Speedometer/Headlight Difficulties in '91 Dynasty

Discussion in 'General Motoring' started by C. Bailey, Oct 11, 2003.

  1. C. Bailey

    C. Bailey Guest

    A week or two ago, I posted a problem I have been having with the
    speedometer in my car. The speedometer works intermittently. When it quits
    working, if I turn on the high beams, it seems to start working again (but
    not all of the time). Low beams usually has no effect on the speedometer.

    Several pieces of advice were given:

    1) I was told it might be the speed sensor. It was replaced with a brand
    new sensor, and there is no change.
    2) I was told the speedometer may be getting voltage or ground from the high
    beam indicator in the dash. My high beam indicator has never worked. I
    took it apart tonight and discovered the bulb was good. To be certain, I
    exchanged it with another bulb. Still did not work. I removed the bulb and
    checked the voltage across the terminals. 2 volts on high beam, 3 volts on
    low beam. Obviously something is the matter with this circuit, but the
    lights will toggle from high to low beam - just no indicator. I don't
    understand how this could be related to the speedometer, but I thought I
    would throw this information out to the experts. I checked the voltage
    across other bulbs on the board, and it was 13.5 volts (car running). I
    also have 13.5 volts between two out of the three terminals on speedometer.
    These terminals are in the same circuit as the voltage gage and not reading
    any resistance between the voltage gage and the speedometer. The voltage
    gage has always been very constant even when the speedometer is misbehaving.
    This makes me think the speedometer is getting appropriate voltage to it.
    3) I was told this car had a resistor on the instrument panel that
    overheats, and melts the solder causing a poor connection. I searched all
    over - the only circuitry with resistors is right on the side of the
    speedometer. I couldn't get access to it, because I was unable to remove
    the needle from the speedometer. However, I could see 95% of the board very
    well, and there was no evidence of overheating, or poor connections.
    4) I was told that the connectors on the back of the instrument panel get
    loose wires. Unfortunately, there is not enough excess wire to pull it out
    of the dash and get a good look at it. However, all wires seem to be
    attached to the connector snuggly.
    5) Wiggle wires. The speedometer was working for me, so I reached behind to
    the connections to the instrument cluster. I wiggled them for a while, but
    the speedometer continued working.

    Does anyone have any other advice for me? Unfortunately, the speedometer is
    working right now, and it's tough to diagnose any more. The only thing I
    can think of is to solder a couple leads off the back of the speedometer so
    I can measure the voltage when I know for certain the speedometer is not
    working. Currently, when the speedometer stops, I have to stop the car,
    pull the instrument cluster out, and probe it with the voltmeter. I have
    been getting good voltage, but each time I have started out again, the
    speedometer is working.

    C. Bailey, Oct 11, 2003
  2. C. Bailey

    StevJensen Guest

    "C. Bailey" wrote
    Symptoms like these generally indicate that there is bad solder joint
    or cracked trace on the circuit board. Even if you are experienced in
    electronics and know exactly what to look for the flaws can be very
    difficult to see and wiggling often provides a fix that lasts several days.
    The oxide gets rubbed out of the flaw and takes some time to reform.

    Use a jewelers loop or strong magnifier and a good light. You are looking for
    hair like discolorations that circle a soldered pin or run across the board
    (anywhere for cracked traces).
    Rotating the board under the light helps as the flaws can be invisible
    from many viewing angles.

    On a board this old you should be able to find several joints that look bad,
    and probably are. Even if they haven't oxidized enough to cause some
    particular trouble yet.

    Connector pins, ground pins and the pins of any heavy object soldered to the
    board are high probability areas for flaws.
    StevJensen, Oct 11, 2003
  3. C. Bailey

    JPeter6981 Guest


    I have a BRAND NEW Speedometer (set at the factory at 23,000 miles) still in
    the original box, that will fit the Dynasty / New Yorker / Imperial's, if you
    are interested. It has never been installed in any car.

    Drop me a line if you would like some more information.


    JPeter6981, Oct 11, 2003
  4. C. Bailey

    C. Bailey Guest

    Here's a little bit more information. I soldered several leads onto the
    board in the instrument cluster. The leads are soldered onto the back of
    the three posts that "plug-into" the speedometer. This allowed me to
    attached my multimeter while driving. One of the leads is a ground, and the
    other is positive. The third lead is the signal (I assume from either the
    speed sensor or ECM??). The voltage between ground and positive was 13.5 to
    14 volts during my drive, whether the speedometer was working or not. The
    high beam/low beam made no difference to the voltage. I then connected my
    multimeter between the ground and the signal wire. I discovered that the
    signal wire is a pulse. Unfortunatley, didn't have a scope to measure.
    However, I put the car in neutral, and rolled it back and forth with my
    foot. Voltage was either 0 or 3.2 to 3.8 volts. Now this is where it gets
    interesting. With the headlights on, this voltage jumps to 5.7 volts!
    Thus, my headlights are definitley impacting the signal the speedometer

    Today was a bit of an unusual day in that the speedometer was working all
    day. I was forcing it to fail, by toggling the high beams on. Usually the
    high beams will make a non-working speedometer work, but apparently they
    will also cause a working speedometer to fail. The longer the high beams
    are on, the longer it would take for the speedometer to start working after
    the high beams were turned off. If they were on for a brief second (flash
    to pass), then the speedometer would start working very quickly after I
    released the lever. If I held the flash to pass for 10 seconds, then the
    speedometer would not work for 5 to 10 seconds after releasing the lever.

    Any thoughts appreciated.

    C. Bailey, Oct 11, 2003
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