Re: Baffled By Voltage @ Coil 888 Merc...
I know I posted on this but I dont see it, its been a bit crazy in here today.
to much estrogen and most speak no english.
a quick way to test coil operating voltage.
go from the I terminal or the power tap area for the resistor wire to the ign coil + post.
with key on engine off,points closed, this will be the voltage drop in the ign circuit to the coil +.
subtract that from battery voltage and you have coil operating voltage.
points open from block to coil + is battery voltage or should be very close.
points closed from engine block to ground with key on and your measureing the voltage drop of the coil,point contact and ground plate and if equipped the coil internal resistor.
to test voltage drop on the entire ign circuit,points closed key on, test between the battery + terminal to coil primary +.
the reason you see the voltage rise as engine speed increases is two fold.
one is battery voltage typically climbs some, the DVA meter will show about 140 volts on the coil + terminal.
and what yor reading from coil+ to engine ground is the circuit trying to rapidly swing from about battery voltage to 0 V and most digital meters will buffer the difference.
if your measuring 4V from coil positive to block ground with the points closed move to the - coil,should see less than .5V with points closed key on.
if ore inspect all conections,point contacts and the ground plate.
an easy way to visualize it is draw it out on paper,straight line.
then apply a voltage at one end of the drawing ending with the ground signal at the other.
if I draw a straight piece of wire from positive to negative with a lght bulb in the middle,then I put 2 test points in front of the bulb and 2 test points afrter.
plug it in the lamp is lit.
place a meter lead one on the positive leave it there the rest of the test. the other on point 1,if no resistance is noted I get 0V,point two is the same,positive side of the bulb,the same,negative side of the bulb I will get a small voltage,this is the voltage drop in my wire so far.
points 3 and 4 will read the same as the - side of my bulb unless there is a bad connection or anydevice that drops voltage.
now subsitute a resistance wire,coil windings,point contact resistance and ground plate for the test points an bulb.
