QUESTION - My ignition system
is not working correctly and I have never tried to fix one that
has points before. I think maybe the coil is in backwards. Can
someone help? I have a negative earth system.
ANSWER - Here's a brief
description of the primary ignition circuit for a negative earth
1. Switched 12V to + side of coil.
2. - side of coil to insulated connector on side of distributor.
3. From insulated connector to condenser wire and insulated side of points.
4. When points are closed, from contacts on insulated side of points through
contacts of grounded side of points then through ground to the negative side
of battery. Note: The secondary high voltage circuit provides high voltage
when the points are closed. How long they are closed affects how much high
voltage is generated.
1. Coil centre lead to centre top of distributor cap. Spring
contact inside centre top of cap to centre of rotor. along
brass bar at top of rotor to whatever outside cap contact it
is pointing to.
2. From outside cap contact through spark plug wire to insulated
centre contact of spark plug.
3. From insulated centre contact of spark plug, through the
gap to the grounded contact then through ground to the negative
side of battery.
Think about the cap and rotor as a rotary switch that switches
the high voltage to the correct spark plug. All the contacts
need to be insulated with no cracks, oil film or carbon tracks.
There is a tiny gap between the end of rotor and the cap contact
where a small spark is generated. When the contacts become pitted
it is time to replace the cap and rotor. If the ends are just
black & rough, a light sanding gets them back into action.
If you have carbon tracks that you can see or what looks to be
shallow cracks in the plastic replace the cap & rotor. There
is a spring loaded centre contact inside the cap. It needs to
make solid contact with the rotor. If the centre contact is damaged
or the spring not working, replace the cap.
Think of the points as an on-off switch that switches the secondary
high voltage on and off at just the right time to send a spark
to a plug as the piston nears the top of its compression stroke.
The distributor is the switch operator for both switches. It
operates each switch at exactly the right time to produce a spark
and get it to the correct plug at just the right time. The shaft
turns the rotor and has a four lobe cam (one lobe for each spark
plug) near the top that opens and closes the points.
The points sit on a movable platform that moves the points in
relation to the distributor shaft cam lobes. The movement of
the platform advances the timing (The faster an engine turns
the more advanced the spark needs to be). There are mechanical
weights and springs that move the plate, advancing the timing
the faster the distributor turns until it reaches it maximum
limit. This is the mechanical advance.
There is also a vacuum advance that moves the plate when there
is a high vacuum at the base of the carb. The static setting
of the distributor, both advances and the point gap determine
when the spark will reach the plugs. If any of these things are
badly worn, not working, or set out of spec the spark will arrive
at the wrong time and the engine will not run right.
The common thing to go out of spec is the point gap. It should
always be one of your first suspects. today's points are much
lower quality than they used to be and they go bad or out of
adjustment more often than the days when all cars had points.
Parts that go bad during use and require periodic replacement:
1. Spark plug wires: Plug wires need to be changed periodically.
Carbon core (resistor) cables need to be replaced every two
years. Solid core (wire) cables should be replaced every four
to five years.
2. Spark plugs
3. Distributor points, cap and rotor & condenser. Note:
a condenser seldom goes bad, but it has a tendency to build
up oxidation at its ground contact which reduces its effectiveness.
Trouble shooting the ignition system:
IMPORTANT NOTE: IGNITION PROBLEMS AND PROBLEMS
DELEVERING ENOUGH FUEL TO THE CYLINDERS EXHIBIT THE SAME SYMPTOMS. Do
not get hung up spending a lot of time chasing a problem in
either the fuel or ignition system without a quick check of
the other to make sure it is OK. Its possible to end up replacing
the entire ignition system because the fuel filter needs replacing
or the fuel pump needs rebuilding. Or rebuilding the fuel system
because the points slipped. And you can end up rebuilding both
systems when you have a burnt valve. It is always a good idea
to run a compression test before spending a lot of money trying
to track down a problem in either the ignition or fuel systems.
Symptom: The engine periodically
misses a beat during idle. You may hear a periodic pop or snap.
Engine may stumble coming off idle and smooth out at higher RPMs.
Usual Cause: short in the high voltage
secondary circuit going to a cylinder. You could have a bad spark
plug, spark plug wire or a conductive build up inside the cap
between a outer cap contact and ground. Plug wires are the most
common guilty party. Examine the inside of the cap from any signs
of tracking. Replace cap & rotor if needed. Replace all the
plug wires if one has a problem. A bad plug is the least likely
cause, but replace if a new cap & wires do not solve the
problem. If you replace everything and there is still a problem
run a compression test.
Symptom: Engine just cuts out instantly
for no apparent reason then cuts back in or leaves you stranded.
Checking the cap & wires can make things start working again
for no apparent reason.
Usual cause: broken wire in the
primary circuit, most likely between the coil and distributor.
The wires vibrate when the engine is running. Sometime they fatigue
and break right where the connector crimp is. You end up with
two pieces of wire touching inside of the insulation. As the
wires vibrate they make contact & loose contact. From the
outside the look OK, but if you bend the wires you may feel that
one end bends easier right at the connector. Check with a volt
meter or continuity lamp. First, remove the distributor cap and
make sure the points are open. If they are not open, put the
gearbox into 4th gear and move the car until the points open.
With the ignition switch in the on position place a voltmeter
between the insulated side of the points and ground. It should
read a little over 12 V (the light on a continuity tester should
be lit). If it is, then wiggle the leads near the connectors
at each side of the coil and at the base of the distributor.
If the 12V goes away you have found a bad wire & replace
What to do is there is no voltage at the insulated
side of open points:
If there is not 12V on the insulated side of the points you
need to back track the primary ignition circuit one connector
at a time until you find it. If you find 12V at the connection
between the points and outside distributor connector, look
for a a missing or bad connection between the points and connector.
For the rest you need to disconnect the wire going to the points
at the base of the distributor.
Check for 12V at the disconnected wire where it would connect
to the distributor base. If you have 12V at the connector then
your points are shorted or your condenser is shorted. If you
have just installed a new set of points you might have accidentally
shorted the insulated side. Sometimes new points are bad.
If you do not find 12V on the wire connector that connects
to the distributor, check the coil connector that connects
the other end of the wire. If there is 12V there, replace the
wire. If there is not 12V, check the other connector on the
coil. If there is 12V there, then there the primary circuit
of the coil is open and you will need to replace the coil.
If you still do not have 12V with the ignition switch turned
on, you will need to trace each connection back until you find
Symptom: Engine just does not start
and there is weak or no spark at the plugs. (a low battery or
starter motor with shorted windings can cause the voltage to
be too low to create a spark when trying to start the engine)
Likely cause: If you are not out
of petrol, the ignition switch is in the on position, your battery
is fully charged, you have a good ground connection between the
engine and battery ground and your starter motor does not have
shorted windings then it could be any part of the ignition system.
First check the point gap (This should always be your first
check). If the gap looks OK, remove the distributor cap, put
the engine out of gear, mechanical brake set and have someone
turn the the engine over while you watch the inside of the distributor.
Verify that the rotor is turning, the brass bar at the top is
still there and that the points open and close as the lobes move.
Also visually inspect the centre contact of the cap to assure
that the sprung contact is still sprung and in good condition
and that there are no carbon trails or shallow cracks leading
sway from the centre contact.
With the starter motor not turning use a volt meter or continuity
lamp to verify that there is 12V (lamp is lit) when the points
are open and zero volts when the points are closed. You place
one lead on the insulated side of the points and the other on
a good ground. If there is 12V when the insulated contact is
open AND closed then either the points are not really closing
or there is no electrical path between both contacts. Readjust
points or replace them if the contacts are bad.
If there is NO 12V on the insulated side of the contacts you
need to trouble shoot the primary circuit using the method previously
If the primary circuits checks out as OK, the points are working
correctly and the points have the proper point gap, the fault
could be either the secondary circuit or the timing is off by
a long way (usually if the timing is way off it will try to start
or a cylinder will fire at a wrong time). The parts of the secondary
circuit that will keep anything from firing are the coil, high
voltage wire between the coil and distributor, centre connector
of the distributor cap and the rotor. If a visual inspection
doesn't identify the bad part, swap out components with new ones.
Symptom: Engine starts and idles
OK but runs crappy off idle.
Likely electrical causes (Beware
electrical, fuel & low compression can cause the same symptoms):
First check point gap and condition and fuel filter. This is
something that can easily be caused by a weak coil, bad spark
plug wires or advances not working. Replace old spark plug wires
as a matter of course. Disconnect a spark plug wire from the
plug. Insert a short bolt about the same diameter as the
spark plug top. One that will be held into the insulated connector
and not fall out. Holding it with an insulated tool, engine
running, place the bolt attached to the spark plug connector
near a bare ground and look at the spark in subdued lighting.
If you see a yellow spark the coil or centre plug wire needs
replacing. The spark should be an "electric blue" if
the coil is good. Turn the engine off for the other tests.
With the cap removed visually inspect the distributor. Try to
wobble the shaft (point gap be changed by lateral shaft movement).
There should be no side to side wobble. Replace distributor if
there is. Check the cap for cracks or carbon tracks between the
posts, wear on the center spring loaded post and deterioration
of the side posts.
Check the vacuum advance. Disconnect the distributor
vacuum line at the carb base and put a suction on the tube while
looking at the distributor plate the points are sitting on. Use
of Mity Vac or just suck on the tube. The plate should move when
you put suction on the tube and stay in the moved position for
as long as you maintain suction. If it doesn't, replace the vacuum
advance subassembly or the entire distributor. While you have
the Mity Vac in hand, if you have power brakes put a vacuum on
the brake booster to make sure you do not have a bad booster
diaphragm (could cause the same or similar symptoms). You can't
suck enough volume for this test.
Check the mechanical advance. This is easiest
done with a timing light and the vacuum advance disabled by disconnecting
an end of the vacuum advance tube. You should be able to see
the timing advance as you move off idle with the timing lamp.
If your points are properly adjusted, your primary and secondary
circuits are working OK, your advances are working and your initial
timing is OK trouble shoot the fuel circuit and run a compression
Symptom: Engine just runs crappy
with no or uneven idle and low power
Likely electrical causes (Beware
electrical, fuel & low compression can cause the same symptoms):
This is something that can easily be caused by a weak coil, bad
spark plug wires, slipped points, bad cap & rotor, or condenser.
Or a burnt valve, low compression, problems with the fuel system
or an intake vacuum leak including a bad power brake diaphragm.
First check point gap and condition and fuel filter. Second
check the distributor timing, the advances (by the method previously
described and make sure that the vacuum line between the distributor
and carb is intact and not leaking.
If it is not obviously slipped points or timing, this is a good
time just to perform a major tune up with new cap, rotor, points,
plugs, spark plug wires and condenser. If the problem is still
there, check for a yellow spark using the method previously described
and replace the coil if needed. If you still have the problem
after a major ignition system tune up, checking the coil, timing,
advances and vacuum line it is time to trouble shoot the fuel
system and run a compression test.
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