that the negative bolt was loose. It would start, but sounded like it was struggling.

    On another note. My 850 had a draining battery problem. I hooked up the charger over night and the next morning pulled the batteries. the one on the passenger side was warm and only held 10 volts. the other one was holding 12v. So bad battery. replaced the bad one with an 800cca from NAPA. no more power issues.

  11. #11
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    Quote Originally Posted by wolfebrian2120 View Post
    battery connections can be tricky i had an issue with my truck where it seemed as tho the battery was going. It has side mount terminals and it turned out that the negative bolt was loose. It would start, but sounded like it was struggling.

    On another note. My 850 had a draining battery problem. I hooked up the charger over night and the next morning pulled the batteries. The one on the passenger side was warm and only held 10 volts. The other one was holding 12v. So bad battery. Replaced the bad one with an 800cca from napa. No more power issues.

    ive replaced both my batteries twice in the past 5 years... Clearly the issue is with the proper voltage not getting to my batteries...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  12. #12
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    Quote Originally Posted by CDO5OO1A View Post
    Yes. Left side (Driver USA) left positive cable comes to front of car near brake master cylinder.

    Right side (Passenger USA) right positive cable comes to front of car near electric starter.


    I would investigate oxidized and loose fitting terminals on the left side engine bay first because that is where the alternator is.

    The ground strap between the engine and the body must be HIGHLY conductive (no rust, no oxidation) because the alternator uses its case as the ground reference and the alternator is bolted to the engine. Sometimes parts get painted and the paint is enough to screw up a high conductance
    ground path.

    Hopefully light bulbs will be going off in your head or your trusted mechanic will have an ... AH HA moment.


    ... One other thing is anodized parts. The real kicker is clear hard ano. When aluminum parts are clear hard anodized many mechanics are baffled by the electrical gremlins.

    Awesome thank you ! question, where would that ground strap be on the left side? is it visible?
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  13. #13
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    Quote Originally Posted by CDO5OO1A View Post
    The ground strap between the engine and the body must be HIGHLY conductive (no rust, no oxidation) because the alternator uses its case as the ground reference and the alternator is bolted to the engine. Sometimes parts get painted and the paint is enough to screw up a high conductance ground path.
    A SoCalEights member had a no start condition that was ultimately traced to a severely corroded ground cable.
    1993 850Ci.....18 years & 165,000 miles and counting!

  14. #14
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    thank you ALL for your most helpful comments, its really appreciated...

    today i replaced my grounding strap (it was one of those braided straps) with a brand new 1/4" cable with metal loop ends... i wire brushed all the terminals too.

    then i removed and clean the 2 hot terminals beside the fuse box in the same fashion.

    Then i went into the trunk and disconnected both batteries and ALL associated cables (including ground) and cleaned them all in the same fashion.

    Then reconnected the works and started the car... but first with my meter i took a reading on the battery off and read 12.60 volts. after starting my car, the voltage stayed the same... it did not jump up with juice from the alternator as it should have....

    so now what?? something is preventing the 14.5 volts my Alternator discharges from getting to my batteries
    !!

    Any ideas?

    thank you in advance and Happy Thanksgiving to my Canadian brethren...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  15. #15
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  16. #16
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    nope checked that too... maybe GM??
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  17. #17
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    Quote Originally Posted by CDO5OO1A View Post
    What you start looking for now is a section of cable that is heat damaged. When a piece of copper gets hot it loses its conductivity and is more a resistor than a conductive wire. You can get an inexpensive small clamp on current meter and start looking for current flow in the main wire coming from the alternator. Smaller gauged wires suffer from this heat damage before heavier gauges.

    Thermal cycling with the intrusion of oxidation can also cause the crimps of the terminals to loose conductivity to the copper wire they are crimped to.

    I am NOT saying that this is the problem. These are just a couple of possibilities with a 20+ year old wiring system.

    How do you know that your alternator was putting out 14.5 volts at the alternator's main terminal today?
    Makes sense... is the cable under the carpet?? does it go directly to the battery or veer off or split off too?

    the guy that rebuilt my Alternator installed a potentiometer on it to crank out 14 plus volts and my mechanic confirm the voltage after he installed it... thats how this issue came to light... other possibility is the main General Module... ordered a rebuilt from Wuffer, he says a similar issue in our club was resolved by the new GM...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  18. #18
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    Hello Starship, would you please comment in the leaking diff thread. I have the same leaking diff problem.

  19. #19
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    Before you worry about removing carpets - the first place the alternator connects to is the B+ post under the bonnet. I would suspect that this post will be at the same voltage as the batteries when the engine is running.

    Giving a mechanic a potentiometer to twiddle is a recipe for disaster - and I will give you odds of 10:1 that your problem is simply the alternator - either dead or you mechanic has adjusted it to a point where it doesn't give any current out!
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  20. #20
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    Quote Originally Posted by Timm View Post
    Before you worry about removing carpets - the first place the alternator connects to is the B+ post under the bonnet. I would suspect that this post will be at the same voltage as the batteries when the engine is running.

    Giving a mechanic a potentiometer to twiddle is a recipe for disaster - and I will give you odds of 10:1 that your problem is simply the alternator - either dead or you mechanic has adjusted it to a point where it doesn't give any current out!
    thanks for your input Timm... when i checked the voltage at the B+ post, yes its the same as the batteries with motor running however, my mechanic confirmed the 14.5 volts right at the alternator after he installed it...

    thats where this witch hunt started from...

    ergo the thought of the GM......


    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  21. #21
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    AFAIK, it is a single fat RED cable from the alternator to the B+ post - so should be easy to measure across. On the E32 I have seen the B+ post come a cropper, it has a bunch of cables bolted to it below the post - and corrosion can make loosen all of them - it wasn't a pretty sight.

    But, I have only seen this once (Shogun will remember I'm sure), and never on the E31 - my money is still on a dead alternator.
    Timm..2007 E64 650i Individual Sport..1999 E31 840ci Individual Sport..ex owner of 2000 E38 740..1999 E38 740i V8 M62..1998 E38 735i V8..1993 E32 730i V8..1988 E28 518i


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  22. #22
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    UPDATE ON ALTERNATOR...

    had my 8 in the shop to change a few hoses and fix heater console... same Mech as installed my Alternator few months ago... at my request i asked him to search out for broken or missing wiring, and he was successful. Had a plastic sheath at front of block with wiring inside that had broken wires inside and was determined these were a contributing factor to voltage loss at batteries. Wires repaired and voltage issue resolved.

    Now 14+ volts at the Alternator, same at the B+ post under the hood and 13.8 at the batteries at IDLE...

    thank you all for your valuable input !
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  23. #23
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    How did those wires get broken?

  24. #24
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    Wow! Congrats on your discovery.
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