<VV> Meter is Better Re: Bulb over Meter WAS Late model headlight switch

[FrankCB at aol.com]

Ken,
 
IMHO, the best way is to turn ON the circuit and begin measuring the  
voltage at the light bulb (or whatever) and find out at what point the 12 volts  
(nominal) disappears or is so low it doesn't do much good.  The problem  
with using a test light bulb is that its current drain can be very small  
compared to the current drain of the item that doesn't work.  For example,  if 
you have a poor connection at the battery terminal, your test light bulb can  
light just fine, but when you try to run 100 amps through that connection to 
 activate the starter, the voltage drops so much that there's not enough 
left to  make the starter turn over.  A voltmeter instead will show a major 
voltage  drop at the poor connection (say 10 volts) when the starter is 
activated leaving  only a volt or two at the starter terminal which won't do much 
good!
 
So keep the circuit activated and measure where the BIG voltage drop  
occurs.
 
Frank Burkhard
 
 
In a message dated 11/18/2010 7:25:10 A.M. Eastern Standard Time,  
kenpepke at juno.com writes:


I  have thought about that over the years.  Can it be possible for modern  
meters sense the presence of voltage / potential without any current  draw?  
I don't own anything sensitive enough to find any amperage at all  through 
the volt meter.
Ken P  


**************************
"David B. Neale"  <david.neale3 at ntlworld.com> wrote:


"Past experience tells me  that a better tool is a circuit tester which has 
a light bulb.  In cars  that have been around for a long time it is 
possible to have enough dust /  dirt collected on the fuse to leak voltage and show 
as a working circuit even  though there is less than enough amperage to 
energize the circuit!    
Ken P"

---------------------------

You are essentially  correct, Ken; especially with a modern meter, which 
will draw so little  current almost any circuit path around and through an 
open-circuit, (blown),  will register, possibly,  full battery voltage on the 
load side of the  fuse.b This is where, if a meter must be used, an 
old-fashioned,  low-resistance, (read cheap), meter will give best results.

It would  make sense when measuring with a meter  to have any device that 
was fed  through the fuse switched on. so that the load's attempt to draw 
power would  indeed cause any meter to read zero in the case of measuring on 
the load side  of a blown fuse.

And yes, a bulb is much, much better than a meter for  the job; and a 
buzzer, too, for those times when you are upside down in the  car, head wedged 
underneath the dashboard, eyes having diifficulty focusing,  and unable to 
monitor a bulb, let alone a meter.

David

'65 Monza  140 4-speed convertible in  leafy Leicestershire, heart of a 
wet, cold  Autumnal  England





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