distributor itself. A crankshaft or camshaft rotating
sensor (fig. 4-40) is used to provide the electronic
control unit with piston position and engine speed. This
signal is used to trigger the correct coil at the correct
time for high-voltage spark. There are several types of
this system currently on the market. For testing and
repair, consult the manufacturers maintenance
manuals. Use only the correct tools and testing
equipment when working on these units.
As an automotive electrician, you will be called on
to troubleshoot the conventional, transistor, and
electronic ignition systems. The instruments you need
to pinpoint problems in a conventional ignition system
include the simple voltmeter and ohmmeter. Although
an engine analyzer simplifies the troubleshooting of
electronic ignition systems, you can do so with a
volt-ohmmeter (0 to 20,000-volt/ohm range). Better yet,
you may use an ignition scope tester since it can test
system components while the engine is running.
To troubleshoot a conventional ignition system, you
must conduct separate tests on the primary circuit (low
voltage) and the secondary circuit (high voltage). The
primary circuit carries current at battery voltage,
Figure 4-40.-Components of the distributorless ignition
system found in some General Motors products.
whereas the secondary voltage could be as much as
Primary Circuit Tests
Using a simple voltmeter, you can check a 12-volt
primary circuit as follows:
1. Hookup the voltmeter between the switch side
of the ignition coil and a good ground. The engine must
be at operating temperature, but stopped, and the
distributor side of the coil grounded with a jumper wire.
(See fig. 4-41.)
2. With the ignition switch on, jiggle it and watch
the voltmeter. The switch is defective if the meter needle
fluctuates. The voltmeter should read a steady 5.5 to 7
volts with the points open on systems using a ballast
3. Crank the engine and watch the voltmeter. It
should read at least 9.6 volts while the engine is being
4. Remove the jumper wire from the coil; then start
the engine. The meter reading should be 5 to 8 volts on
a ballast resistor system while the engine is running.
5. Stop the engine by turning off the ignition
switch. Hook up the voltmeter between the distributor
side of the coil and ground. Remove the high tension
wire from the coil and ground it.
6. Close the ignition switch and slowly open and
close the breaker points by bumping the engine. When
the points make and break the voltmeter should read
between 4 and 6 volts. Normally, with the engine
stopped and points opened, the reading will be 12 volts;
with points closed, the reading will be near zero volts.
If while the engine is cranked, the voltmeter reading
Figure 4-41.Testing ignition primary circuit.