Because anything electrical will already be earthed back to the battery via
a connection to the chassis or else it wouldn't run in normal operation. If
you connect a direct positive feed and the device runs it tells you that
both the device and the earth side is ok. If the device still doesn't run it
could either be broken or the earth side is not making contact. In that case
you need to add a negative feed to check the device fully.
--
Dave Baker

You can probably clip the aligator to a nail and stick that in the socket. I
use a piece of thick heavy bared copper wire.

As for the radiator fan motor not comming on when you probe it, that might
be due to the location of the thermal switch which only completes the
circuit when the rad heats up. I don't know enough about them to say for
sure.

--
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William R Watt    National Capital FreeNet    Ottawa's free community network
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Many auto items even lamps have both a positive drive wire and a (Often
black) ground wire thei ensures the ground stays good even in old age
when the mounting frame connection has rusted.

If you only use it for testing and stop when it burns your fingers it
will be fairly safe.

For a permanent rewire you should use wire as thick as was there from
the factory.

An inline fuse right close to the battery connection could save you from
connecting to a wrong pin and starting a fire in the test lead.

I've only got one question.  :)

Why not use a test lamp (low wattage bulb with earth lead attached)
rather than jumper leads?  In most cases it'll tell you as much, and
more, with a lot less risk of setting your car alight.  Or worse
still, providing a path for the smoke to escape from delicate
electronic components -- around which an LED is an even safer option.

Jumper leads can be fine for bench testing but need to be used with
extreme caution for in car diagnostics.

--
John H

I have some old harnesses around and have taken some of their plugs to
make ends for my jumper cables.  I have a spade and a pole one so far.
The rest have been ok with needles or paper clips.

You might want to go to a wrecker and tell them you want some old plug
pigtails.  (plug/socket and 3" or so of wire)  They should be cheap.

You also can get the plastic holders for inline glass fuses for about a
buck or so.  The kind they use on radios or CB's.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos:  Non members can still view!
Aug./05 http://www.imagestation.com/album/index.html?id=2120343242
(More Off Road album links at bottom of the view page)

julie, i'm not really sure exactly what your fascination is with all
these jumper wires but i've been a tech. in excess of 35 years now and
i still dont have any one standard "fix"  that will fit every
application . the truth is, every connector is different, from the size
and shape,to the availiable space , to whether it is male or female.
etc....  just get yourself  a little asst. of different guages of wire
(prefferably some that isn't too stiff) don't really sweat the guage of
the wire. if the wire starts to get hot while testing,then you have a
problem. testing purposes are not for extended periods of time. in
other words if you "jump" a blower motor assembly, don't let it run for
5 minutes. if it comes on and runs for 45 secs. then its o.k.
as far as fuses are concerned, you should use them on every jumper
wire. these are not expensive. go to your local auto parts store and
ask for some circuit breakers , they come in many different amps. pick
up a 10, 20,and 30 amp. then just cut your jumper and attatch somewhere
in line . they have 2 "LEGS" coming off  each breaker. these are really
inexpensive ($2) .
as for RADIO SHACK well, they know absolutely crap about  automobiles.
and the way most cooling fans operate, they have a full time positive
feed to the fan motors. the relays  and temp sensors work off of a
ground. they supply ground to the circuit. this is oposite from your
blower motor.  today's electronics are all networked and contain many
"PULL-UP" and "PULL-DOWN" circuits. you should learn the difference
because if you don't , it's simply a matter of time before you really
wish you hadn't tried jumping something. destruction is inevitable.