Oscilloscope Patterns and Waveforms

Electronic Ignition Distributor:

BACKGROUND AND APPLICATION INFORMATION:
Magnetic Crankshaft Position (CKP) and Camshaft Position (CMP) sensors are found on many North American, Asian and European-built cars and trucks, ranging from Toyota Corollas to Mercedes Benz flagships. They are analog sensors. They are still around because they are very reliable, perform adequately, and are inexpensive to manufacture. These sensors can be found mounted inside a distributor, or at the front, middle, or rear of the crankshaft or camshaft. They are two wire sensors, but many times their two wires are encased in a shielded, wire-braid conduit. This is because their signal is somewhat delicate, in electronic terms, and is susceptible to Electromagnetic Interference (EMI or RFI) from high voltage spark plug wires, car phones or other electronic devices on the vehicle. EMI and RFI can distort the signal's Critical Dimensions and create an "electronic communication" breakdown, which could in turn, cause a driveability problem.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the sensor wire labeled (-) or "low" in the wiring diagram.
2. Connect the "CH1" probe to the sensor wire labeled (+) or "high" in the wiring diagram.
3. Set the voltage scaling to 2 V/div. Alter as needed later.
4. Set the time base to 10 ms/div (1/100th of a second per division). Alter as needed later. A time base of 20 ms/div may be needed to diagnose no-starts due to slower cranking RPM.
5. Select "GND" coupling and position the trace on the division line in the center of the DSO display.
6. After ground position is set, select "AC" input coupling.
7. Select "NORMAL" acquire mode.
8. Set the "TRIGGER" mode to "AUTO." Set the "TRIGGER SLOPE" to trigger on a negative slope.

EXERCISE THE SENSOR
Crank or start the engine.

Good Crankshaft Position Sensor Pattern (Typical):

EXPECT THIS WAVEFORM RESULT
There are many AC waveform shapes produced from the different types of camshaft and crankshaft position sensors. There is nothing more effective than having a reference waveform to compare to when determining if a magnetic cam or crankshaft position sensor signal is good or bad. Generally, the oscillations (the "ups and downs" in the waveform) may not be perfect mirror images of each other above and below the zero level mark, but they should be relatively close on most sensors. The amplitude of magnetic CMP and CKP sensors will increase with increasing RPM; the higher the RPM, the taller the waveform. And, as RPM increases, frequency increases, meaning that more oscillations show up on the DSO display. The diagnostician should look for assurance that the critical dimensions of amplitude, frequency and shape are all intact, repeatable, regular and predictable. This means that the amplitude of the peaks has to be sufficient, the time between pulses repeatable, and the shapes repeatable and predictable.

The 5 Critical Dimensions


IMPORTANT NOTE: Beware that peak voltages, as well as the shape of the waveform, will vary substantially from one type of sensor to the next. Also, since an integral part of the sensor is a coil, or winding, their failures can be very temperature dependent. In most of these cases the waveform will become much smaller or very deformed, and the engine will stall, misfire or cut out at the same time the waveform takes on new shape. In general, the most common type of AC sensor failure is where the sensor won't generate a signal at all -- it's inoperative. It's boring to look at, but if the waveform stays flat while cranking the engine, make sure the circuit isn't grounded, check the DSO and sensor connections, make sure the proper parts are spinning, then condemn the sensor.

FOR MORE INFORMATION
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix