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Chart C-1D:

Chart C-1D Wiring Diagram:

CIRCUIT DESCRIPTION:

The manifold Absolute Pressure Sensor measures the changes in the intake manifold pressure which results from engine load and rpm changes; and converts these changes into a voltage output. The ECM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changed, the output voltage of the sensor also changes. By monitoring the sensor output voltage, the ECM knows the manifold pressure. A low pressure (low voltage output) will be around 1-2 volts at idle. While higher pressure (high voltage output) will be around 4.5 - 4.8 volts at wide open throttle. The MAP sensor is also used under certain conditions as a barometric pressure sensor. This allows the ECM to make certain adjustments for altitude. The ECM uses the MAP sensor to adjust fuel delivery and ignition timing.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Check the current atmospheric pressure and select the value in the atmospheric pressure column in the diagnostic chart. Call the local weather bureau, airport flight service or weather information. Consult your telephone listing for the correct number. You can also Scan the barometric pressure from a good known vehicle.

a. Be sure to use the same DMV for all measurements.

b. Disconnect the MAP sensor harness connector and connect the positive lead of the digital voltmeter (DVM) to terminal "C" (5 volt reference) and the negative lead to terminal "A" (sensor ground). With the ignition "OFF" note the actual 5 volt reference voltage.

c. Select the column in the diagnostic chart which matches the 5 volts reference value measured in STEP b. Follow the 5 volt reference column into the signal voltage area down until it intersects with the line for the atmospheric pressure reading taken in STEP 1.

d. Record the MAP sensor signal voltage value from the chart.

e. Connect 3 jumper wires between the MAP sensor and the harness connector.

f. Connect the positive lead of the DVM to terminal "B" MAP sensor voltage to terminal "A" sensor ground.

g With ignition "ON" and engine "OFF" note the map sensor signal voltage. The voltage noted in this step should be +/- 0.4 volt of the chart value recorded in STEP d. If the voltage is not within the these limits, replace the MAP sensor.

DIAGNOSTIC AIDS:

Comparing atmospheric pressure reading of a good known vehicle with the same sensor is a good way to check the accuracy of a suspected bad sensor. The value should be within +/- 0.4 volts.