Interesting trick.

I test coil packs and wires by using my inductive timing light.   By putting the clip on each wire one at a time while the engine is running

Can find a lot of misfires that way.

My guess is that short piece of vacuum tubing allows for what is called a series gap.  when he gets the test probe close enough to the coil, the gap was "jumping"  from the coil to the wire (or in some cases in reverse as a lot of those coil packs only fire one time and one bank is negative to positive: google wasted spark ignition)  anyway when he gets the probe close enough, the electricity takes the path of least resistance and jumps to the probe , instead of the wire.    


Can learn a lot from the old time mechanics, like when I watched one diagnose a wrist pin by spraying water into the carb.   Lots of old tricks going by the wayside with all the computer technologies......still cant compete with instincts and skillfully learned experience.  

edit: a little info about igntion system types by a spark plug maufactuer
http://www.ngksparkplugs.com/docs/tech/New%20Ign%20Systems-Long%20Life%20Plugs.pdf

Last edited by Greg B (6/27/2013 11:45 AM)

funny thing this guy is younger than Me!!  prolly 40ish!  He used to work at autozone here in town, thats where I got to know him!  he just decided to open his own shop and is doing quite well!  of course with that type of knowledge he prolly will continue