Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 34:

MAP Sensor Circuit:

CIRCUIT DESCRIPTION
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The Engine Control Module (ECM) receives this information as a signal voltage that will vary from about 1 - 1.5 volts at idle to 4.5 - 4.8 volts at Wide Open Throttle (WOT).

A scan displays manifold pressure in volts. Low pressure (high vacuum) reads a low voltage while a high pressure (low vacuum) reads a high voltage.

If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the Throttle Position (TP) sensor to control fuel delivery.

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

1. Diagnostic Trouble Code (DTC) 34 will set when:

^ Engine running less than 1000 RPM.
^ MAP sensor signal voltage is too low (less than 12 kPa of pressure or greater than 26 inch Hg of vacuum) for less than one second.

OR


^ Engine running greater than 1000 RPM.
^ Throttle position greater than 21%.
^ MAP sensor signal voltage is too low (less than 12 kPa of pressure or greater than 26 inch Hg of vacuum) for less than one second.

2. If the ECM recognizes the high MAP signal, the ECM and wiring are OK.

3. The scan tool may not display 12 volts. The important thing is that the ECM recognizes the voltage as more than 4 volts, indicating that the ECM and CKT 432 are OK.

DIAGNOSTIC AIDS
An intermittent open in CKT 432 or CKT 474 will result in a DTC 34. With the ignition "ON" and the engine "OFF," the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude.

Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Readings should be the same +/- 0.4 volts. Also, Chart C-1D MAP Sensor Voltage Output Check can be used to test the MAP sensor. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

IMPORTANT
Make sure electrical connector 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 indicates a bad connector or connection. If OK, replace sensor.

^ Refer to Chart C-1D MAP Sensor Voltage Output Check for further diagnosis. Chart C-1D Manifold Absolute Pressure (MAP) Output Check