<VV> 140 stalling (now circuit tracing)

[John Dozsa]

> Subject: Re: <VV> 140 stalling
> Date: Fri, 12 Aug 2005 21:22:12 -0700
> From: Larry Forman <Larry at forman.net>
> To: Karl Behrens <gold1969corvair at yahoo.com>, virtualvairs at corvair.org
> 
> At 07:56 PM 8/12/2005 -0700, Karl Behrens wrote:
> >Well, I finally figured out my 140 stalling issue. At first, most
> >mentioned that the car was suffering from vapor lock. Well, here is what
> >the car is doing now! I have a 1969 500 coupe 140/glide. The car can sit
> >idle in my driveway, and runs fine. After about 30 min the car dies. This
> >is not in gear, just sitting there running. Well, the coil during normal
> >running is only getting 10 volts. But, when it stalls the coil is only
> >getting 6 volts! The alternator is putting out 15 volts, the battery is
> >getting 15 volts. The voltage regulator is putting out 15 volts to the
> >battery. So..I am at a loss. I am running the Crane Fire Ball Igntion I
> >purchased through Clark's. Now, when I noticed the car stalled, I found
> >that it blew 4 fuses. When I replaced the fuses, and stepped on the brake
> >pedal, I heared a sizzle, and the fuse blew again! I have purchased an NOS
> >Column switch to replace my orginal one. I disconnected the original
> >switch to keep from blowing fuses. Would the switch ca!
> >use the
> >  low voltage to the coil, or what??
> >Thanks,
> >Karl

SNIP

> 
> So my guess is that you need to search for a short.  I have heard of people
> taking a small buzzer and placing it in place of the fuse with maybe
> another fuse in series with it to keep from completely frying things.  When
> you energize the circuit, the buzzer will buzz.  Now you take a small
> pocket AM radio and tune it to the top of the band with no signal
> present.  The buzzer is supposed to generate radio frequency energy and you
> should hear it in the radio.  Now you can go along the wiring from the
> buzzer following it toward the circuit load or lamps or whatever, until the
> radio noise stops.  This should be the approximate location of the
> short.  It makes sense to me with my radio and electronics
> background.  Note, however, I have not actually used this equipment that
> way.  I have sources of more sensitive instruments to do the same
> thing.  FWIW, the very old HP Current Tracer, whose model number I forget,
> which was designed as a digital troubleshooting piece of equipment in the
> 70s, can be used to do the same thing if the circuit is not powered but
> driven with a pulse generator.
> 

SNIP


While I've not tried the buzzer technique either, I can see how it can
work.  The buzzer needs to be a old mechanical type door buzzer, not a
modern solid state piezo electric type.  The buzzer mechanical arm that
makes the noise is constantly making and breaking the circuit.  When the
circuit flow is broken the inductance in the coil tries to keep the
current flowing, but it can't because there is an open circuit.  A broad
spectrum of radio frequency (RF) energy is released.  This energy is
dissipated by radiation and conduction.  The conducted energy is sent
down the wire.  The radiated energy is what interferes with radio
reception.  For a buzzer and AM receiver to work as a wire short
detector you would have to minimize the radiated energy of the buzzer
coil.  The buzzer would need to be enclosed in a metal can that was
grounded to the vehicle chassis.  The conducted RF energy could then be
sensed by the portable AM radio.  Placing the ferrite antenna of the
radio along the wire would receive the wire conducted RF energy that is
now being broadcast.  In theory you could trace the exact path the
current flows including detours caused by shorts.  Anyone tried it?

John "EE" Dozsa