Question:How can I tell what's causing a "no-spark"
condition?
Answer:The best way is to methodically check each component
until you've isolated the fault. But many people start replacing
things until they eventually stumble on whatever was causing their
problem (which is a very expensive and time-consuming way to solve
a no start problem).
If you're certain the ignition system is not creating a spark
(Further information is available on getting a spark.), you can assume that either the coil
is defective or something in the electronic circuitry that controls
the coil is defective.
Most distributor type electronic ignition systems have either
a magnetic pickup or "Hall effect" sensor that generates
a signal as the distributor shaft rotates. On distributorless
ignition systems, the trigger signal is generated by a Hall effect
or magnetic crankshaft position sensor (and cam position sensor
in many applications). The trigger signal goes to the ignition
module, which switches the ignition coil on and off to fire the
spark plugs. If the pickup or Hall effect switch is not producing
a signal, or if the ignition module is not processing the signal,
the coil won't fire.
COIL CHECKS
Ignition coils are pretty simple (just a set of copper windings
around an iron core sealed in plastic), so there's not much that
can go wrong with them. They do fail occasionally, but usually
the part that fails is something more vulnerable like the ignition
module, the pickup in the distributor or the wires that connect
to the pickup or the module.
The coil can be ruled out as a possible cause by checking
it's "primary" and "secondary" resistance
with an ohmmeter. Primary resistance is checked between the positive
and negative coil terminals. As a rule, primary resistance should
be two ohms or less. Secondary resistance is tested between the
high voltage terminal and the negative terminal. Secondary resistance
should be high, ranging anywhere from 8,000 to 20,000 ohms. The
exact specs will vary from one application to another, so refer
to a manual for the specifications for your vehicle.
NOTE: On some distributorless ignition systems, the individual
coils in the coil pack assembly can be replaced if only one coil
is defective. On others, the coils cannot be replaced separately
and the entire unit must be replaced if any coil, or the coil module,
is bad.
WIRING CHECKS
CAUTION: Don't disconnect or unplug any connectors without
first making sure the ignition is off. Breaking a connection
while voltage is flowing in the circuit can create a voltage surge
that may damage electronic components.
Check all the connectors at the distributor, module and coil
to make sure they're tight and corrosion-free. It doesn't take
much to disrupt the primary ignition circuit. A visual inspection
won't necessarily reveal all such problems because "invisible"
corrosion can create enough resistance to disrupt a low voltage
circuit. You may have to back probe both sides of a connector
with an ohmmeter to see if there is resistance in the link.
Next, remove the distributor cap (if your engine has one)
and inspect the wiring connections on the pickup (and module if
the module is located inside the cap as is the case with most
GM systems, or on the side of the distributor as is the case with
many Ford systems). The flexing of the distributor can sometimes
cause hairline cracks in the wires or their insulation creating
an open in the circuit.
PICKUP CHECKS
If you don't find any problems here, check the resistance
of the pickup in the distributor (refer to a manual for the exact
specs). An open or short here can prevent the pickup signal from
reaching the module. The exact test procedures are so varied
that we'll summarize by saying that most checks involve using
an ohmmeter to measure the resistance of a magnetic pickup or
a voltmeter to check voltage readings in the Hall effect sensor
circuits. If the readings are out of range, the pickup or sensor
needs to be replaced. With Hall effect sensors, it's also important
to make sure the sensor is receiving voltage because it can't
generate a signal without voltage. Refer to a manual for the
specific diagnostic procedure for your vehicle.
NOTE: On engines with computerized controls, loss of the
pickup signal will usually set a fault code in the computer.
On vehicles with distributorless ignition systems, a bad crankshaft
or camshaft sensor will also set a code. To check for such a
code, you'll need a scan tool -- unless the computer has a "manual"
diagnostic mode that displays "flash" codes (blinking
lights) when certain terminals on the diagnostic connector are
jumped or grounded. Refer to a manual for the code retrieval
procedure.
MODULE CHECKS
Ignition modules frequently fail for three reasons: heat,
vibration or voltage overload. Excessive heat can damage sensitive
electronic chips. GM ignition modules and Ford TFI modules, in
particular, rely on a layer of grease under the module to help
carry heat away from the electronics. If someone replaced the
module and forgot to apply the grease, the module may have failed
due to overheating. The leads that connect the module to the
wiring are also vulnerable to breakage, often as a result of vibration.
Finally, a module can be destroyed if the high secondary voltage
in the distributor somehow finds a path to the module.
In some applications, modules can be checked by using a high
impedance (10 megohms) digital ohmmeter to measure resistance
between various terminals. If any of the measurements are out
of spec, the module is defective and needs to be replaced. But
on most applications, a process of elimination is used to isolate
a bad module. Or, a "known good module" is substituted
for the old one to see if it cures the problem. If the engine
starts, the old module must have been bad. If it still doesn't
start, then it's something else. The problem with this technique
is that the new module may be damaged if there's a short in the
wiring that caused the old module to fail.
COMPUTER CHECKS
Most electronic ignition systems will start even if the computer
is defective or disconnected. The computer plays an "intervening"
role and modifies spark timing by delaying or advancing the pickup
trigger signal or module switching of the coil. In spite of the
fact that many people blame the computer for anything that goes
wrong, the fact is the computer is pretty reliable. Most problems
can be traced to opens in the wiring harness or loose or corroded
connectors.
A scan tool is required to check the computer for ignition-related
fault codes, unless the system provides a manual flash codes.
Refer to a manual for the diagnostic procedure for your vehicle.
