Speedometer/Headlight Difficulties in '91 Dynasty

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.

Chris

 

"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.

 

Chris,

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.

Thanks

Jeffrey

 

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
gets.

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.

Chris