This page details the troubleshooting I did to determine the fix for the
When the speedo went bad I had an opportunity to have a shop replace the
sensor with a known good one. They did this for me and found no difference
in the behavior of the speedo. This ruled out a bad sensor. I removed the
instrument cluster and removed the speedo unit and carefully looked over
all aspects of it looking for a problem and I couldn't find any.
I left the speedo non-functioning for nearly two years before joining the
mailing list and reading about others with the same problem. I decided again
to attempt to repair the faulty speedo. What I was noticing is that it would
work intermittently, but then usually die again soon. My initial thoughts
were that the exposed portion of the sensor wire had aged and cracked, causing
an intermittent connection. I traced the sensor wires from the speedo sensor
to where they enter the car. This is under the rear seat on the passenger
side. I removed the wires from the sensor, cut, and soldered new wires to
the old, and pulled these through the protective sleeve into the passenger
compartment. I then soldered them to the existing wires. Took the car for
a drive, and no speedo.
The wires were attached to each other like lamp cord ( zip-cord) one was
brown ( the ground ) and the other was brown with a red stripe ( the signal
). I decide to trace the wires through the car to determine if there was
a break somewhere. The wires proceed from the rear seat along the passenger
side rocker panel and then turn towards the drivers side of the car right
in front of the passenger front seat. From there, they head about two inches
forward of the shifter, in front of the drivers seat and to the drivers
side rocker panel. They then head towards the wire bundle in the front at
the fire-wall. From there, they go towards the instrument cluster white
connector. About 4 inches before the connector, the wires are crimped to
the smaller wires used with the connector that mates with the instrument
cluster. I used these crimp connectors as a test reference point for determining
if the speedo unit was functioning properly. The teal arrows in the picture
below show the location of these crimp connections.
I used a fluke 87 DMM attached to these connectors to monitor the speedo
signal both in the garage and while driving. What I noticed during driving,
was that there was a frequency associated with speed, but the numbers I
was getting didn't make sense and the speedo was not working at the time.
I talked to some electrical engineers and they suggested that the speedo
needs to be powered to work properly, so the signals I was getting were mostly
noise. This narrowed the problem to the fact that the speedo sensor was
not getting power. This lead me to look closely at the instrument cluster
and all the connections the speed signal makes with the instrument cluster.
I pulled the speedo unit out and pushed the spring type contacts on the
unit closed to help make a better connection. When I had the speedo unit
out, I noticed that some of the solder joints looked as if they had been
touched up. The explanation for this is that these joints are what hold
the speedo circuit board to the speedo unit. I re-flowed these joints and
put the speedo back. I took it for a test drive and it worked for a short
time and died. After some discussions with Bryon Pinkston ( pinkston_at_aros.net),
He told me about the lead for the cruise control from the back of the instrument
cluster behind the speedo unit. I decided to move my test point to this
contact on the back of the speedo in an attempt to get closer to the origin
of the problem. I took out the speedo unit and soldered a wire to the cruise
output signal. At that time I also decided to re-flow all the solder joints
on the speedo circuit board. I put the unit in and hooked up the DMM and
took the car for a ride. The test point is shown below with a yellow arrow
on the bottom of the picture.( notice the green test wire) and the speed
input from the sensor is the yellow arrow on the left in the picture.
It worked!!! I turned on the DMM and checked the frequency, it was roughly
twice the vehicle speed. ie at 20mph it read 40 Hz, at 60 mph it read 120
Hz ...etc. The voltage across the wires was about 7.5-8.5 Volts DC ( if
I recall correctly). I left the setup connected for a couple of days and
in the mean time told Bryon about the re-flow procedure. He did the re-flow
to his speedo unit, and it fired to life. I think the problem is solved.
as of writing this I have about 1000 mi on the fix through rain, cold weather,
bumpy roads... and it still works fine.
Back to the speedo repair page.
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