Chapter 34 Advanced Diagnostics 671 Copyright by Goodheart-Willcox Co., Inc. Figure 34-21. Manifold absolute pressure sensor can also be checked with a scope. A—Here a scope is probing through a connector to test the MAP sensor. The other lead is grounded. B—Signal frequency should increase with engine speed with a digital MAP. C—Amplitude should increase with engine speed with an analog MAP. Probe to Correct Wire A MAP Sensor Wires to ECM Scope Ground Signal frequency increases as throttle is opened (vacuum decreases). As the throttle closes, the frequency decreases. B The Lower Horizontal Lines Should Almost Reach Ground Peak-to-Peak Voltage Should Equal Reference Voltage Voltage Drop to Ground Should Not Exceed 400mV If the Voltage Drop Is Greater Than 400mV, Look for a Bad Ground at the Sensor or ECU The Upper Horizontal Lines Should Reach Reference Voltage Voltage Transitions Should Be Straight and Vertical High Engine Load Low Engine Load As the Throttle Plate Opens, Manifold Pressure Rises (Manifold Vacuum Lowers) A Low Voltage Level Indicates Low Intake Manifold Pressure (High Vacuum) C A High Voltage Level Indicates High Intake Manifold Pressure (Low Vacuum) Figure 34-22. Note the basic method for testing analog and digital airflow sensors. A—Jumpers are being used to allow power to remain connected to the sensor. Probe service manual recommended pins or wires. B—As airflow increases, analog airflow meter should produce more voltage. C—With a digital airflow meter, signal frequency usually increases with engine speed and airflow. Airflow Sensor Ground Scope Harness to ECU Test Jumpers A Damping Action Caused by Air Flap Movement Airflow into the Intake Manifold Is Increasing Wide Open Throttle, Maximum Acceleration Idle Air Bypass Compensating Airflow into Intake Manifold B The Lower Horizontal Lines Should Almost Reach Ground The Upper Horizontal Lines Should Reach Reference Voltage C Peak-to-Peak Voltages Should Equal Reference Voltage