electrical gremlin... bad ground?

Hi all, i'm new here. I've been amazed at how much xj knowledge there is on this forum... so i have a question. I've just gotten an 82 650 seca (non turbo) and done some tuning to get it running after 1. it sat in a shed for 17 years and 2. the guy who bought it out of the shed did some pretty dumb things to it.

but now it purrs and pulls, but there's just this one electrical issue. The electronic tach doesn't work. i have tested the wires.

before starting up, key on engine off gets me:
ground wire to frame: 0 ohms
hot wire to frame: 12 volts
signal (pulse) wire to frame: 12ish volts
headlight off

key on engine running gets me:
ground wire to frame: 30 ohms
hot wire to frame: 12-14 volts depending on rpm
signal (pulse) wire to frame: pulse (checked with light bulb)
headlight on

after shutting down before shutting off key, (key still on, engine off) gets me:
ground wire to frame: 140 ohms
hot wire to frame: 12 volts
signal (pulse) wire to frame: 12ish volts
headlight on.

I tried a jobber tach meant for a 4 cylinder engine and it worked but only read half (by my guess) of actual rpm. maybe only because it's not meant for use with the wasted spark ignition system?

tried a tach off an xs1100, and it reads only about 1/5th of actual rpm.

What the H is going on here???

 

the ohms to frame is changing because as the conditions change the circuit changes through the connectors, more connectors more resistance
did you ever see it work? at all, does the needle bounce if ya hit it
17 years, might have some cob webs in it

 

are you telling me that it's normal for the ground to vary between 0-140 ohms? it also seems weird to me that the headlight would not come on with the key, until the engine is started...? i have taken apart the tach and it's clean as a whistle inside, and the needle turns freely, but all it does is twitch when it's hooked up.