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Flakey speedometer fixed - at last!



"Flakey" speedometer problem solved!  (I hope for good.)

With *many* thanks to Steve Buchholz and Mike LaRosa.

For about a year now, I've been fighting the "Battle of the Flakey
Speedometer".  I think the problem has finally been solved and the cure
found.  At least, the repairs have held now for at least two weeks.

The car:  1989 200TQ with electronic analog (not digital display) instruments.

The symptoms:  The speedometer has "bounced" from a low indicated speed to a
higher indicated speed, neither of which is even close to the actual speed
as indicated by the tachometer.  The period of oscillation varies with
speed.  There appears to be about a 25% duty-cycle.  The speedometer seems
to be "on" about 25% of the time and "off" the rest.  The problem was
intermittent, coming and going apparently in relation to proximity to the
dealer service department.  If I got within two miles of the dealership the
speedometer would work perfectly.  At first the problem was extremely
intermittent.  It would happen once in a while.  A nuisance but not really a
serious problem.  As time progressed the problem became more and more
frequent until it was virtually all of the time except when near the dealership.

In addition to the speedometer woes, when the speedometer isn't working then
the cruise control doesn't work either.  Neither does the in-dash trip
computer.  Well, it "works" but the outputs are meaningless.  GIGO!

The diagnosed cause of the problem:  

An intermittent poor contact between one or more the four contacts of the
speedometer head and the printed circuit board which drives the speedo head.
Contact is made with two pairs of pins (one pair near the top and one pair
near the bottom) which extend from the speedometer head and pass through
connectors in the pc board.  These pins are mounted to a small pc board
which is part of the speedometer head itself.

The "cure":  

Obtain a mated pair of high quality four pin connectors.  (I used a pair of
connectors from a Radio Shack telephone extension line - RJ-11's - any other
high quality four conductor connector would also be suitable.  Something
smaller than the RJ-11 connectors might be preferable.) 

Remove the instrument cluster from the dash and bring it inside to a
convenient work surface and disassemble it.  I would presume that if you've
been having this problem for any length of time then removal of the cluster
and its disassembly require no directions.

Cut the four conductor cables attached to the connectors at about 8 to 10
inches length (decide for yourself the proper length for the cut) and strip
the outer covering about 4 inches (10 cm) and the wires about 1/4 inch (2-3
mm).  Mechanically secure (for strain relief) these wires and solder them
into the circuitry.  Use a low power soldering iron.  No more that 30 watts.
You don't really want to destroy the pc board.  Believe me - you DON'T want
to do this.  (I didn't.)

One (the male RJ-11 cable end - it's smaller) four conductor cable is
soldered into the speedometer head itself using the solder lugs to which the
connector pins themselves are already soldered.  The pins may be removed or
they may be simply left in place.  Leaving them in place will minimize the
likelihood of breaking a pc trace in the speedometer head.  Removing them
provides a convenient means of providing strain relief for the conductors.
A bit of improvisation is in order here.

The other four conductor cable is soldered into the larger pc board so that
the wires are connected electrically to the pc traces to which the pin
connectors are already soldered.  Be sure that the conductor colors match
location for location.  If the pins have been removed from the speedometer
head the wires may be passed through the center holes of the connectors and
soldered to the pc board on the other side.  If the pins have not been
removed then this pathway is not available and another must be used.  Improvise.

Reassembly of the instrument cluster as now modified requires a bit of
"MacGyvver work".  

The male RJ-11 connector must be able to pass through the plastic case of
the instrument cluster.  There is already a small triangular opening near
the speedometer head which can easily be enlarged using your handy dandy
Swiss Army knife or other small saw blade.  Pass the RJ-11 connector through
this opening.  Strain relief is now easy since this cable passes beside the
spring retainer which must be pressed to separate the pc board from the rest
of the assembly.  The cable fits tightly between this "plastic spring" and
the rest of the piece.

The back of the cluster is mounted using several screws, three of which
attach a hinge piece to the assembly.  Two of the wires passing through the
connector centers (if this route is used) interfere with replacing this
hinge piece where it belongs.  More MacGyyver work is required here.  A
simple slot sawed into the side of the hole which is supposed to surround
the pin connector works well.  The wires will pass through this slot upon
reassembly.

Once the cluster reassembly is complete, make the final connection between
the connectors (RJ-11's in my case) and tape the loose ends down so that
bouncing and vibrations of these ends will not lead to metal fatigue
breaking of newly soldered in leads.  Then put the instrument cluster back
into the dash.

Smaller connectors would allow these connections to be made internally.  If
this is done, greater care must be taken if it is ever necessary to re-enter
the cluster.  I simply used what was readily available out in the boondocks
where I live.

Mistakes to avoid:

Don't break copper pc board traces.
Don't loose a piece of insulation down inside the speedometer head itself.
This can produce yet another problem, a "sticking" speedometer.

A caution:

Don't ask how I know of mistakes to warn about.  I don't intend to answer
such questions.  :-)

Bob
   **********************************************************
   *  Robert L. Myers  <rmyers@olie.wvitcoe.wvnet.edu>      *
   *  Chair, Department of Chemistry, WVIT                  *
   *  Home:(304) 574-2372        Office: (304) 442-3358     *
   *  Obligatory references for Quattro and Sleddog Lists:  *
   *       My Huskies like to ride in my '89 200TQ.         *
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