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Thread: Fluctuating voltage

  1. #1
    Join Date
    Apr 2013
    Posts
    111

    Default Fluctuating voltage

    I have been through a lot trying to get my charging system working properly (BDA engine).

    First: My car stopped charging, due to there being only one bolt holding my alternator on (the one it pivots on, so the one to lock down the belt tension wasn't there). Ok, easy fix.

    Next: Belt was stretched just enough to make the alternator hit the steering shaft when trying to tighten the belt. So belt slipped, and strange vibration came through the steering wheel when I hit bumps. New belt fixed that.

    Now: Yesterday the volt meter read 12 volts, or even a bit below, when my wife and I went out for a ride. I noticed it and turned back home. Was definitely not charging, as it cranked slowly when I went to restart it. Today I checked it all over, looked over wiring, checked for continuity from alternator to battery + terminal. All seemed fine. Started it up, and voltage was 13-14 volts. Went for a 5 mile ride, and through that ride the volt meter fluctuated between approx. 12.0 and 15.5 volts. Not normal - right?

    Faulty voltage regulator? I had an alternator shop test the alternator a while back, and it was fine, but now I'm thinking that the voltage regulator is intermittent, and happened to work well when the alternator was tested. Should I just buy a new alternator? Anything else anyone can think of?
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

  2. #2
    Join Date
    Sep 2017
    Location
    Midlands of SC
    Posts
    28

    Default

    Do you have a good clean chassis to body to battery ground cable(s)?

  3. #3
    Join Date
    Apr 2013
    Posts
    111

    Default

    I'll check. By "body", do you mean the aluminum skin, or the frame?

    Quote Originally Posted by SCse7en View Post
    Do you have a good clean chassis to body to battery ground cable(s)?
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

  4. #4
    Join Date
    Jan 2020
    Posts
    96

    Default

    Also, check that all connectors on the alternator are plugged fully in and tight!
    1987 Caterham 1700 Supersprint
    2002 Porsche Boxster
    2004 Porsche Boxster S
    1941 Dodge Luxury Liner
    2009 Mercedes CLK 350 Cabriolet
    2017 Chevrolet Volt

  5. #5
    Join Date
    Sep 2017
    Location
    Midlands of SC
    Posts
    28

    Default

    By "body", do you mean the aluminum skin, or the frame?
    To the frame.

  6. #6
    Join Date
    Apr 2013
    Posts
    111

    Default

    I had three connectors to the alternator: Heavy cable that attaches to a stud with a nut, and two spade connectors. Based on this thread:

    http://www.usa7s.net/vb/showthread.p...ght=alternator

    and what someone told me when I called the company that makes the alternator, and what the guy who tested my alternator told me, I disconnected both of the spade connectors. Seems they were a legacy from the crossflow that used to be in this car. So there's just the bolted-on wire, and it's nice and tight.

    Quote Originally Posted by Anker View Post
    Also, check that all connectors on the alternator are plugged fully in and tight!
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

  7. #7
    Join Date
    Apr 2013
    Posts
    111

    Default

    This morning I checked the ground cable that goes from the engine block to the frame. Sanded the surface on the frame that it bolts to, to make sure there's no paint in the way. Cleaned and reattached. The cable was a little bit frayed at the crimp connector (though still plenty of strands intact), so at some point I'll replace that cable just to be certain.

    On my starter the terminal that the smaller wire (not the big red power cable) attaches to has some looseness. Hard to explain, but it looks like it is attached to the starter with solder, and the spade connector juggles around. So I fastened that down with a wire-tie, thinking that maybe a poor connection there is somehow affecting charging.

    Started it up and drove around the block. Volt meter read from 12.5 to 15 or so. At this point I'll take it our for a drive every day and notice what the volt meter says.
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

  8. #8
    Join Date
    Apr 2013
    Posts
    111

    Default

    I took a bunch of measurements today with my voltmeter, and here's what I learned:

    - Voltage gauge in the car reads 0.1 to 0.6 V lower than my multimeter (VOM).

    - Battery voltage (per my VOM) was 12.7-12.8 before starting.

    - Checked for parasitic current losses, and didn't detect any.

    - With engine running, before it warms up, voltage is around 12.5 at idle, and 14.0 or higher when revving.

    - MOST IMPORTANT: Once the engine warmed up, the voltage measured with VOM at the battery, and at the alternator + terminal, was 12.2. 12.0 with the lights on. Revving the engine did not budge this voltage - meter needle (yes - it's an old, non-digital VOM) did not move AT ALL.

    - NOTE: I did measure a voltage drop of 1.0-1.2V between the alternator + terminal and the battery + terminal when running cold. The scale on my VOM made it hard to read this, so I'm not 100% confident in this datapoint, but I did see the needle move a little. Once the engine warmed up this voltage difference went away - no needle movement at all.

    So - it charges when the engine is cold, but stops charging completely when warmed up. Bad alternator/voltage regulator? Other ideas?
    Last edited by BlueBDA; 05-30-2020 at 03:18 PM.
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

  9. #9
    Join Date
    Feb 2012
    Location
    Roswell GA
    Posts
    974

    Default

    Is there an auto parts store near to you that has an alt. tester? I don't remember if you said anything about that earlier.
    Life happens while you're making other plans
    1995 Caterham Crossflow RHD
    1990 Miata
    2007 GMC Sierra WT
    2013 Mini (Her's)
    2014 Outback
    JOHN DEERE D130

  10. #10
    Join Date
    Apr 2013
    Posts
    111

    Default

    I did have it tested at an alternator shop, and it was fine. But I realize now - it was cold. The problem occurs when the engine heats up.
    1997 Caterham, BDA
    1991 Miata, supercharged
    2017 VW GTI

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