DTC 64

***UPDATED BY TSB # 90-6-27 DATED APRIL 1990


CIRCUIT DESCRIPTION:

The manifold absolute pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts at idle, when manifold pressure is low (high vacuum), to 4.5 - 4.8 volts at wide open throttle (low vacuum or high pressure).

If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the throttle position sensor (TPS) to control fuel delivery.




NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

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

1. Code 64 will set if:

^ Engine rpm less than 600.
^ Manifold pressure reading less than 13 kPa.
^ Conditions met for 1 second.

OR

^ Engine rpm greater than 600.
^ Throttle angle over 20%.
^ Manifold pressure less than 13 kPa.
^ Conditions met for 1 second.

2. This test to see if the sensor is at fault for the low voltage, or if there is an ECM or wiring problem.

3. This simulates a high signal voltage to check for an open in circuit 432. If the test light is bright during this test, circuit 432 is probably shorted to ground. If "Scan" tool reads over 4 volts at this test, circuit 474 can be checked by measuring the voltage at terminal "C" (should be 5 volts).


DIAGNOSTIC AIDS:

^ An intermittent on circuit 474 or circuit 432 can cause a Code 64. With the ignition "ON" and the engine "OFF," the manifold pressure is equal to the atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of altitude.


^ Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same +/- 0.4 volts. Also CHART C-1D can be used to check the MAP sensor


^ Make sure electrical connection remains securely fastened. Check all connections. Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than 0.1 volt indicate a bad connector or connection. If ok, replace sensor. Refer to CHART C-1D, MAP sensor voltage output check for further diagnosis.