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 system:
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
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
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
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
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
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
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 as needed.
What to do is there is no voltage at the insulated side of open
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 it.
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
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
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
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.
FIRING ORDER: 1-3-4-2
DISTRIBUTOR ROTATION: Counter clockwise (turn counter clockwise
POINT GAP: .014-.016
TIMING: 6 BTDC (More if engine worn and timing
PLUG TYPES: For both 7:1 an 8:1 engines up
through 1984, CHAMPION RN11YC4 (nearest to original) or NGK
BP5ES (a good option).
PLUG GAP: .032
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