Sensor Reference (VREF) Test

Chrysler SBEC Engine Controller:

BACKGROUND AND APPLICATION INFORMATION
This DSO setup and test procedure can be applied to most all VRef circuits on 1980-and-up Electronic Powertrain Management systems. This setup and test procedure verifies that the sensors using the VRef signal as their power source are getting the quality of power they need to operate properly. Many Manifold Absolute Pressure (MAP), Throttle Position Sensor (TPS), Engine Coolant Temperature (ECT), Intake Air Temperature (IAT) and other sensors and circuits use a 5 volt VRef as their power source. 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 look at what is happening. The DSO is set in SPIKE DETECT acquire mode so it's sampling the signal superfast, looking for small or intermittent glitches.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the Powertrain Control Module (PCM) ground circuit, negative battery post or the engine block.
2. Connect the "CH1" probe to the VRef circuit to be tested. Use a wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 2 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 "SPIKE DETECT" or "MIN/MAX" acquire mode.
8. "TRIGGER" mode is usually set automatically (Roll Mode).

EXERCISE THE CIRCUIT UNDER TEST:
Make sure power is switched on to the PCM so the VRef circuit is on. The amplitude of the VRef waveform should stay very close to 5 volts under all conditions. It is common to see a few tenths of a volt increase from KOEO (Key On Engine Off) to KOER (Key On Engine Running). This is normal. If an intermittent problem is suspected, wiggle the wiring harness or tap very lightly on the PCM.

Good 5 Volt Voltage Reference Pattern (Typical):

EXPECT THIS WAVEFORM RESULT:
If there is a failure in the circuit, the waveform's amplitude will change. 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), usually indicating a PCM problem.

The 5 Critical Dimensions

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