Secondary Ignition Wires and Distributor
In our last episode we spent the entire day checking out the spark
plugs. Is that efficency? Actually, it took more time to explain
than it really did to do the inspection. At any rate we decided that
a compression test was in order so we could verify the internal engine
status. That way we wouldn't buy any tune up parts until we were sure
that the engine was in reasonable mechanical condition. Our tests
showed a fictional compression reading of 85, 80, 82 and 86 psi. That
is well within accepted parameters of a 15 psi spread. The spark
plugs had a light gray tinge to them and the gap was not excessive.
Each one also fired with a sharp bluish white spark when we tested it
with each separate tractor plug installed its secondary wire and
properly grounded against the block. We decide to clean, regap, and
reinstall the plugs as the wear on them was very little. Of course we
could have replaced them and not done any of the above, but we're
frugal and didn't want to spend unnecessary money.
The next step is to examine the secondary ignition wires; commonly
called spark plug wires. These items transmit a high voltage pulse
from the distributor to each spark plug. Contrary to popular belief
these wires are quite sensitive. Most secondaries have a soft copper
core wire surrounded by stainless steel or carbon impregnated thread
mixed with an elastomer type conductor. The outside covering of a
heavy layer of insulation prevents the 12,000 to 25,000 volts from
bleeding out when the wire is carrying current. The resistance type
wires do not handle sharp bending or jerking and can break internally
thereby ruining the wire. Routing of wires near exhaust systems and
high temperature areas can cause the exterior coating to dry out and
crack. Excessive exposure to oil and antifreeze can chemically break
down the coating causing it to swell and get slippery or slimy.
Pulling on the wires to release them from the spark plugs can separate
the conducting material causing internal arcing and/or can pull the
steel plug connectors off the wire ends leaving them dangling off the
plug. Any of these misuses can cause the wires to leak voltage and
either misfire to another plug wire, cause carbon tracking in the
distributor, cause an intermittent fire at the plug, or no fire at
all. Keep them clean and separated from each other with the proper
wire clips available at auto stores. When removing the wires from the
plugs make sure and grasp the rubber boot, not the wires. Secondary
ignition wires have the habit of looking good, but not necessarily
being good. They can be the cause of irritating engine 'poofs' that
simply can't be located or can make an engine run erratic and
backfire. There is a real quick way of identifying bad secondary
wires on an engine. Run the engine on a moonless night away from all
forms of light. When your eyes get adjusted to the darkness watch the
plug wires while on the high idle setting. Bad wires will send eerie
flashes of violet light from one plug wire to another. An extremely
bad set of wires with moisture on them will give quite the light show.
A more sophisticated way, though, is to measure the wire resistance
using an ohmeter. A general rule of thumb is 8,000 to 12,000 ohms per
foot. This will also test the wire for continuity ensuring that there
are no breaks in the copper wire core. An alternative way to check
the wires in lieu of an ohmeter is to simply put a test light in
series with a voltmeter (or a small 12 volt test light) and the
tractor battery. The glowing light or a voltmeter reading the same as
the battery indicates continuity of the copper core. A follow up test
is to remove the plug wire, start the engine and run a grounded test
probe along the plug wire. A wire with a crack or puncture in the
insulation will arc a spark to the grounded probe. Replace that wire,
it's trouble!
The other end of the secondary ignition wire is installed in the
distributor cap. This cap, made of hard plastic or bakelite,
distributes the appropriate pulse of voltage to the correct spark plug
as determined by the rotor inside. Proper inspection of the cap will
reveal hints about the intergrity of the ignition system. Make sure
when removing the plug wires from the cap that you grasp and gently
twist the boot as you pull. It might be a good idea to label the
wires with tape and a magic marker so they reinstall without error.
Visually inspect the cap towers. They should be clean and shiny.
Many times the secondary wire to the coil will have heavy corrosion in
the terminal tower. This causes erratic operation or a no run
condition. If corrosion is present replace the cap and secondary wire
boots as they are probably cracked or loose and let moisture into the
tower. Before wiping the cap off check for carbon tracking along the
tower tops to the cap base, usually where the retaining clips are
located. Bad plugs or secondaries will cause an occasional or regular
spark to occur from any one of the towers to the nearest ground....the
mounting clip. The evidence of this happening is a small powderlike
carbon trail that resembles a hairlike tree root running from top to
bottom of the cap. Replace the cap if this is present. Now turn the
cap over and inspect the inside. Check the center carbon terminal and
the individual terminal posts for cracking or burning. Carbon
tracking on the inside will look similar to outside except that the
hairline fracture lines will run from terminal post to post; evidence
of cross-firing, backfiring, or missing. If the posts are burned or
grooved the cap should be replaced. Mild scaling, caused from the
spark leaping from the rotor to the post, can be scratched off with a
sharp object. Try not to groove the terminals as they are probably
made from aluminum and are quite soft. When replacing the cap it is
best to replace the rotor also. If reusing, gently scratch the rotor
prong so as to remove any oxidation or scaling.
The ignition points act as a switch to open and close the primary
ignition circuit. When the points are closed, current flows through
the ignition coil and builds up a magnetic field. When the points are
opened, current through the coil is stopped and the magnetic field
collapses, creating a high voltage current. Sound simple? It is if
you understand how electricity works. But we're only interested in
getting the points set correctly so we won't worry about how it
works...just as long as it works. But first, let's take a closer look
at the distributor itself.
It's easier to clean and inspect the distributor if it is removed and
put in a vise on the workbench. It is understandable, though, if one
doesn't feel comfortable pulling this rotating heart from engine.
Getting it reinstalled and timed correctly can be a hair pulling
experience if you've not done it a few times. But for those of you
who are adventurous, I'll walk you through it. In order to save
reinstallation time set the distributor cap back on the unit and take
note where number one plug wire tower is. Mark the distributor
housing side with a chalk mark that corresponds to this tower. Pull
the cap back off and note where the rotor firing prong is. Rotate the
motor by hand (it's easy without the plugs in) so the rotor prong
lines up with the chalk mark (also note the rotor rotation direction).
You might note that when they align, the timing marks on the
crankshaft pully will line up with the timing cover arrow. This means
that number one piston is at top dead center on the compression stroke
and the distributor is firing on that spark plug terminal. Also note
that the points are just opening....you can tell because a high spot
on the rotating cam on the d-shaft has pushed against the rubbing
block at the point base forcing the points to separate creating a gap.
When the spark jumps the gap it causes the coil to fire the high
voltage spark to the cap rotor and from there to the correct cylinder.
With the motor timed to number one you can now remove the mounting
bolt and fork that retains the distributor in the motor housing.
Observe how the rotor shaft will rotate a little as you remove the
distributor. This is because the drive gears are usually tapered and
as you pull up on the distributor the unmeshing of the gears causes
the shaft to turn. Remember when you reinstall the shaft you will
need to be off center to the mark you made by a little to compensate
for this gear taper. Make sure and disconnect the battery ground
cable so the kid doesn't roll the engine over while you're working at
the bench. If you forget, and he does, don't dispair. You can retime
the engine by placing your finger over number one spark plug hole,
then roll the motor over so you feel compression trying to blow your
finger off. Jockey the crank to align the timing marks back up and
this will be top dead center (TDC) for number one piston. When putting
the distributor back into the bore align the rotor tang with the mark
on the distributor casing that you made....make sure to offset the
rotor a little to compensate for the gear mesh.....and slide it in.
Some engines have a tang at the bottom of the d-shaft which runs the
oil pump. Make sure this tang engages the pump shaft. If it doesn't
then put a little downward pressure on the shaft and then rotate the
engine a quarter or half turn. The shaft will then drop into postion.
Now that the distributor is in the vise observe the inside condition.
Is it clean or is it full of oil, dirt, or carbon debris? If there is
oil in it then the bushing or seal at the bottom of the distributor is
bad and needs to be replaced. Wobbly motion of the shaft in the
housing bore will confirm this. Excess carbon indicates the points
have been arcing too much or there has been a lot of cross firing
under the cap. Dirt and debris can be caused from an opening in the
side of the cap and/or a dry rubbing block at the base of the points.
A little bit of high temperature grease on the block facing the cam
rotation will reduce that problem. Remove the points and condenser
and set aside. Underneath the mounting plate there may be a spark
advance system of springs and counterweights. Make sure they freely
rotate and that the springs aren't broken. Moisture buildup tends to
rust these gizmos up a lot so you may have to break the distributor
down a little farther to get to them. Take note of the proper order
as you disassemble. The configuration may look a little different on
the bench as it looked in the unit. Wire brush and clean the pieces
in solvent before reassembling. Some newer tractors may have a vacuum
advance diaphram. Make sure the rotating linkages are free to move
and check the diaphram by sucking on the vacuum port. The vacuum
plate will rotate as pressure decreases. Hold the pressure on with
your tongue for a few seconds to see if the diaphram has any small
cracks in it. Now lets check the points. If the points are a dull
slate color and if wear is not excessive they can be cleaned with a
point file and reinstalled. Don't use sandpaper or emery cloth as
this will scratch the point surface and leave small grit particles
imbedded in the points. Use a clean file as this will remove a small
layer of metal creating a good flat surface to fire against. If the
points are burned or pitted they need to be replaced. If one side of
the points is severely pitted and the other side has metal buildup on
it then you need to check and make sure the correct capacitor (or
condensor) is installed. Also check the coil polarity. Most engines
have a wire running from the negative terminal of the coil to the
distributor. It is common practice to replace the condensor when the
points are replaced. A leaky one can cause excessive point arcing
and/or metal transfer on the point surfaces. Reinstall the
refurbished or new point set in the distributor. Rotate the shaft so
the cam lobe separates the points, creating a gap, and measure the gap
opening with a wire type feeler gauge. Set to specs and tighten the
mounting screws. Make sure and keep the rubbing block lubricated with
a small dab of high temperature grease. Install a new rotor, line up
the casing marks and reinstall the distributor in the housing....don't
forget to compensate for the tapered gears. The mark on the
distributor casing should now line up with the rotor tang. If it
doesn't then you are probably off one tooth. Pull it out and
reinstall it again. The plug firing order is usually stamped
somewhere on the engine block. Locate it there or in the manual and
reinstall the plug wires accordingly. You know where number one plug
is on the cap and you wrote down the d-shaft rotation, so install the
wires in their respective order and attach them to the corresponding
plug. With the current distributor alignment the tractor should start
and run. Additional timing should be checked and adjusted with a
timing light. Attach the light pickup to number one plug wire, start
the engine, and aim the light on the crankshaft pully timing marks.
Adjust the distributor according to specs. If no light is available
then you can static time the engine. It won't be as close a timing
light but it will be next best. Look in the book to determine the
exact firing timing for that motor. Most of them are 4 or 6 degrees
before TDC. Rotate the timing mark on the crank pully to line up with
the housing cover arrow according to the specs. Hook up a spark plug
to the number one secondary wire and hold it against a ground. With
the ignition on, rotate the distributor in the direction of normal
distibutor rotation and watch for the exact moment that a spark occurs
at the plug. If it caught you by surprise back the distributor up and
do it again. The engine is now timed properly. Tighten the
distributor mounting bolt and secure all the necessary wires.
One last thing to check in this section is the integrity of the
primary wires running to the coil and distributor. Check to see that
they are securely fastened and have no exposed strands from broken or
crimped insulation. The coil should be clean and dry; check the
termial tower for oil or corrosion. If oil is present it indicates
internal leakage and the coil should be replaced.
Now that we've slapped new plugs and points in we'll go on, in our
next segments, and look at filters, hoses, and belts.
Contributing Author - Curtis Von Fange
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