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Here is an easy test to check your speedo input signal and wiring harness:
With ignition off. If you take off the dash surround and undo the four cluster screws then lift out the cluster and tilt it to see the back.
You should be able to see the speedo signal and power inputs, which are four silver phillip drive cheese head screws. Looking from the above, (for TB42E) top one is marked 2P, the bottom is IGN. The side ones have a bit of Japenese followed by - and +. These are the speedo signal inputs.
To get a good measurement, I made two 70cm long test leads, by stripping a half centimetre of the insulation from each end. Loosen the -ve and +ve. Slip one lead under the -ve and tighten the screw, making sure that no strands of copper are loose and able to contact any other part of the circuit.
Do the same for the+ ve. Re position the cluster and do up the four anchor screws. I taped the other ends of the test leads to the probes of a digital multimeter and set the dial to AC volts. DO NOT LET THE WIRES TOUCH EACH OTHER OR EARTH DURING THE TEST.
Get an assistant to drive. See that you get a small voltage which should rise to approx 5 volts as the speed increases 60 kph. When you have finished, remove the leads and re tighten the screws, firmly, but do not over torque them.
If the signal is ok, then another unit from a wrecker or Ebay may be the go.
It is fairly easy to swap out the speedo so you can retain your odometer. Pop the black cluster shroud and clear cover off.
Unplug the odometer connector. Undo the four screws, (and also another near the high beam and indicators) and gently lift out the gauge.
The inputs are circled in yellow. The arrows point to the grey and blue input wires.