Power Supply (B+) Test

BACKGROUND AND APPLICATION INFORMATION:
This same Digital Storage Oscilloscope (DSO) setup and test procedure can be applied to a lot of different automotive circuits that use B+, or battery voltage as their power source, such as power supply circuits (to the Powertrain Control Module (PCM) and other control modules, such as cruise control, Antilock Brake System, climate control, etc.), temperature switches, throttle switches, vacuum switches, light switches, brake switches, cruise control switches, etc. The relatively slow time base of 500 ms (milliseconds) allows a glitch to stay on the screen for 5 long seconds, so the diagnostician can get a good long look at what is occuring in the circuit. The DSO is set in "SPIKE DETECT" acquire mode so it's sampling the signal at warp speed -- about once every 40 billionths of a second. At 500 ms/div however, the DSO displays the information slowly, so the diagnostician has plenty of time to interpret it.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the negative battery post or the engine block.
2. Connect the "CH1" probe to the B+ circuit to be tested.
3. Set the voltage scaling to 5 V/div.
4. Set the time base to 500 ms/div (1/2 second per division).
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 "NORMAL" acquire mode.
8. "TRIGGER" mode should be set automatically (Roll Mode).

EXERCISE THE SENSOR, SWITCH, OR CIRCUIT UNDER TEST:
Make sure power is switched on in the circuit so that the sensor, device or circuit is operational. Exercise the sensor, device or circuit while paying particular attention to the amplitude of the signal. It should stay in a predetermined voltage range for a given condition. In most cases, the amplitude of the waveform should stay at B+, or battery voltage, when the circuit is on, and go to zero volts when the circuit is off or the switch is activated.

Good Battery + Pattern (Typical):

EXPECT THIS WAVEFORM RESULT:
If there is a failure in the circuit, the waveform's amplitude will change when it is not supposed to; for example, when the switch in the circuit is not being operated. Possible violations of the "Critical Dimension" rule to watch for are spikes to ground (meaning there is an open circuit in the power side, or a voltage short to ground), upward spikes (meaning possible open in ground side), or out of range voltage (either too low or too high).

The 5 Critical Dimensions

FOR MORE INFORMATION
Description of Automotive Signals

Labscope Quick Reference Appendix