Powertrain Controls - ECM/PCM

Code 34 Diagnostic Chart:

Manifold Absolute Pressure (MAP) Sensor Wiring Schematic:

CIRCUIT DESCRIPTION
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which results from engine load (intake manifold vacuum) and RPM changes; and converts these into a voltage output. The Powertrain Control Module (PCM) sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output voltage of the sensor also changes. By monitoring the sensor output voltage, the PCM can calculate the manifold pressure. A lower pressure (low voltage) output voltage will be about 1.0 to 1.5 volts at idle. While higher pressure (high voltage) output voltage will be about 4.5 to 4.8 at Wide Open Throttle (WOT).

When the engine is not running, the manifold absolute pressure is equal to the barometric pressure. When the ignition is turned "ON", and before the engine is started, the PCM "reads" the MAP sensor signal and uses this to calculate barometric pressure, and make base adjustments for different altitudes. The PCM uses the map sensor signal to control fuel delivery and ignition timing.

NOTE: Manifold Absolute Pressure and manifold vacuum are inversely proportional. "HIGH" manifold pressure is "LOW" vacuum, and "LOW" manifold pressure is "HIGH" vacuum.

DTC 34 WILL SET WHEN
^ Engine running.
^ No DTC 21.
^ MAP less than 14 kPa.
^ Engine RPM less than 1200 or TP sensor greater than 15.2%.
^ Above conditions met for 0.2 second.

ACTION TAKEN (PCM WILL DEFAULT TO)
Malfunction Indicator Lamp (MIL) illuminates. The PCM will substitute a fixed MAP value and use the Throttle Position (TP) sensor to control fuel delivery.

NOTE: Tech 1 will not show defaulted value.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This step determines if DTC 34 is the result of a hard failure or an intermittent condition.
2. Jumpering harness terminals "B" to "C" (5 volts to signal circuit) will determine if the sensor is at fault, or if there is a problem with the PCM or wiring.
3. The Tech-1 scan tool may not display 5 volts. The important thing is that the PCM recognizes the voltage as more than 4 volts, indicating that the PCM and CKT 432 are OK.

DIAGNOSTIC AIDS
An intermittent open in CKT 432 or CKT 416 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 PCM 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 volt.

^ 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.

NOTE: Make sure electrical connector remains securely fastened.

^ CHART C-1D can be used to test the MAP sensor. Refer to "C-CHARTS/CHART C-1D MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR OUTPUT CHECK" for further diagnosis. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

^ If no problems can be found, and code is recurring, refer to "DIAGNOSIS BY SYMPTOM/INTERMITTENTS". - Intermittent Malfunctions

NOTE: After repairs use Tech 1 scan tool "Fuel Trim Reset" function to reset long term fuel trim to 128.