At 12:47 AM 3/2/2009, Scott Daniel  - Turbovans wrote:
>very good.
>tell us  more about what a logic probe is and how to use it.

<http://www.allspectrum.com/store/product_info.php?products_id=392>

This site has a nice one for sale, $20, and an excellent picture if
you click to enlarge the tiny one.  I have to go shovel snow now, but
I'll read over the description later and try to fill in the
blanks.  One thing that may help -- in digital electronics, a logic
level is a voltage.  A voltage near the supply voltage means yes, or
one.  One near ground means no, or zero.  In between there is a zone
that means invalid, bad signal.  These levels can change
extraordinarily rapidly, much too fast to see.  Logic probes display
High, Low, or invalid, and they stretch out the blink so it's long
enough to see.  Many families of logic circuits have rigid voltage
requirements, but the one called CMOS works over a wide range of
voltages, and the levels are defined as fractions of the supply
voltage instead of fixed voltages.  CMOS is therefore an ideal family
for automotive use.

The Hall-effect sender in the distributor is not CMOS logic, but its
behavior is similar enough that a logic probe set for CMOS should
work fine.  When all is well, both lights will light.  When the music
stops, either high, or low, or no lights will be on.

Normally hooking a long wire to the end of a logic probe is a bad
idea, but this circuit operates slow enough that I think running it
up to the dash will not be a problem except maybe from the guy with
the monster amplifier on his CB radio (depending on the impedance of
the Hall-effect circuit, which I haven't time to look up).  And using
RG-174 coax with shield grounded at one end would probably cure
that.  RG-174 is lovely stuff, about as thick as regular spaghetti,
very flexible.

  --
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"