The speedo drive is still under investigation! I jacked a rear wheel up last week and let the engine idle in gear while I checked the drive unit for the flashing LED denoting a signal. Nothing. So it looks as though there is no signal being generated until some time into the journey. To further prove this I connected my rather elderly 'scope up to the sensor, and went for a drive.
Amazingly, the internal (Ni-cad!) battery still holds a charge so I was able to check out the signal on the move. With the speedo working, I get the pattern below:
I've not yet been able to monitor with the speedo inoperative, but would expect a flat line like in the first photo, although it's possible that under certain situations, electrical noise could swamp the signal, hence the reason for carrying out the checks.
I had a bit of trouble tracking down a suitable speed sensor - I'd bought a universal one but that was not compatible with the cruise control system. Bearing in mind this was 20 years old I wasn't confident of finding spare parts, but I eventually discovered it was an Australian manufacturer of the kit, and they even listed spare parts. Bad news is no reply to my emails on shipping to the UK, but MorrisItalSLX has come to my rescue and is going to post one on to me
As I'm coming up to the magic 200k miles I don't want to miss any, so important to get my odometer working properly!