DISTRIBUTORLESS and COIL-ON-PLUG IGNITION SYSTEMS
General Motors introduced a new kind of ignition system that
uses no distributor back in 1984 on the Buick 3.8 liter
turbocharged V6 engine. Called "Computer Controlled
Ignition" or "C3I", the same system has went through several
generations and is now found on numerous GM engines. Similar
distributorless ignition system (DIS) are used by Ford, Chrysler
and most of the import vehicle manufacturers.
By eliminating the distributor, distributor cap, rotor and
pickup, DIS increases ignition reliability and reduces the need
for service and periodic maintenance. Most distributorless
ignition systems do not allow for manual timing adjustments
(though some do have an adjustable crankshaft position sensor).
This eliminates the possibility of misadjusting the timing. No
distributor cap means no carbon tracing or cracking, and less
chance of moisture-related misfiring. No rotor means no arcing
HOW IT WORKS
A distributorless ignition system is something like the
ignition system on a motorcycle. The ignition coil is connected
directly to the spark plug, and the plug fires every revolution
of the engine.
Because distributorless ignition systems have no distributor,
the timing signal that's needed to trigger the ignition module is
generated by a Hall effect or magnetic crankshaft position sensor
mounted on the engine. This sensor performs the same
basic function as a pickup coil or Hall effect switch in a
distributor. Most applications also have a separate camshaft
position sensor so the computer can distinguish top dead center
(TDC) of the compression stroke from TDC of the exhaust stroke.
On some of these applications, the signal from the camshaft
sensor is also used to generate the pulses that open and close
the fuel injectors.
On Ford 2.3L DIS applications, a "dual" Hall sensor is mounted on
the crankshaft behind the timing belt cover. The sensor provides
both a crank reference signal (Profile Ignition Pickup or "PIP"
signal) and a camshaft signal (Cylinder Identification or "CID"
The signal from the crankshaft position sensor (and camshaft
position sensor) are monitored by the engine computer and
ignition module. The signal triggers the ignition module to fire
the ignition coils, which in turn send spark to the plugs. If
the crankshaft or camshaft position sensor signal is lost either
because of a sensor failure or wiring problem, there will be no
spark and the engine won't start.
The firing of the coils in a distributorless ignition system
is controlled by an ignition module located in the base of the
coil pack, or elsewhere in the case of applications where
individual coils for each cylinder are used. Spark timing is
usually regulated by the engine computer. In some
applications, the ignition module controls spark timing below 400
to 700 rpm while the engine is cranking, then turns the job over
to the computer once the engine starts. The ignition module may
also have a built-in "failsafe" capability that allows it to
provide a fixed ignition timing signal (the "limp-in" mode) in
case the spark control signal from the ECM is lost.
Most distributorless ignition systems have a separate
ignition coil for each pair of spark plugs (two coils for a four
cylinder, three coils for a V6, four coils for a V8, and in the
case of a V10 five coils). Each coil fires its two spark plugs
simultaneously. The paired spark plugs are located in opposite
cylinders (such as 1-4 and 2-3 on a four cylinder, or 1-4, 2-5 &
3-6 on a V6) so that one cylinder fires on its power stroke while
the opposite cylinder fires on its exhaust stroke. The spark
plug that fires in the cylinder during its exhaust stroke does
nothing and is referred to as the "waste spark." There is little
voltage resistance in the waste spark cylinder, so most of the
firing voltage is available at the plug in the compression
cylinder that needs it. When the engine turns another revolution
and the cylinders reverse roles, the same process repeats with
both plugs again firing simultaneously. One spark ignites the
mixture while the other does nothing.
A somewhat different setup is found on some engines such as
GM's Quad Four, 1990 & up Nissan 300ZX and '92 & up
Maxima with the DOHC 3.0L V6. On these engines, individual coils
are located directly over each spark plug. On Saab's Direct
Ignition (SDI) system, each spark plug has its own coil. But the
coils run on a primary voltage of 400 volts, which is stepped up
at the ignition module.
GM DIS APPLICATIONS
In the early Buick 3.8 liter C3I system, a Hall effect cam
position sensor is used in place of a distributor to provide a
valve timing reference signal to the ECM. A second Hall effect
sensor mounted on the crankshaft pulley tells the computer when
each cylinder reaches TDC. The computer then figures out how
much timing advance is needed according to coolant temperature,
throttle position, engine load and rpm, and signals the ignition
module to charge and fire the coils accordingly. The system also
has a detonation sensor that can retard timing when high loads or
heat cause the engine to rattle.
The Direct Ignition Systems used on 3.0 liter V6 engine uses
a combination cam/crank sensor, while the 2.0 and 2.5 four, 2.8
V6 and Quad Four use a single magnetic crankshaft position
sensor. The magnetic crankshaft sensor reads notches machined
into a ring cast in the middle of the crankshaft. The ring has
one notch for each cylinder, all evenly spaced with an extra
notch offset 10 degrees from the others to generate a TDC "snyc
pulse." By comparing the time between pulses, the DIS module can
pickup up the odd pulse and recognize which notch is for which
cylinder to calculate proper ignition timing.
The Integrated Direct Ignition (IDI) system used on the Quad
Four is unique in that the ignition coil and module assembly is
mounted directly over the spark plugs between the cam towers in
the cylinder head. There are no spark plug wires so there's no
easy way to check for the presence of a spark. To get at the
plugs, the plastic Quad Four cover between the cam towers has to
be removed and the IDI assembly pulled off the plugs. Like the
other direct ignition systems GM uses, a pair of coils are used
to fire all four cylinders.
When an engine with a direct ignition system fails to start, the
first thing you should do is determine if the no-start is due to
On most DIS systems (except the GM Quad Four and Saab
systems), you can quick check the ignition system to see if the
engine is getting spark by pulling the plug wire off one of the
spark plugs and connecting it to a plug tester. On the Quad Four
and Saab, the IDI coil assembly has to be lifted off the plugs so
ignition cables can be connected between it and the plugs or a
If you get a spark when the engine cranks, you can rule out
an ignition problem. The engine's not starting because of a fuel
or mechanical problem.
Note: On the Buick V6 Type 1 C3I application and others with
Sequential Fuel Injection (SFI) that use a cam position sensor to
pulse the injectors, loss of the cam sensor signal can prevent
the injectors from working.
If there's no spark when you crank the engine, then there's
an ignition, wiring or sensor problem. The question is where?
Check the other cylinders (1, 3 & 5 on a V6, or 1 & 2 on a four)
to see if any of the other plugs will fire. If all the plugs are
dead, the problem is in the ignition module, ECM, crank sensor or
wiring harness. The next step is to scan the vehicle's computer
system for fault codes.
On GM C3I systems, a Code 41 means the system has lost the
signal from the cam position sensor. A Code 42 on any of the C3I
or DIS systems means the electronic spark timing (EST) signal
from the ECM has been lost. Loss of the EST signal to the
ignition module should still allow the engine to start and run.
But because the module will be running without the benefit of the
computer EST signal, timing is fixed and engine performance will
suffer accordingly. A Code 43 means trouble in the detonation
You'll have to refer to the appropriate diagnostic charts in
the service manual and run through the various circuit checks in
order to isolate the offending component. Remember to clear any
stored trouble codes from memory afterwards.
If a quick spark check finds some plugs are firing but others
are not, then a bad coil is likely at fault. The engine may
start, but run poorly or with a bad miss. Remember, cylinders
1-4 and 2-3 share the same coil on the four cylinder engine while
cylinders 1-4, 2-5 and 3-6 share coils on the V6. A bad coil,
therefore, will take out two cylinders.
If, for example, you find that cylinders 3 and 6 are dead on
a V6 but the others spark, then the coil that supplies voltage to
cylinder 3-6 is probably defective. On Type 1 C3I systems, the
three coils are part of a single assembly meaning if any are bad
the whole trio has to be replaced. On Type 2 C3I and DIS
systems, the individual coils can be replaced separately if
It's also important to check the ignition wires to make sure
an open or shorted wire isn't causing the problem instead of a
weak or dead coil. Wire resistance should not exceed 30,000 ohms
on either cylinder.
To confirm a faulty coil on a Type I coil assembly, remove
the six Torx screws that attach the coil trio to the base module.
This will expose the coil leads. Each coil has a common blue
feed wire (positive) and a different colored control wire
(negative). Primary coil resistance can be checked by hooking up
an ohmmeter between the coil leads. Primary resistance should
read 0.7 ohms. Secondary resistance between each coil's high
voltage terminal and the negative control wire lead should be
Next hook up a test light between the control wire and blue
feed wire under the suspect coil. When you crank the engine, the
light should blink if the module is supplying the coil with
voltage. If the coil fails to fire, the problem is in the coil
itself. If the light fails to blink, the problem is in the
module. Replace the defective component.
On the Type 2 systems with replaceable coils, first check the
suspect coil for carbon tracking, then replace it with one of the
good coils. If the plugs now fire, you've confirmed your bad
coil diagnosis. But if swapping coils makes no difference, then
the module needs to be replaced.
DIS SENSOR CHECKS
The Hall effect cam position and crank position sensors on GM
Type 1 C3I V6 engines can be checked by unplugging the 14-wire
connector on the module and connecting a voltmeter's positive
lead to terminal L (cam sensor signal) and the negative lead to
terminal K (cam sensor ground). Cranking the engine should
produce a fluctuating voltage signal that ranges from zero up to
8-12 volts. If you remove the cam sensor, it's position will
affect injector timing but not ignition timing so make sure it is
properly aligned. The crank position sensor can be checked by
connecting the positive voltmeter lead to terminal G (sensor
signal) and the negative to terminal H (sensor ground) at the
module 14-wire connector. Again, you should see the voltage
reading fluctuate from zero to 8-12 volts when the engine is
On Type 2 DIS systems, the crank position sensor feeds
directly into the ignition module. To test the sensor,
disconnect the three-way connector from the DIS module and check
harness terminals A and B with an ohmmeter. Resistance should
read 900-1200 ohms. Then reset the meter to read volts and crank
the engine. The reading should be greater than 0.1 volts if the
crank position sensor is good.
COIL-OVER-PLUG (COP) IGNITION SYSTEMS
Placing individual ignition coils directly over each spark plug eliminates
the need for long, bulky (and expensive) high voltage spark plug cables.
This reduces radio frequency interference, eliminates potential misfire
problems caused by burned, chaffed or loose cables, and reduces resistance
along the path between the coil and plug. Consequently, each coil can be
smaller, lighter and use less energy to fire its spark plug.
It's also a known fact that the resistance of spark plug cables goes up
as they age. This increases the voltage required to fire the spark plugs,
which eventually leads to misfiring if the resistance gets too high.
Eliminating the plug wires improves ignition reliability as the miles add
up and reduces the risk of ignition misfire that could damage the catalytic
converter (unburned fuel makes the converter run too hot).
From a performance standpoint, having a separate coil for each cylinder gives
each coil more time to recharge between cylinder firings. With single coil
distributor systems, the coil must fire twice every revolution of the
crankshaft in a four cylinder engine, and four times in a V8. With a
multi-coil system, each coil only has to fire once every other revolution
of the crankshaft. This provides more saturation time for a hotter spark,
especially at higher RPM when firing times are greatly reduced. The result
is fewer misfires, cleaner combustion and better fuel economy. This is
especially important with today's Onboard Diagnostics II (OBD II) systems
that monitor misfires and turn on the Malfunction Indicator Lamp (MIL) if
misfires exceed a certain threshold.
A hotter spark also makes spark plugs more resistant to fouling and helps
100,000 mile plugs go the distance. A multi-coil ignition system also
improves idle stability and idle emissions.
Another advantage of COP ignitions is that it also improves the engine's
ability to handle more exhaust gas recirculation (EGR) to reduce oxides
of nitrogen (NOX) emissions so the engine can meet lower emission standards.
The Powertrain Control Module (PCM) can make timing adjustments in each
cylinder more quickly as operating conditions and engine loads change.
On Chrysler, Honda, Toyota and many other imports, the coils are mounted
directly over the spark plugs. Many of these have the thin "pencil" style
coils that extend down into recessed wells in the engine's valve covers.
On other applications, such as GM's Quad 2.2L Four, the individual coils
are mounted in a cassette or carrier that positions the coils over the
On late model Corvette, Camaro and other V8s, a "Coil-Near-Plug" (CNP) setup
is used because of the location of the spark plugs on the side of the
cylinder head. There isn't room to mount a coil directly over each spark
plug, and the heat from the nearby exhaust manifold would create a problem.
So the individual coils are mounted on the top of the valve cover and
attached to the plugs by short plug wires. This provides most of the same
benefits of a COP ignition system, but adds short plug wires to the system.
Even so, because the plug wires are so short they are less apt to chafe from
engine vibration. The shorter length also means less of an increase in
resistance as they age.
THE BRAINS OF THE SYSTEM
On most of the newer COP systems, the coil switching function is handled
entirely by the PCM and there is no separate ignition module -- though there
may some additional electronics and diodes built into the top of each coil
depending on the application.
The PCM receives a timing signal from the crankshaft position sensor (CKP)
and camshaft position sensor (if equipped) to determine engine speed, firing
order and timing. It then looks at inputs from the throttle position sensor
(TPS), airflow sensor, coolant sensor, MAP sensor, vehicle speed sensor and
even the transmission to determine how much timing advance to give each
cylinder. Most of today's COP ignition systems are capable of making timing
adjustments between cylinder firings, which makes these systems very responsive
and quick to adapt to changing engine loads.
Diagnosing late model "Coil-Over-Plug" (COP) ignition systems isn't a whole lot
different that diagnosing older distributor and distributorless ignition systems.
The only major difference is that the coils are mounted directly over the spark
plugs and there are no plug wires. That means you have to use some special tools
to pick up an ignition signal if you want to look at ignition patterns on a scope.
COP COIL DIAGNOSIS
Though COP coils are very reliable, an individual coil may sometimes fail (multiple
coil failures are rare!). Coils heat up when voltage passes through them. Over
time, the combination of heat and voltage may break down the insulation between
the windings, coil housing or tower. This may weaken or kill the spark altogether,
causing ignition misfire, a loss of power and a huge jump in hydrocarbon emissions
(which may also damage the catalytic converter!).
If a coil problem is suspected, the coil's primary and secondary resistance should
be measured with an ohmmeter. If either is out of specifications, the coil needs
to be replaced. A coil tester that actually saturates the coil is the best kind
A short that lowers normal resistance in the primary windings will allow excessive
current to flow through the coil, which may damage the PCM driver circuit. This
may also reduce the coil's voltage output resulting in a weak spark, hard starting,
hesitation or misfire under load or when accelerating.
Abnormally high resistance or an open circuit in a coil's primary windings will
not usually damage the PCM driver circuit, but it will reduce the coil's secondary
voltage output or kill it altogether.
A short that reduces resistance in a coil's secondary windings will also result
in a weak spark, but will not damage the PCM driver circuit.
An open or higher than normal resistance in a coil's secondary windings will also
cause a weak spark or no spark, and may also damage the PCM driver circuit due
to feedback induction through the primary circuit.
Caution: Never disconnect a COP coil from its spark plug while the engine is
running. Doing so may damage the coil or other electronics.
FINDING THE BAD COIL IN A COP or DIS SYSTEM
When a coil failure occurs on a distributor ignition system, it affects all
the cylinders. The engine may not start or it may misfire badly when under
load. The misfire may also jump from cylinder to cylinder. But with COP
ignition systems, a single coil failure will only affect one cylinder. A
coil failure should set a DTC and turn on the MIL lamp.
On vehicles with OBD II, the misfire monitor should detect any misfire problems
and set a fault code that identifies the misbehaving cylinder. A misfire code
P0301, for example, would tell you cylinder #1 is misfiring. But is it spark,
fuel or compression?
If the coil is shorted or open, you should also find a code that would indicate
a bad coil for the same cylinder. If there is no code, you should measure the
coil's primary and secondary resistance with your DVOM, and/or observe the
primary and secondary ignition patterns on a scope. You should also remove
and inspect the spark plug. Check the spark gap and look at the deposits on
the plug to see if the misfire is due to carbon or oil buildup.
If the coil and spark plug appear to be okay, the misfire may be due to a
weak or dirty fuel injector (check the injector's resistance, voltage supply
and current), or a bad valve (check compression).
NOTE: If an engine with a COP ignition system cranks normally but won't
start because there is no spark, the problem isn't one or more bad coils.
More likely, the fault is a bad crankshaft or camshaft position sensor or a
voltage supply problem to the coils in the ignition circuit.
COP COIL ADAPTERS
Observing COP ignition patterns on a scope requires using an inductive pickup
that attaches directly to the top of the coil, or adapter cables that go between
the coils and spark plugs. An inductive pickup that attaches to the coil will
allow you to observe the secondary ignition output from one coil at a time.
There are also simple inductive pickup tools that flash or illuminate when they
pick up the strong magnetic field produced by an ignition coil. Though not as
accurate as a scope that can display actual KV readings, this type of tool can
help you quickly find a dead coil.
When a coil is replaced, the connector should be cleaned and checked for corrosion
or looseness to assure a good electrical connection. Corrosion can cause resistance,
intermittent operation, or loss of continuity, which may contribute to component
failure. Applying dielectric grease to coil connectors that fit over the spark
plugs is also recommended to minimize the risk of spark flashover caused by moisture.
If an engine is experiencing repeated coil failures, the coils may be working too
hard. The underlying cause may be high secondary resistance (worn spark plugs or
excessive spark plug gap), or in rare cases a lean fuel condition (dirty injectors,
vacuum leak or leaky EGR valve).
On high mileage engines with COP ignitions, new plugs should also be installed if
a coil has failed, or the original plugs show signs of fouling or are near their
recommended replacement interval (45,000 miles for conventional plugs, 100,000 miles
for long life plugs).