Alternator or ECU computer or some other sensor?

Discussion in 'General Motoring' started by Charles Henderson, Dec 8, 2003.

  1. 1990 Civic DX, 4 Dr., 5 Sp., 1500cc

    I've got the Battery light on in the dash pretty much all the time now.
    It was intermittent, not coming on for a half hour or so after startup,
    and either coming on or going out under certain engine load conditions,
    but now it's there all the time.

    Honda Service Manual has an Alternator testing procedure that first
    begins by removing the main connector from the Alternator and checking
    for voltage to ground at two different pins inside the connector.

    Unfortunately, mine fails the second of those voltage tests, and in that
    case the test procedure refers you to the ECU troubleshooting section at
    that point. Which is a bit of a problem because I don't have the special
    interface cable needed to access the ECU circuits for test purposes, and
    get this: the test procedure for the ECU says "Substitute known-good
    ECU; if problem disappears, replace ECU" at various points in the

    Hmmm. I haven't priced it officially yet, but that thing costs in the
    neighborhood of $250 bucks! I can't afford to buy one just for testing
    and troubleshooting!

    And if I *had* a known-good ECU, what would I need a testing procedure
    for in the first place? Swapping it out would *BE* the testing procedure!


    I think the problem is that the ECU has an economy circuit in it that
    shuts down the Alternator under certain load conditions to save fuel.
    Pretty neat idea, except when it malfunctions!

    My question: How can I determine for certain whether it is the ECU
    malfunctioning, or if it's some other load/condition sensor giving it
    faulty data? And I suppose it still could be the Alternator itself...

    Keep in mind I don't have a "known-good" ECU to follow the official
    diagnostic procedure, and I'm not sure if I can go buy one, use it for
    testing, and return it if it turns out not to be the culprit.

    Any ideas?



    --Charlie Henderson
    Charles Henderson, Dec 8, 2003
  2. Charles Henderson

    N.E.Ohio Bob Guest

    I think you need a simple volt, amp, load test at your corner parts
    store. Follow their advise. bob
    N.E.Ohio Bob, Dec 8, 2003
  3. Yes, perhaps. You're right, I didn't do the full field tests on the
    Alternator, but remember, I was following the Honda Service test
    procedures. They specified testing for certain voltages through the ECU
    from the Alternator harness *first* - and that test failed.

    And Bob, I appreciate the suggestion; believe me, I wish I could just
    turn it over to someone with all the proper test equipment/new parts.
    But I'll be lucky if I can afford the parts I need... there's nothing in
    the budget for someone else's labor.

    I was hoping these symptoms would be common, and someone here would know
    where the likely failure is - the computer or some load sensor, etc...


    --Charlie Henderson
    Charles Henderson, Dec 9, 2003
  4. Charles Henderson

    N.E.Ohio Bob Guest

    Round here, they test 'em fer free! bob

    N.E.Ohio Bob, Dec 9, 2003
  5. Hey, you're right, Bob! I found a parts dealer who'll bench test the
    Alternator for free. Or a *small* fee. So that'll take my diagnosis a
    little further, with a degree of certainty. But if it tests ok, then I'm
    still left with my original question:

    That circuit in the FI ECU that shuts down the Alternator to save fuel?
    How can I find and test the sensors that trigger that circuit *without*
    replacing the ECU with a "known-good" unit?

    By the way, Honda wants $850 for that ECU, and will *not* allow me to
    return it if it turns out not to be the problem. I can get it
    aftermarket for around $250, I think, but I can't return that, either...


    --Charlie Henderson
    Charles Henderson, Dec 9, 2003
  6. Charles Henderson

    Cosmin N. Guest

    Actually, it makes sense, and that rule applies to all electric components.
    If you plug anything the wrong way, or in a faulty circuit you have a very
    good chance of burning it. Sometimes that is noticeable with the naked eye,
    other times you need to run a full diagnostic to find out if the part
    still works.

    So nobody wants to take any chances on returned electric parts.

    Cosmin N., Dec 9, 2003
  7. Charles Henderson

    N.E.Ohio Bob Guest

    Only time I invested $100. in a junk yard ECU, it didn't fix the
    problem. It's from an 89 LX-i. The ECU and the extra distributor for
    that car are for sale, if anyone wants them. bob
    N.E.Ohio Bob, Dec 9, 2003
  8. Charlie,
    I have a regular voltage testing machine but I was in Kragen's Auto Parts
    store yesterday and saw a $9.00 item called "Auto Charging System
    Analyzer". I believe they sell the same type of item at almost all auto
    parts stores. You can use it to test the battery or test the alternator.
    It comes with detailed instructions. I purchased it as a Christmas present
    for someone. It may not tell you everything you want to know but at least
    it will allow you to test the alternator and battery.
    Bill B. Johnson, Dec 9, 2003
  9. Charles Henderson

    Randolph Guest

    That circuit in the FI ECU that shuts down the Alternator to save fuel?
    The ECU does not actually shut down the alternator, it just lowers the
    output voltage. This is not based on any single sensor but a host of
    conditions including engine RPM, vehicle speed, coolant temperature etc.
    On my '94, stepping on the brakes will defeat this power saving mode and
    bump the voltage up to normal.

    Does you service manual outline what conditions need to be met for the
    lower voltage to be engaged on your particular car? The one from Helm
    does for my '94, but I don't know if Haynes and others go into that much
    Randolph, Dec 9, 2003
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