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From andy42uk

have posted on this subject in the past, and I was wrong about a couple of things, however I now FULLY understand what is happening and why.

The speed pulse signal is feed directly to the speedo head as a bi-polar signal from the sensor in the gearbox.
The speedometer uses this signal for the speedo reading and then CONDITIONS this signal into a square wave (at about ¼ of the original frequency), which is outputted to the rest of the cars electronics, ECU, HICAS Power steering speed sensitivity.
The good news is that the speedo head is modifiable to UK spec. if you are able to work with a soldering iron, and a lot of common sense.
THIS information may well piss off people selling converter ‘chips’ however I feel the Skyline is a special case as with the correct signals the car is MUCH nicer to drive, and feels more stable, than even with the best 5 wire converter fitted.

Firstly strip the dash out, I always remove the top steering wheel cowl then the dash trim as this avoids scratches, remove the dashboard and take it somewhere well light clean etc. split the front cover of by the plastic clips, and undo the 4 screws that hold the speedo head into the cluster, and remover the speedometer.
AT ALL TIMES BE GENTLE.
On the printed circuit board on the back of the head which you will see the foil side of, mostly at the top things are written on the pcb, and there are a set of calibration pads.

What you need to do is this, where the solder pads have ADJ 1-9 clean all pads (on mine 4+5 were joined) and join pads 6 8 and 9

Where J2 is written but no link installed join J2.

What would be the 10th adjust link is called PESL cut this link, (the solder pad is there but the pcb track is continuous), cut it and leave it as an open link.

This will get you right into the ball park, however you may need to play with the adjustment links to calibrate for your car, mine is an auto and I think its dif ratio is different to a manual, also wheel size effects things, so you may need to set up with GPS or something.
I have function generators and was able to do mine 100% accurate on the bench, but it is an easy task to add or remove links on the solder pads.
Adding a link decreases the reading, 1 buy the smallest 9 by the greatest amount, and it is incremental in a binary fashion.

This is the definitive and final answer to this question, I have today tested and tested and then tested again, this information is 100% accurate, joining J2 is what alters the conditioned signal to MPH pulses that removes the limiter, but all things should be done at once
 
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