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Batt voltage dropping at high rpms?

Discussion in 'General Tech and Maintenance' started by FLounder, Jun 29, 2010.

  1. FLounder

    FLounder power hungry VIP

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    My car did something very strange on the dyno this morning. It sits right around 13 volts idling, and as the rpms climb the voltage drops to low 12s. Also, it ran very rich as the volts dropped making terrible power. We suspected the alternator was going bad, since it should be making at least 14v at wot. The battery is brand new. We even checked the old logs and on every other dyno pull we have ever done it was over 14v. Heres where it gets weird and I get confused.....

    The alternator tested excellent on a bench test, making over 14 volts. The belts felt pretty tight and the car was making a full 10lbs of boost, so they couldnt have been slipping right? Well then how the hell am i losing voltage when the alternator is on the car?

    :headsmash:
     
  2. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    my first thought was the alternator is going bad.... then the belt could have been slipping.... you shot both of those down.... :shrug: i got nothin
     
  3. K2e2vin

    K2e2vin Senior Member

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    Maybe bad voltage regulator that wouldn't show on a slower-rpm bench test? Bad ELD?
     
  4. Dual-500

    Dual-500 Well-Known Member VIP

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    That's what I'm thinking. Another possibility is it could be a fuse or wiring issue acting intermittent.

    Ground straps on engine are good?
     
  5. FLounder

    FLounder power hungry VIP

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    yeah the grounds are tight. i am thinking about upgrading the grounds, couldnt hurt.

    thanks for the advice though, it may be some sort of regulator, i need to dig around in my service manual and figure out how to test some stuff.

    its one thing after another with this car lately :mad:
     
  6. Dual-500

    Dual-500 Well-Known Member VIP

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    They seem to go through cycles relative to issues. :)

    The grounding on my Civic uses crimped on connectors - they are subject to corrosion and vibration as well as just workmanship issues for the mechanical contact.

    If/when replacing grounds, I'll sweat solder the connections also. In fact, I've been looking at the engine ground that connects to valve cover. You're right about replacing grounds. A good idea. I've had a couple of coolant leaks under the hood of mine - one led to dizzy failure about 6 months after the manifold coolant line develped a pinhole leak and corroded the dizzy - when I opened it up it was a mess inside.

    I suspect either the regulator in the alternator is squirrely or secondly wiring and I would rate grounds #1 for wiring the way it's acting.

    But who knows, it's gettin wierd when the motor is rockin so vibration may come into play and that adds a whole other dimention.

    good luck with it - intermittent types are the most fun!!!
     
    Last edited: Jul 1, 2010
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  7. Dual-500

    Dual-500 Well-Known Member VIP

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    Read your initial post again - here's what's happening. When you wind it up the load from the ignition system increases thus dumping the battery - because the alternator is wasted and not charging the battery correctly.

    Change the alternator. First look at charging circuit for a fuse - but, I'll all but guarantee it's the alternator.

    I learned this running the Rotax powered kart. No charging circuit on the battery in the kart. Engine is electric start and has electronic ignition. So, when the battery is low, it will start up and run. Then you take off and when it gets to 7-8 grand it lays down, and ignition load increases, it backfires, sometimes flat stalls. Sit there a minute for battery to recover and it all repeats itself.

    Problem was battery had lost capacity and wouldn't take a decent charge.

    Your car is acting exactly the same, only the larger battery isn't laying all the way down.

    Should read 13.5 to 13.8 at idle with a full charge. 13.8-14v when revved up a bit.

    Why it tested good, dunno. Could be wiring, but I'd put my bet on alternator.
     
    Last edited: Jul 1, 2010
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