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Car Alternator voltage is too high (16V+) Can it be reduced?

  1. Aug 29, 2015 #1
    My car alternator runs at 16 to 16.5 volts when I hit the gas, and 15v idling. This is scaring me and a new one is too expensive. It could be my computer or pulleys in the wrong too so I can't risk buying a new one and still having the problem. It turns off my speaker amplifier as soon as I hit the gas pedal. I put in a resistor between the battery positive to my amplifier power line and it reduced the voltage by 1v, so about 15v max now. The amplifier stays on now but there's another problem. The speaker starts stuttering when the volume is up and it has to play sustained low notes, so I think the resistor isn't allowing enough amps through. I need it to allow ~15 amps (amplifier fuses are 15amp), and it seems to only allow around 6 or 7 based on how much work the speaker can do.

    I'm not sure what exactly I need in Ohm's+amps/watts specs for a new resistor to do the trick while lowering my voltage from 16v to around 14v. Perhaps even a couple higher-ohm resistors in parallel to allow more amps through?

    Maybe a resistor is the wrong way to go about it? Voltage Regulators seem to only be low-amp and I don't know a thing about them. I just need a simple way to reduce about 2 volts while still allowing 15 amps through, and I don't know how or the proper math to figure it out. Resistors seem too skinny, so maybe a thicker resistor-wire would work, but at what ohm rating?
  2. jcsd
  3. Aug 29, 2015 #2


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    Sounds like the alternator voltage regulation circuit is shot. If you don't want to replace the battery, radio and amp in the near future take the car to a good shop, let them diagnose what's wrong and spend the money to fix it.
  4. Aug 29, 2015 #3
    Your alternator has a bad voltage regulator.

    You might try to find a used or rebuilt alternator. Don't try adding a resistor or whatever. That's dangerous.

    It's reasonably easy to build a regulator in the lab, but getting one to work consistently in the hostile environment of an auto engine is tricky. Too many things can go wrong.
  5. Aug 30, 2015 #4


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    Back in the day we just messed with the springs and contacts. :wink:
  6. Aug 30, 2015 #5


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    (Solid state!!!!) Regulators are pretty cheap and they're easy to replace yourself. Don't risk running on the old one.
  7. Aug 30, 2015 #6


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    Have you checked the calibration of your voltmeter?
    If you turn on the headlights, are they brighter than expected when the engine is running?
    A dirty connector can give unusual results. Are you measuring the voltage between the battery's lead terminals, the external connectors or between the B+ distribution in the fuses and the chassis as B–?

    Fix your alternator/regulator first. Over-voltage will generate and vent gas and so dry out and damage the battery.

    Without knowing the make, model and year of the vehicle we can have no idea how your system works.

    Do not use series resistors to drop voltage, instead use power diodes. Power diodes have a voltage drop that is less sensitive to variations in current than resistors. But that is not applicable here because you must protect the battery by reducing the peak battery voltage, by fixing the problem.
  8. Aug 31, 2015 #7

    jim hardy

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    at 16 volts it should be boiling your battery electrolyte. Check it, and catch some rainwater to refill if low. Or buy distilled...

    Be careful prying off the caps, wear glasses and an old shirt because it's apt to splatter when cap comes loose.
  9. Sep 7, 2015 #8
    alright thanks for the replies. It's a 2003 chevy tracker and the voltage regulator is built into the alternator so I wanted a work-around to avoid spending 160$ on a new alternator. I checked with multiple multi-meters at multiple locations, it's definitely 16v+. The negative terminal was grounded to painted metal, so I sanded it down and now I'm even higher than 16v. The battery is new and still in good condition. Maybe once the alternator is out I can tinker with it and get it working properly. It was sitting outside for a few years before I bought it recently so I guess I should expect a few problems. I'll take a look at power diodes for my amp power wire until then.
  10. Sep 7, 2015 #9

    jim hardy

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    it's always a good idea to check for something dumb. I always do, and don't mention it unless i find something.

    1. Measure voltage between alternator frame and battery's negative terminal. More than about 1/10 volt says there's a "grounding" issue.

    2. Some alternators are "single wire"(not counting the frame ground)
    and produce around 14.5 volts at their output terminal ,
    assuming there's not much voltage drop between there and the battery.

    Other alternators are "Remote Sense" and use a third wire to measure voltage over closer to battery.
    If that third wire corrodes away,
    or if some hack cuts it to get power for a stereo (happens a lot in poor quality shops)
    the alternator thinks the battery is low so obediently goes to full output trying to charge it back up.

    Here's some Delco-Remy info on remote sense alternators.

    See if your alternator is "remote sense" and if so, use your meter to make sure it sees 16 volts at its remote sense terminal. If you find low voltage there the regulator is doing its job.

    It is possible to replace an internal regulator
    but local auto parts stores only sell whole alternators.

    I once helped a guy who'd replaced his starter battery and alternator, all because he'd unknowingly disconnected a ground wire .
  11. Sep 7, 2015 #10

    jim hardy

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    Last edited: Sep 7, 2015
  12. Sep 7, 2015 #11


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    The problem and solution is always simple and obvious once you finally identify it.

    My last really weird one was an intermittent chassis to engine bonding cable. The alternator B– is connected to the engine, the battery B– is connected to the chassis. Are those always the same?
  13. Sep 7, 2015 #12

    jim hardy

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    every alternator i ever encountered (well except for one) used its metal case for negative. Of course that's bolted to the engine.
    Battery negative wire varies with make and year model.
    (VW's up to early 70's were positive ground !)

    My friend recently had troubles on his GMC truck. His battery negative was wired to fenderwell. A braid that connects his engine to the chassis corroded in two He'd worked around that by adding a wire from battery negative to engine , but encountered strange electrical symptoms like applying brakes shut off the electric fuel pump so he had to shift to neutral at traffic lights. Turned out some other circuits shared that same piece of braid and we had to provide returns for them too.

    Old Fords have one big long wire running all the way from battery's negative post to engine, often to a starter mounting bolt. In the middle of that wire is a metal clamp that looks like it's just cosmetic to keep the wiring tidy., it fastens to to the fenderwell with a good sized sheet metal screw.that's important.
    That innocuous clamp is what ties both the engine and the battery to chassis.
    If you leave it unconnected there's no return path for headlights, starter solenoid and chassis accessories to battery negative.
    The guy i helped had replaced that wire with a generic one that had no grounding clamp in the middle. We retrieved his old cable from the trash , cut it just above the clamp and reconnected engine to fenderwell using the lower half. He was hopping mad at having bought new starter, alternator and battery.

    There's another dumb check i make, measure voltage from engine to chassis sheet metal with a lot of electrical load - headlights and heater fan. I might read it again while cranking . You'll find broken or missing ground straps that way.
    I also measure from center of battery post to the nut that tightens the clamp(cant do on side-post GM batteries). One finds a lot of corroded terminals that way especially in springtime when humidity comes up.

    Sorry, i didnt mean to turn this into a hobbyist discussion.

    But that "dumb checking" is one practical application of the laws of Kirchoff and Ohm. I hope this post stimulates thought and saves somebody a towtruck charge.

    old jim
  14. Sep 7, 2015 #13


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    The worst solution is to ignore the failing regulator and use diodes to drop the voltage. I consider it downright dangerous since eventually the regulator will likely fail completely and destroy the battery and then the entire electrical system.

    Otherwise it is likely the alternator's regulator. Take it to a parts dealer and they can generally check it.

  15. Sep 9, 2015 #14
    The ground seems fine, way less than 1/10th of a volt between the alt casing and the negative terminal. I found the right alternator on amazon for under 100$ so if all else fails I'll try that (part number is 30026479 )

    I can't seem to find much information on it though. I don't think it has remote sensing, and I think the regulator is inside the casing unfortunately. Hopefully it's the brushes inside, I can get replacements easily enough. The wires coming off of the alternator all look okay at a glance, but I'll check better when I take it out. The voltage seems to correlate with how hard I press the gas pedal which makes me think it probably is a regulator problem. I just hope it's not the computer
  16. Sep 9, 2015 #15

    jim hardy

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    Good on you for checking that.

    That alternator shows internal regulator, not controlled by computer.( if i got to the right one. )

    Here's what the connector may look like

    With engine off, unplug the little connector . See if when you turn the key to "on", battery voltage appears at center terminal (of the wiring harness plug.)

    Hint -
    i once encountered a GM alternator with failed regulator. According to the previous owner this was the seventh alternator that car had eaten in as many years.
    I noticed the big white wire connecting to alternator's main output terminal was brown at the last inch adjacent the lug that bolts to the main terminal.
    So i whittled off the insulation and sure enough, of the ten or so wire strands half were broken right at the lug.
    That reduction in area made a hot spot that discolored the insulation. Doubtless it was heating the main terminal bolt, too.
    To see why heat way out there was wrecking regulators i took apart the alternator. Sure enough, that bolt went straight to the internal voltage regulator's heatsink - a bad crimp connection from the factory was killing alternators by slow cooking the regulator.
    The new owner (my son in law) had already bought a new alternator so we fixed the wire and put that one in.

    I ordered a new regulator and put it in the old alternator which i gave to him for a spare after i'd painted it bright pink. After all, it was a Corvette....

    anyhow - check your wire.
    Internal alternator parts are hard to find and yo have to identify your alternator by the cryptic manufacturer code.

    see http://store.alternatorparts.com/delco-3.aspx
    might be one of these


    good luck, keep us posted.
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