An example of a running engine at idle. (my
400, of course) This is looking at the tachometer line of the HEI ignition
system. A standard ignition doesn't look all that different because the
points do the same job as the module in an HEI. The HEI has the coil as
part of the cap to reduce spark loss, and it keeps the coil away from engine
heat better. The switching transistors in the module make for a faster
coil collapse and hotter spark then mechanically moving points can. Points
also move position, and build up arc material on the contacts, reducing
The wave form of the tachometer output line
of the HEI. The lower valleys are the voltage sags as the coil draws current
during buildup of inductive charge, and the tall spikes are the coil's
discharge kickback. This happens at 55 cycles per second (18 milliseconds
apart) at an idle speed of 900 RPM in this engine. These pulses on the
coil's minus side are what a tachometer counts and displays as RPM.
Testing a standard ignition coil. Set the
meter for ohms, and measure across the plus (+) and minus (-) terminals.
It should be between 1.5 and 2.0 ohms. This one is 1.6 ohms. Measure from
each terminal to the coil's case also. The meter should NOT read anything
Measure from the coil's minus (-) to the
inside of the tower. It should be several thousand ohms, and at least 4,000
minimum. This one is 10.25k ohms. (10,250 ohms) If it measures more than
40,000, the coil is bad. (unless it's a special high output coil like an
Accel) Measure from the terminal to the coil's case also. The meter should
NOT read anything. An ignition coil is called an autotransformer. This
just means the coils are tied to a common point. In this case the - terminal.
Right click and select "View image" for
a better look.
Testing the condenser. (capacitor) Open the
points and set the meter on ohms. The meter should start out at several
megohms and drop to 0, then start rising again. Reverse the leads, and
it should do the same again. A steady low reading means a shorted condenser.
No reading at all, (OL on a digital) means an open condenser. A condenser
is a 0.25 microfarad (approx) capacitor with high voltage and heat ratings.
It absorbs the reverse kickback from the coil as the points begin to open.
If the condenser is open, the points will arc over as they open up and
the engine will have weak if any spark, because the coil's current falls
off too slowly. If the points aren't all that old, but have massive
contact buildup, it's going bad. If it's shorted, it's the same as points
that aren't opening at all. If the engine runs but won't rev up past a
certain RPM, the advance plate may have loose rivets and is moving around.
Right click and select "View image" for
a better look.
This is the entire ignition system on most
older vehicles. The key switch has an IGN terminal that stays powered in
every position except OFF. The ACC powers in the ACC and ON positions,
and is OFF during START. The START powers the starter solenoid. The I terminal
on the starter bypasses the ballast resistor during starting to increase
the spark. The ballast resistor is about 2 - 4 ohms and reduces the total
current to the coil to prevent overheating. During startup, the battery
voltage can drop very low, and the coil may not even produce spark without
the starter's bypass terminal. The running engine will measure about 4
to 7 volts on the coil + terminal. This is normal due to the resistor.
The distributor's function is to position the rotor somewhere in the middle
of the cap terminal during cylinder sparking. When things are right, the
rotor moves in position, then the spark jumps the gap as the points
open. Harley Davidsons don't have a distributor because both plugs fire
at the same time. the other cylinder is in it's exhaust stroke and doesn't
care. More cylinders need better coordination than that, hence the distributor.
This problem drives most people crazy. It shuts off
when going down the road, then mysteriously starts running again and may
run for minutes or hours before the next stallout. My tried and true way
to find it is to get 2 test lights. Run wires from the + and - terminals
of the coil into the cab. Extend them if necessary. Have a friend hold
the lights and then go for a drive. The light on the + side will be steady,
and the - side will be dim and/or flickering. This is normal. Have the
friend watch the lights like a hawk. When the ignition cuts out, see which
light goes out.
If it's the flickering light (- terminal)
Look for shorts in the distributor, bare spots in the wire leading to it,
points that are set too far from the cam to open, or a bad condenser. If
it has dual points, one set may set so close that they are intermittently
If the steady light goes out (+ terminal)
Look for shorts or opens on the + wire. Typically, the wire leading to
the starter melts into the manifold and intermittently shorts, kills the
truck, then it moves away again. The ballast resistor may be bad. The firewall
connector may be loose. It could also be a bad ignition switch.
If both lights stay on, or go out at the same
time, then the coil has a heat failure problem. Replace it. They heat
up, then short internally for a few minutes.
There can be several things that cause this, but
9 out of 10 times it isn't the carburetor. I can't count the times I've
seen someone start turning the idle jets when the truck won't fire up.
Spare me please.
A bad or missing resistor bypass line can do
it. Put a test light on the + side and watch it during starts. If the light
all but goes out, use a clip lead to connect straight to the battery for
full coil power as a test.
A failing condenser will cause weak spark.
If the distributor was removed, it may have been
put in wrong. Yes, it can make a difference where it lands as it's reinstalled.
If it's too far off, the coil will fire during correct timing with the
rotor all but out of position. If it's a half inch away from the cap terminal,
it will either not fire, not start easily, or rev up much if it does start.
If you can't seem to time it, pull it out and start over with TDC and then
reset it in the hole.
Bad spark plug wires are not too uncommon, but
they don't all fail at once! They degrade over time, but not catastrophically
and all 8 (or 6). same for plugs. Measure the wires. They should be several
thousand ohms depending on the type used. They should be within 10% of
each other though.
Buildup on the points will cause the timing to
move off, and if it's bad enough, the points will fail to switch anymore.