After Market Tacho Not Accurate Reads Double

Thought I might drag this thread up, as I have a related question...and this is my first question on the forum, so go easy!

I installed a VDO tacho, and at the same time a new coil. I followed the instructions below, power source from 'engine' fuse and signal from negative on coil.

https://docs.google.com/open?id=0B6wnvyI__yzHel90dzBBaUlUaEtzWW02aWxxdTVFUQ

I drove on the highway for about 40km and as I was waiting at lights on exit ramp the engine died. I managed to get it started after 10 minutes, then I bunny jumped for about another km before it died again and I couldn't get re-started. After diagnosis, I found the coil had died. I took it back to REPCO and got a refund, and re-installed the old coil (which was still in working order). I never re-hooked the tacho back up, and have been running fine ever since.

I originally thought the problem was just a dodgy coil, but then decided to check the wiring etc. to see if I had done something wrong. All wired correctly. I didn't want to re-hook the tacho back up, because I hate breaking down in traffic and thought maybe the tacho had stuffed the coil somehow...

Tonight, I was curious after reading this thread so I measured the signal wire (currently the tacho is all wired up to power etc. but the signal wire usually connected to the negative on coil is un-hooked). I connected positive probe of voltmeter to signal wire and grounded negative. To my surprise (with engine running) the voltmeter had a steady reading of 0.20 Volts. This seems wrong, and I'd like anyone's idea on whether this signal wire should be reading a positive voltage, and could this positive voltage have a detrimental effect on a coil after driving a few kms.

Thanks in advance. Andrew.