After verifying there’s 5 volts to the power wire, probe the signal wire (usually the middle one) and check the voltage on your multi-meter. At 0 in.Hg vacuum the output should be 4.7 volts. Hook a vacuum pump up and check voltage while applying vacuum. At 5 in.Hg the signal should be 3.9 volts, at 10 in.Hg it should be 3.0 volts, and at 20 in.Hg it should be 1.1 volts. More voltage equals a richer condition, less is leaner. Keep in mind even a small variance (0.2 volts) can make a big difference in air/fuel ratio and drivability.