Re: zero volts

[Simon Favre <favres@xxxxxxxxxxx>]

Maybe I can shed a little Electrical Engineering light on this subject.
One poster was right that there are fairly large currents running around
in a car.  Evan a small excess resistance can cause a significant
voltage drop.  What's worse, the excess resistance causes heating at the
contact, leading to premature failure, or melting of the insulation in
extreme cases.  I believe what you want to measure IS the voltage across
the connection while current is flowing.  The standard way of measuring
very small resistances is with a Kelvin, or 4-point probe.  You pass a
known current through the outer pair of contacts, and measure the
voltage with the inner pair of contacts.  That way, the contact
resistance does not affect the measurement.  All the current passes
through one pair of contacts whose voltage drop is irrelevant, and the
contacts measuring the voltage pass no current.  The same thing is
applicable to a car.  Connect the negative lead of an accurate DVM to
the car frame.  Connect the positive probe to the engine block.  Crank
the starter a few seconds, and you can measure the voltage drop across
your main ground strap.  You might be surprised.

The problem with trying to measure such small resistances with an Ohm
meter is that the contact resistance swamps the actual resistance.  When
current is actually flowing, the resistance may also change due to local
heating.  You can't measure this with an Ohm meter.

You can also measure drop from the battery this way.  If you pull one of
the headlight sockets, you can connect the negative lead of the DVM to
one of the "hot" tabs on the bulb.  You might have to pull it out a
little from the socket.  Connect the positive lead of the DVM right onto
the battery terminal.  Turn on the headlights.  You might be surprised
again.  As you reconnect the positive probe closer and closer to the
bulb, any single point of excess voltage drop should make itself known.

Since poor grounds are the cause of so many ills on some Alfas, it makes
sense to actually measure voltage across a supposed ground connection
while it is running, or with all accessories turned on.  Do your
instruments go wacko when the light are on?  Guess what, there are bulbs
in your instrument cluster.  The current needed to light those bulbs can
cause significant voltage drop across the cluster ground strap, causing
erratic or inaccurate instrument readings.

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