Oscilloscope Patterns and Waveforms
Import Distributor:
BACKGROUND AND APPLICATION INFORMATION:
The ignition coil primary signal can be one of the most important diagnostic signals in powertrain management systems. The ignition coil primary signal has always been a valuable diagnostic tool. This is still true. Some of the most effective diagnostic uses of this signal are for driveability problems, such as no starts, stalls at idle or while driving, misfires, hesitation, cuts out while driving, etc.
Many times the critical diagnostic path to pinpointing a problem successfully has the same question in it: "Is the problem ignition or fuel related?" When a vehicle is cutting out or stalling, etc., and it is not known if the engine is starving for fuel or if it's momentarily losing spark, the ignition primary signal is often very useful in eliminating one or the other.
The DSO is particularly effective on the ignition primary signal in instances when the driveability problem only occurs while driving or only occurs intermittently, because the portable DSO can be taken on a road test. Unfortunately, some newer EI (distributorless) systems have coils mounted in such a way that it is relatively difficult or impossible to get at the ignition coil primary circuit to connect the DSO. On those systems it is advisable to use the appropriate DSO probe adapter and connect to an ignition secondary signal. In some cases where the ignition coils are on top of the spark plugs (like the GM 2.3L Quad-4), installing temporary spark plug wires between the coils and the plugs can assist in testing.
Connect and Setup the Digital Storage Oscilloscope (DSO):
1. Connect the "COM" probe to a good ground source, such as the negative battery post or engine block.
2. Connect the "CH1" probe to the ignition coil primary signal (the pulsing side of the ignition coil). Use a wiring diagram to obtain the pin number or the color of the wire.
3. Set the voltage scaling to 10 V/div (20 V/div may work better on some systems, and to see the height of the coil release spikes).
4. Set the time base to 5 ms/div. Alter as needed later.
5. Select "GND" coupling and position the trace on the second division line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" acquire mode.
8. Set "TRIGGER" mode to "AUTO".
Exercise The Circuit:
With the key on, engine running, use the throttle to accelerate and decelerate the engine or drive the vehicle as needed to make the driveability problem occur.
Make sure that the Critical Dimensions of amplitude, frequency, shape and pulse width are all consistent, repeatable, and operating properly.
Good Ignition Coil Primary Signal Pattern (Typical):
Expect This Waveform Result
The duty cycle (dwell) should change when the engine is accelerated. See the pointers on the sample waveform to help determine coil on-time (dwell) and to identify the spark burn line. Additionally, more pulses (on/off cycles) should be seen on the screen of the DSO as engine RPM increases.
The current limiting circuit on some vehicles will switch in about 2 milliseconds after the coil is turned on, making the waveform lift about 8 volts off the ground level, until the circuit is turned off and the tall spike is produced. Generally look for the tall peaks to have equal amplitude. Also look for the proper shape, more in terms of it being consistent and repeatable. Pay close attention to whether the frequency of the waveform is keeping pace with engine RPM, and that the pulse width (dwell) changes when engine load and RPM changes.
Possible defects and Critical Dimension violations to watch for are inconsistent spike heights, dwell not changing with engine RPM and load, and instances where the waveform gets inconsistent (or goes completely flat) while the driveability problem is occurring.
The 5 Critical Dimensions
For More Information
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix