UK Car Repair: 1994 Nissan Sunny Tropic, electronic voltmeter, diagnostic scanner

Question
I have one of the above: its only done 65K miles and the speedo has stopped.
Fuses all Ok; Tried instrument panel from a scrapyard:no joy: checked sender unit for resistance and output via AC electronic voltmeter rotating it by pillar drill at about 850rpm: gave 0.4volts output. Sender connections to speedo head OK.  Taken to an autoelectric guy who can't find fault.
Any ideas? Everything else works very well but my son says I'll have to buy a new car as it will fail MOT!!!  

Answer
The problem with using a yard Speedo to test the system is the fact that most times you are unable to verify if the Speedo you pulled works before you pull it. And, if the wiring guy you took it to looked it over and couldn't find fault in it you will need to take it to someone who is more capable. The .4 volts is not important. The sensor is Hall Effect, meaning it produces an on/off pulse as a magnet on the shaft passes an inductive coil. It is not designed to generate power it is only a switch. The Speedo has a 5.0v reference signal that is universal with all sensors and the speedometer sensor pulses this signal in relation to how fast the magnet or magnets pass the inductor. You cannot test it with a volt meter. You can only test it with an oscilloscope. It is not uncommon for the magnets to die or the inductor to short. I would make the investment in taking it to a garage and having it hooked to a diagnostic scanner. The car has a speed limiter and if it has an automatic transmission it uses the signal from the speed sensor to determine when to shift gears. So a diagnostic scanner should be able to log a failure at the sensor. If it is an automatic and the shifting is still consistent, you may need to look at getting a refurbished gauge cluster.
It is commonplace to see Nissans with fried speedometers. It's the quality of the parts they used. Get the car to a qualified repair center, not auto electric. Just a repair shop with a good track record. The auto electric guy you took it to should have been able to do this job with the proper equipment. Let a garage sort it out. If your kid gives you any grief about keeping this car, tell him you will have a tachograph installed to replace the speedometer. The Tachograph will show speed as well as recording how fast he was going and how long he was at that speed. That usually keeps them in line; my dad did it to me when I was in high school.

Happy Motoring!

John