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Resolved Charging issues

Posted by aaronth777, May 11, 2019

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  1. aaronth777

    aaronth777 Probationary Member

    27
    8
    Joined Apr 12, 2015
    Charlotte, North Carolina
    Hello all, I have a few questions regarding the charging system in a 98 gsx. I've had some charging issues so I replaced my alternator with a new one, not remanufactured. When the car starts and it's cold, it'll put out a full 14v. However as the car warms up and I drive it around, the voltage slowly drops to 12.1v. When the fan comes on and I turn the headlights on, it'll drop to 12.0v, and stay there, only going back up to 12.1 when the fan turns off and I turn the headlights off. I tried a few things and found that if it's running at 12ish volts and I unplug the 4-wire harness from the alternator, it'll immediately jump up to 13.5 volts and slowly climb to 14. So this tells me that the alternator is fine, it's the wiring or the signal it's receiving from the ecu.

    My question is what should the 4 wires be reading on the 4-wire alternator harness? I know the green and white wire both go to the ecu. Should they be grounded, 12v+, or just floating? I've read that if the white wire is grounded then the alternator will be in "low voltage mode", only putting out 12v, but not sure about the green wire. The red wire goes to the 40a headlamp fuse, and it's putting out 12v+ regardless if the headlights are on or not. I have aftermarket HIDs if that makes a difference or not. And then the black/yellow wire goes to the alternator relay, which I do not have in my car. My battery light works in my dash (it's slightly dimmer than the rest of the lights, idk if that matters), so I read that I don't need this relay for the alternator to work properly. My car also has a code 1500 "generator fr terminal circuit malfunction" if that makes a difference. Any help is appreciated!
     

    Street Build 648  1

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  2. stillrunning

    stillrunning Proven Member

    102
    47
    Joined Oct 10, 2013
    brandon, Florida
    I forget off hand which wire does what but the two wires in the schematic going to the ECM are to allow control of the alternator voltage regulator.
    One is the field feedback that the ECM uses as a reference
    The other the ECM applies a duty cycle to ground which reduces alternator output
    When the ECM grounds the terminal at 100% duty cycle the regulator output is reduced to ~12.6v low
    I tried unhooking the ECM field feedback and the grounding wire, the alternator put out upwards of 15+v at higher rpm
    Problem was the voltage regulator control was very on and off without the ECM duty cycle control helping to smooth output
    Even though I had plenty of voltage it spiked and dropped constantly which was noticeable in the lights dim and bright
    If you disconnect either the field feedback or grounding control it will set a CEL as well unless you have an option to disable the check

    Technically I think you only need one of the wires to power the regulator for the alternator to actually function though
    How smooth the output will be in your case, can't say
    I think one of my issues with erratic voltage regulation without the ECM control was that I had a 140amp aftermarket alternator
    I'm curious if the factory alternator would have a more stable output on the factory regulator without the ECM control
     

    Street Build 418  3

    1997 Mitsubishi Eclipse Spyder GST
    fwd · manual · 2G DSM
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  3. Mham2k

    Mham2k Proven Member

    177
    25
    Joined Feb 8, 2008
    Dallas, Texas
    Is your belt tight? I had issues for a while, once I snugged up my belt they went away.
     

    Street Build 326  1

    1996 Eagle Talon TSi AWD
    awd · manual · 2G DSM
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  4. aaronth777

    aaronth777 Probationary Member

    27
    8
    Joined Apr 12, 2015
    Charlotte, North Carolina
    Thank you for the information. So it looks like the green and the white wire both go to the ECM with the white being the field feedback and the green being the control wire that gets grounded from the ECM. I would try unplugging it altogether and drive it, but I don't want to damage anything by running 15+ volts. Based on your post, maybe one of these wires is being grounded somewhere or not sending a good signal to/from the ECM. I'll do a few more tests and post my results. That actually makes sense that one of these wires would be bad since I have an error code for the generator circuit, and you said if I disconnect one of the wires then it throws an engine code.

    Also yes my belt it tight. Like I said, it works perfectly fine if I unplug the wiring harness.
     

    Street Build 648  1

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  5. aaronth777

    aaronth777 Probationary Member

    27
    8
    Joined Apr 12, 2015
    Charlotte, North Carolina
    Oh okay, that makes sense. I'll check the wires for continuity and making sure they're not grounding out somewhere in the middle. If they're good then I'll suspect the alternator, as I've had the same issue with two different ECMs. The 90amp alternator from a 1997 Galant bolts up like the factory one, correct? I may as well upgrade from the 60amp factory unit if I'm getting a new one.

    Also can anybody confirm that the red wire coming from the 40a headlamp fuse is putting out constant 12+v, regardless if the headlights are on or not? I thought it would be switched so the alternator knows when to put out more power
     
    Last edited: May 12, 2019

    Street Build 648  1

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  6. aaronth777

    aaronth777 Probationary Member

    27
    8
    Joined Apr 12, 2015
    Charlotte, North Carolina
    Okay so I unplugged the wire harness from the ECM, jumped the green and white wires going to the alternator, and tested for continuity at the alternator harness, and it had continuity. I also tested them individually for continuity to ground in case they were shorted out somewhere, but there was no continuity so they aren't grounded anywhere. That means my wires are good, so that narrows it down to only a bad voltage regulator in the alternator, or something wrong in the ECM.

    With the car running and the alternator wires plugged in, the car was running at around 12.4v. I unplugged it any it immediately jumped to 15v. I did some voltage tests on the alternator wire harness with it unplugged and here were the results (the positive and negative is directly on the battery):

    White to positive: 12v
    Red to positive: 0v
    Black/yellow to positive: 1.7v
    Green to positive: 10v

    White to ground ~100mv
    Red to ground: 15v
    Black/yellow to ground: 13.3v
    Green to ground: 4.94v

    Does this seem like a bad ECM, or bad alternator voltage regulator?
     

    Street Build 648  1

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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  7. d1prospect

    d1prospect Proven Member

    217
    28
    Joined Mar 14, 2004
    Glen Bunie, Maryland
    Correct, 97 Galant alternator is a direct fit
     

    Drag Race Build 3K  15

    1995 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  8. aaronth777

    aaronth777 Probationary Member

    27
    8
    Joined Apr 12, 2015
    Charlotte, North Carolina
    Just an update, the car is charging correctly now. I went to swap ECMs, but before I did that i adjusted my engine fuse box and it immediately jumped from 12 to 14 volts. I still don't know if it was a wire grounding out or maybe a lose wire/relay, but the problem is gone. To anybody reading this thread in the future, make sure your fuse box is properly mounted and the wires aren't laying against the metal, potentially grounding out.
     

    Street Build 648  1

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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