Very odd ground problem

Unrestored-original '74 Z1-A.  Right turn signal spontaneously quit working. Only the front bulb energized.  Rear bulb tested good.  Left side works fine.  Ground, right?  Well, yeah, but not where we'd ever suspect.

The cause of this problem took an hour+ to find; longer than we'd care to admit.  But' who'd-a-thunk it?


In the pic above, we see the suspect stem/signal assy. at the bottom with the hot wire exiting the hollow stud.  Above that is a disassembled OEM stem of uncertain vintage to show how they're assembled. 

One would believe a steel stud screwed deeply into the stem would create a sound path for ground continuity, right?  Except when it doesn't...

The sleeve of the bulb socket is grounded to the turn signal body, which is (s'posed to be) grounded through the stem/hollow stud assy. to a black/yellow washer/pigtail at the chassis mount..  That pigtail plugs into a dual B/Y connector on the rear main harness.  B/Y is ground throughout on Z1's.

Long story short, there was no continuity between the hollow stud (green arrow) and anywhere on the stem or turn signal body (red arrows).  Had the meter set to 200 Ω scale.  At first it was difficult to believe what the meter was telling us.  Even tried a different meter.  Same.  In hindsight it might have been interesting to go to the 2 MΩ scale to determine if it was truly open & not just very high resistance, but that didn't occur to us.

  Note the flat on the hollow stud in the bottom stem.  The instant we moved the stud using a wrench, continuity returned.  We tried to unscrew that stud but could only turn it a few degrees in either direction.  Acted like it was staked but couldn't seen any staking.  Didn't try too hard 'cause we didn't want to break anything.  We would have liked to have gotten it out to see if there was corrosion inside the stem's threads.  A squirt of WD-40, worked it back & forth a few times & we called it good. 

There's no flat on the stud with the other stem, but it unscrewed by hand.  Peering into the threaded hole of the other stem other we could see where the chrome plating ended 5mm or so into the hole, with what looks like bare metal (aluminum?) past that.  Shouldn't matter anyway, 'cause chrome & aluminum are good conductors.

So the true cause remains a mystery.  We don't like tech mysteries; they have a habit of coming back to haunt us.  At least now we'll have the experience*   to draw on quickly.  Hope the description of uncommon ground problem helps someone in the future.

Good Ridin'
slmjim & Z1BEBE

*  ex·pe·ri·ence
     /ikˈspirēəns/
practical contact with and observation of facts or events.
encounter or undergo (an event or occurrence).

And yaddayaddayadda...  Please allow us to offer an additional definition of experience:
Experience is what you get when you didn't get what you wanted.


 

These are the joy's of "chassis grounding" on older vehicles. You can also test grounds with a voltmeter (tell me if I'm "teaching my granny to suck eggs") black lead to battery ground, red lead to the ground side of the bulb holder - anything more than 0.3v indicates a resistance in the ground line

The ground circuit on older bikes seem to pop up as the as the neglected and weak point in the electrical system. On my 1980  kz440 k ran into an issue with my turn signals after swapping out the handlebars.... on inspection I found that they where switched on the ground side of circuit and the switch had no ground wire ... it grounded thru the handlebars,  which ment that my steering stem bearings were part of the circuit which is probably one of the biggest no, no of bike wiring.
The image shows how I attached dedicated ground to left hand switch assembly 
Also could not see a dedicated ground from engine, to frame , to wiring harness so I added one.